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Echoes Through Time: The Historical Origins of the Droplet Dogma and its Role in the Misidentification of Airborne Respiratory Infection Transmission

Echoes through time: the historical origins of the droplet dogma and its role
in the misidentification of airborne respiratory infection transmission
J.L. Jimenez,1L.C. Marr,2K. Randall,3E.T. Ewing,4Z. Tufekci,5T. Greenhalgh,6D.K.
Milton,7R. Tellier,8J. Tang,9Y. Li,10 L. Morawska,11 J. Mesiano-Crookston,12 D. Fisman,13
O. Hegarty,14 S. J. Dancer,15 P.M. Bluyssen,16 G. Buonanno,17 M. Loomans,18 W.
Bahnfleth,19 M. Yao,20 C.Sekhar,21 P. Wargocki22 , A. K. Melikov22, K.A. Prather23
1: Dept. of Chemistry and Cooperative Institute for Research in Environmental Sciences, University of
Colorado, Boulder, CO USA
2: Dept. of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA, USA
3 Dept. of English, Virginia Tech, Blacksburg, VA, USA
4: Dept. of History, Virginia Tech, Blacksburg, VA, USA
5: School of Information and Library Science, University of North Carolina, Chapel Hill, NC, USA
6: Dept. of Primary Care Health Sciences, Medical Sciences Div., Univ. of Oxford, UK
7: Dept. of Environmental Health, School of Public Health, University of Maryland, USA
8: Dept. of Medicine, McGill University, Canada.
9: Dept. of Respiratory Sciences, University of Leicester, Leicester, United Kingdom
10: Dept. of Mechanical Engineering, University of Hong Kong, Hong Kong, China
11: International Laboratory for Air Quality and Heath, Queensland University of Technology, Brisbane,
12: Goldman Hine LLP, Toronto, Ontario, Canada
13: Dalla Lana School of Public Health, University of Toronto, Toronto, ON, Canada
14: School of Architecture, Planning & Environmental Policy, University College Dublin, Ireland
15: Dept. of Microbiology, Hairmyres Hospital, Glasgow, and Edinburgh Napier University, UK
16: Faculty of Architecture and the Built Environment, Delft University of Technology, The Netherlands
17: Dept. of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Cassino, Italy
18: Dept. of the Built Environment, Eindhoven University of Technology (TU/e), The Netherlands
19: Dept. of Architectural Engineering, The Pennsylvania State University, University Park, PA, USA
20: College of Environmental Sciences and Engineering, Peking University, Beijing, China
21: Department of the Built Environment, National University of Singapore, Singapore
22: Department of Civil Engineering, Technical University of Denmark
23: Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA.
Competing interests: The authors declare no competing interest.
Funding acknowledgements: M. Yao was supported by National Natural Science
Foundation of China grants (21725701, 22040101) and Guangzhou Laboratory
(EKPG21-02). T. Greenhalgh was supported by a Wellcome Senior Investigator grant
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The question of whether SARS-CoV-2 is transmitted by droplets or aerosols has been
very controversial. We sought to explain this controversy through a historical analysis of
transmission research in other diseases. For most of human history, many diseases were
thought to transmit through the air, often over long distances and in a phantasmagorical
way, and often in error (e.g. malaria, cholera). Building on the germ theory of disease
developed in the mid 19th century and on the demise of miasma theory, prominent public
health official Charles Chapin in 1910 urged the public health community to focus on
contact and droplet infection. However, he introduced a major error in the process: that
ease of infection in close proximity is associated exclusively with large “sprayborne”
droplets that fall to the ground quickly, and he deemed airborne transmission as very
unlikely. This new paradigm became dominant, leading to systematic errors in the
interpretation of research evidence on transmission. For the next five decades, no disease
was accepted by the general medical and infection control communities as airborne, until
tuberculosis (which had been misclassified as droplet) in 1962. Chapin’s paradigm
remained dominant and only a few diseases were widely accepted as transmitted by
aerosols before COVID-19: those that were clearly transmitted over long distances or time
scales. Resistance to the idea of airborne spread of a respiratory infection is not new. In
fact, it has occurred repeatedly over much of the last century and greatly hampered
understanding of how diseases transmit.
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The COVID-19 pandemic motivated an intense debate over the modes of
transmission of the SARS-CoV-2 virus, involving mainly three modes: impact of
“sprayborne” droplets on eyes, nostrils, or mouth, that otherwise fall to the ground close to
the infected person; direct or indirect (“fomites”) touch of contaminated surfaces followed
by contact with eyes, nose, or mouth; and inhalation of aerosols that can remain
suspended in the air for hours (“airborne transmission”) (1, 2).
Public health organizations such as the World Health Organization (WHO) initially
declared the virus to be transmitted in large droplets that fell to the ground close to the
infected person, as well as by surface touch. The WHO emphatically declared on March
28, 2020 that SARS-CoV-2 was not airborne (except in the case of very specific so-called
“aerosol-generating medical procedures”) and that it was “misinformation” to say
otherwise (3). This advice conflicted with that of many scientists who stated that airborne
transmission was a significant contributor (e.g. 4, 5, 6). Over time, the WHO gradually
softened this stance: first, conceding that airborne transmission was possible but unlikely
(7); then, without explanation, promoting the role of ventilation to control spread of the
virus, which is only useful for controlling airborne pathogens (8); and, finally, declaring on
April 30, 2021 that transmission of SARS-CoV-2 through aerosols is important (9).
Although the WHO technical lead for the pandemic response admitted that “the reason
we’re promoting ventilation is that this virus can be airborne,” she also admitted to
avoiding using the word (10). The Centers for Disease Control and Prevention (CDC) in
the United States followed a parallel path: first, stating the importance of droplet
transmission; then, in September 2020, briefly posting on its website an acceptance of
airborne transmission that was taken down three days later (11); and finally, on May 7,
2021, acknowledging that aerosol inhalation is important for transmission (12). However,
CDC consistently used the term “respiratory droplet,” generally associated with large
droplets that fall to the ground quickly (13), to refer to aerosols (12), creating substantial
confusion. Neither organization highlighted the changes in press conferences or major
communication campaigns (14). By the time these confusing partial admissions were
made by both organizations, the evidence for airborne transmission was overwhelming,
and many scientists and medical doctors were flatly stating that airborne transmission was
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not just a possible mode of transmission, but the predominant mode (15). By Aug. 2021,
the CDC was comparing the transmissibility of the delta SARS-CoV-2 virus with that of
chickenpox, an extremely transmissible airborne virus (16).
The slow and haphazard acceptance of the evidence of airborne transmission of
SARS-CoV-2 by major public health organizations contributed to a suboptimal control of
the pandemic; whereas the benefits of protection measures against aerosol transmission
are becoming well established (17–20). Quicker acceptance of this evidence would have
encouraged guidelines that distinguished rules for indoors and outdoors, greater focus on
outdoor activities, earlier recommendation for masks, as well as rules for mask wearing
indoors even when social distancing could be maintained, ventilation, and filtration. Earlier
acceptance would have avoided the excessive time and money spent on measures like
surface disinfection and plexiglass barriers, which are ineffective for airborne transmission
and, in the case of the latter, may even be counterproductive (21).
Why were these organizations so slow, and why was there so much resistance to
change? A previous paper considered the issue of scientific capital (vested interests) from
a sociological perspective (22). Avoiding costs associated with measures needed to
control airborne transmission, such as better personal protective equipment (PPE) for
healthcare workers (23) and improved ventilation (24) likely played a role. Others have
explained the delay in terms of perception of hazards associated with N95 respirators
(23) that have however been disputed (25) or because of poor management of emergency
stockpiles leading to shortages early in the pandemic (e.g. 26).
An additional explanation not offered by those publications, but which is entirely
consistent with their findings, is that the hesitancy to consider or adopt the idea of
airborne transmission of pathogens was, in part, due to a conceptual error that was
introduced over a century ago and became ingrained in the public health and infection
prevention fields: a dogma that transmission of respiratory diseases is caused by large
droplets, and thus droplet mitigation efforts would be good enough. To understand the
persistence of this error, we sought to explore its history, and of airborne disease
transmission more generally, and highlight the key trends that led to droplet theory
becoming predominant.
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Focusing mainly on infections acquired through the airways (such as tuberculosis,
smallpox, measles, and influenza) and others that were thought historically to transmit
through the air (such as malaria and cholera), we collected historical theories and models
of disease transmission from the ancient Greeks to the present day. Beginning with
sources on this topic that were known to the authors, we used backward-tracking
(pursuing references of those sources) and forward-tracking (tracking the source forward
in Google Scholar to see which subsequent sources cited it). We also used literature
searches in PubMed, Google Scholar, and Web of Science, as well as consultation with
experts to identify other key papers on the same topics. We used hermeneutic methods to
produce a narrative synthesis of this literature, building a progressively richer picture of
how the transmission of particular diseases had originally been conceptualised and what
empirical evidence had led scientists to revise the model of transmission. To refine our
interpretation, we explicitly sought disconfirming studies (e.g. we looked for ones that
challenged prevailing models and assumptions). Further details of the search strategy are
available from the authors.
Disease transmission throughout most of human history: miasmas and infective air
Humanity has been wrestling with the mystery of disease transmission for over two
millennia. After all, figuring out how contagious diseases spread is difficult. When a person
falls ill, we need to consider which of the many things they did (and in particular, to which
infectious agents they were exposed to) led to infection. As we will see, time and again,
this difficulty made it hard to tell exactly how people became sick, and led to incorrect
theories of transmission becoming entrenched, and it was then very difficult to dislodge
them despite strong evidence in support of a rival theory. Transmission through the air is
especially difficult to precisely pinpoint, given that the infectious particles are invisible and
air moves with the least restrictions, compared to e.g. transmission through water, food,
hands, or mosquitoes .
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Hippocrates in ancient Greece first proposed that diseases were caused by
imbalance of humours in the body, which could be triggered by a “miasma” transmitted
through the air: "Whenever many men are attacked by one disease at the same time, the
cause should be assigned to that which is most common, and which we all use most. This
it is which we breathe in" (27). Postulated imbalances between humors also gave rise to a
theory of personality types, for example “melancholia” was ascribed to an excess of black
bile (“melaina chole”). Throughout much of subsequent human history, the belief persisted
that diseases were transmitted through the air. Because the actual causative agents of
airborne diseases remained a mystery for centuries, explanations were given in general
terms such as “miasmas,” or “bad air” (28), as illustrated by the etymological root of the
term malaria (from “mala aria”, medieval Italian for “bad air”). Some origin theories were
more specific than others. For example, Roman scholar Marcus Terentius Varro (116-27
BCE) wrote that swamps were a particular breeding ground for minute creatures that “float
in the air and enter the body through the mouth and nose and there cause serious
diseases” (28). Based on these considerations, it became a policy of the Roman Empire
to drain swamps, which removed breeding grounds for mosquitoes and reduced the
incidence of malaria, an example of a mistaken theory inadvertently giving good results.
Regardless of whether transmitted or triggered by bad humours or minute creatures,
airborne infections were generally not viewed as contagious and transmitted from human
to human. Rather, infection was believed to simply flow through the air and strike people
Persian physician Ibn Sina in his Canon of Medicine in 1025 summarized the
classical grecorroman miasma theory, but also blended with it the idea that people could
transmit disease to others by breath (29). However, the theory of person-to-person
transmission of disease via infection was not clearly formulated until Italian physician
Girolamo Fracastoro (1478-1553) proposed it in 1546 (30). This idea was built upon a
“seeds” theory by Galen of Pergamon, a prolific Greek physician and writer (162 to 203
CE) (31). Galen’s seeds theory had not caught on, probably because he expressed it
somewhat tentatively, and his more extensive writings continuing Hippocratic humoral
theory overshadowed it (32). Interestingly, Fracastoro’s book proposed that the seeds of
disease causing contagion, or “seminaria” as he called them, transmitted through three
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modes: direct, indirect, and at a distance. Contagion at a distance was, he suggested, the
strongest, stronger even than direct contagion. From his writings, these seeds could be
interpreted as chemicals rather than living organisms.
In 1590, less than half a century after Fracastoro’s writings, spectacle-makers Hans
and Zacharias Janssen invented the microscope. This invention was quickly used by other
scientists to discover microorganisms (33). Molds were discovered by Robert Hooke in
1665, who published his famous Micrographica in 1667 (34). Bacteria were discovered by
Antoni van Leeuwenhoek in 1676. These discoveries were a notable step forward; they
demonstrated the ubiquity of tiny living creatures too small to be seen by the naked eye
and yet potentially capable of causing diseases. What ensued after Fracastoro’s
pronouncement, however, was a centuries-long debate between “miasmatists,” who held
fast to the idea that diseases floated through the air over distances, and “contagionists,”
who accepted person-to-person spread of disease (35).
Because, as stated earlier, it is very difficult to determine from where someone got
infected, the debate failed to reach a resolution Observations of outbreaks would
sometimes note that quarantine did not work, suggesting the miasmatists were right. On
the other hand, people were not always struck down from afar, suggesting that perhaps it
was contagion causing the illness. A middle ground was eventually proposed, called
"contingent contagionism", which was a qualified way of rejecting the application of the
term "contagious disease" for a particular infection. Contingent contagionism could hold,
for example, that cholera, or typhus, was not contagious in a healthy atmosphere, but
might be contagious in an impure atmosphere (36). This idea, derived from observation,
therefore captured some grains of truth, since for example airborne diseases are much
more contagious in indoors locations with poor ventilation (15).
Florence Nightingale (1820-1910) was a convinced miasmatist, and wrote in her
Notes on Nursing: “What does 'contagion' mean? It implies the communication of disease
from person to person by contact. [...] There is no end to the absurdities connected with
this doctrine. Suffice it to say that [...] there is no proof [...] that there is any such thing as
'contagion’” (37). However, she collaborated with contingent contagionists on sanitary
measures. She reduced infection rates with hygiene, ventilation, increasing the distance
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between beds in hospitals, and creating an “isolation ward” for tuberculosis patients. She
encountered significant resistance from her superiors and the British government to her
reforms (37–39).
Snow, Semmelweis and the public health establishment
In 1854, a cholera epidemic struck London. The public health establishment believed
it to be caused by a miasma. English sanitary reformers such as Sir Edwin Chadwick, who
initiated many modern public health practices (40), found miasma theory appealing, as it
appeared to explain the prevalence of diseases in the undrained, filthy, and foul-smelling
areas where the poor lived, and helped justify their efforts to address those conditions
John Snow, a wealthy doctor but an outsider to public health, whose work in
anesthesia made him familiar with the behavior of gases, realized that the spread was not
consistent with what would be expected for a gas. He noticed how cases had clustered in
a specific London borough and persuaded the local council to remove the handle of the
Broad street water pump, which halted the epidemic (42). However, by the time he did
this, the epidemic was already in decline and so the Board of Health in the end refused to
accept contaminated water as the explanation, issuing a report stating “[w]e see no
reason to adopt this belief [that cholera was water-borne],” and dismissing Snow’s
conclusions as mere “suggestions” (43). Snow died before his discovery was accepted.
The Sanitarians had strong incentives for rejecting water as the source of cholera. To
remove the sources of the miasma (filth), they had spearheaded the effort to build sewers
that dumped raw sewage into the Thames, the source of much of London’s drinking water,
thus effectively helping the spread of cholera. They had much to lose by admitting cholera
transmitted through water, including their prestige.
Ignaz Semmelweis was another pioneer of disease transmission who was also
initially ignored as having proposed things too radical for the establishment of the time to
accept. Working in Vienna in 1847, he showed that handwashing greatly reduced deaths
by childbed fever in a maternity clinic (44). However, his ideas conflicted with established
medical and scientific beliefs that still described diseases as due to an imbalance of
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humours. Not helping matters, his colleagues resented not only his brash style but also
the implication that they were hurting their patients by not handwashing, and he was
largely ignored, rejected, or ridiculed. Although his data were compelling, he was
dismissed from his hospital and harassed by the medical community in Vienna so much
that eventually he was forced to move to Budapest. After some years there, he broke
down, was interned and beaten by the guards, and ultimately died from an infected
wound. As with Snow, Semmelweis never saw the fruits of his work, as the importance of
handwashing to reduce infection was only accepted by the medical community after his
death. In an ironic turn, Semmelweis’ name lives on not only for his advances of hand
sanitation, but also in the term “Semmelweis reflex”, which has been coined to describe
the reflex-like tendency to reject new evidence or new knowledge because it contradicts
established norms, beliefs, or paradigms (45).
Second half of 19th century: germ theory
In 1833, In the second half of the 19th century, Pasteur and Koch offered evidence to
support their germ theory of disease. In 1861, Pasteur conducted experiments disproving
the spontaneous generation and proving there are viable microorganisms in the air (46).
However, germ theory was not accepted overnight, and it too encountered much
resistance. For example, experiments by others in which water containing organic matter
was boiled in a vessel, but microorganisms still appeared (later shown to be due to an
imperfect seal or insufficient boiling time) created significant controversy at the time (47).
But by the late 1880s, miasma theory was waning in popularity, and in 1888 the Institut
Pasteur was created in Paris, reflecting the ascendancy of germ theory. Viruses were first
discovered in the 1890s (48, 49). A “golden era” followed, with the identification of the
actual microorganisms that cause many infectious diseases.
The discovery and identification of the organisms causing different diseases did not,
however, eliminate the great difficulty in conclusively determining the mode by which they
transferred from one person to another. For example, French physician Charles Laveran
identified in 1880 the malaria pathogen, but the manner of transmission was still thought
to be through the air. American physician Albert Freeman Africanus King first proposed
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that malaria was transmitted by mosquitoes, but encountered general skepticism. In 1883
he presented a list of 19 facts that supported mosquitoes as the vector of malaria
transmission. However, the theory was not accepted until 1898 when British surgeon
Ronald Ross provided definitive evidence, confirming the presence of the malarial
parasites in mosquitoes, and demonstrating transmission of bird malaria by mosquitoes
In the 1890s Carl Flügge in Germany set out to disprove the then-dominant
transmission theory for tuberculosis, one of the major infectious diseases of the time. Most
experts believed that tuberculosis was transmitted when dust of dried sputum (spit) that
had landed on floors, blankets, bowls, and other contaminated objects was dispersed into
the air. In contrast, Flügge thought that it was not the dried secretions from the sick that
caused infection, but rather fresh secretions that people were exposed to in air before
they reached the ground (50). Some contemporaries of Flügge thought this meant
tuberculosis was transmitted only through large droplets, which were easily visible to the
naked eye (51). However, although the term "Flügge's droplets" has been used to
describe only those large particles that fell to the ground quickly near the infected person
and that were assumed to dominate transmission (e.g. 52), that does not accurately
capture Flügge’s results. Rather, Flügge and collaborators used the term “droplet” to refer
to fresh particles of all sizes, including aerosols for which the researchers waited 5 hours
to settle from the air on their collection plates.
Investigation of airborne infection continued. In 1905, microbiologist M.H. Gordon
was commissioned to study the atmospheric hygiene of the UK House of Commons after
an epidemic of influenza among members. He famously performed the following
experiment: after gargling with a broth culture of Serratia marcescens (formerly known as
B. prodigiosus and other names; environmental strains produce a bright red pigment
making colonies unmistakable, and the bacterium has often been used as biological
marker), he loudly recited passages from Shakespeare in an empty House to an audience
of agar plates, in order to investigate the spatial reach of pathogen-containing aerosols
and droplets. Although growth of colonies was more numerous on plates near the
speaker, cultures were apparent on some plates over 21 m away (53, 54). However,
progress was hampered by the limitations of the experimental techniques available.
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Charles Chapin, Contact Infection, and the Key Errors
The critical point in this history of the understanding of airborne disease transmission
is the work of prominent American epidemiologist, Charles V. Chapin. Chapin worked
only a couple of decades after the germ theory was accepted, during a period of intense
research on pathogen transmission. It was a fluid time, following a major paradigm shift, in
which it was easier to change the dominant scientific discourse than during normal times
(55). He summarized the evidence of transmission of different diseases in his 1910
seminal book, "The Sources and Modes of Infection" (56). Based on his own success with
infection prevention, he conceptualized "contact infection," i.e. infection by germs that did
not come from the environment, but came from other people through direct contact or
close proximity. Chapin believed that contact infection was the main mode of transmission
of many diseases. But like any new theory, his encountered resistance: "I have sometimes
been told I lay too much emphasis on contact infection,” he wrote, although "until recently
very little attention has been paid to it." He was no doubt aware of the resistance faced by
Semmelweis, Snow, Pasteur, Koch, King, and many others, and realized the need to
make his case forcefully if he was to convince his colleagues of the importance of contact
Chapin also reviewed the possibility of airborne infection. Important diseases such as
cholera and malaria, that were for centuries thought to be transmitted through the air, had
undergone paradigm shifts that supported other routes of transmission. Nevertheless,
airborne transmission was considered likely enough in public health to warrant a response
from Chapin, and the miasmatic ideas of phantasmagorical disease transmission through
the air were still in the public's mind. As Chapin admitted at the end of that chapter, the
lingering belief in airborne infection was the main obstacle he encountered to promote his
ideas of the importance of contact infection: “If the sick-room is filled with floating
contagium, of what use it is to make much of an effort to guard against contact infection?
[...] It is impossible, as I know from experience, to teach people to avoid contact infection
while they are firmly convinced that the air is the chief vehicle of infection.”
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Chapin was aware of the work of Flügge and at the UK House of Commons showing
transport of germs for considerable distances and floating in the air for hours. He also did
realize that airborne infection may explain infection in close proximity. But he argued that
ease of infection in close proximity was better explained by “spray-borne” droplets, the
large visible droplets considered by Cornet and others. He argued that since germs began
to die or lose their virulence outside of the body, the closer we were to others, the greater
the chance of infection. There were many opportunities for "transfer of secretions"
between people during close contact, including from asymptomatic cases, which had been
identified by Koch for cholera (57), or as in the famous case of "Typhoid Mary," an
asymptomatic cook who infected 53 people with typhoid fever in New York City in 1907
(58). Chapin stated that “[t]here is no evidence that [airborne transmission] is an
appreciable factor in the maintenance of most of our common contagious diseases.” And
critically, he turned absence of evidence into evidence of absence. “We are warranted
then, in discarding [airborne transmission] as a working hypothesis, and devoting our chief
attention to the prevention of contact infection,” he concluded. “It will be a great relief to
most persons to be freed from the specter of infected air, a specter which has pursued the
race from the time of Hippocrates.”
Neither Snow nor Semmelweis were highly recognized in public health before their
major discoveries and, as is often the case, faced more resistance to their ideas (59).
Chapin was much better positioned to change the paradigm of transmission, as the
long-serving Health Officer of Providence and also thanks to the success of his emphasis
on contact transmission in reducing infections in a new hospital. In 1927 he became the
President of the American Public Health Association. His ideas about the dominance of
contact infection and the implausibility of airborne infection were widely adopted in the
fields of public health and infectious diseases. Chapin was described in 1967 as "the
greatest American epidemiologist" by Alexander Langmuir, the first and long-time director
(1949-1969) of the epidemiology branch of the CDC, and as late as the 1980s Chapin’s
views were dominant there (60). Critically, Chapin’s unproven hypothesis was accepted as
true: ease of infection in close proximity is accepted proof of transmission from spray
droplets. This key error conditioned the evolution of this field over the next century.
Chapin’s ideas were still dominant at the start of the COVID-19 pandemic.
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No natural disease is airborne (1910-1962)
Influenza, thought in the 15th century to be caused by the noxious influence of winter
constellations (“influenza delle stelle”), can cause severe pandemics when a significantly
different strain emerges through genetic shift. The most severe pandemic by far in the 20th
century was that of the 1918 "Spanish Flu". In the early stages of the 1918 influenza
epidemic, a warning from the US Surgeon General published in newspapers across the
United States warned that of “germs being carried with the air along with the very small
droplets of mucus, expelled by coughing or sneezing, forceful talking, and the like” (61).
The dangers of infection thus justified public health recommendations for the public to
cover their coughs, avoid crowds, and wear masks in the same room as infected persons.
There was some evidence that ventilation and outdoor air reduced transmission, which
suggested airborne transmission. For example, some cities such as Chicago implemented
public health measures strongly focused on ventilation, including in schools, churches,
and rooms where patients were being treated; places of public gathering, such as
dance-halls and theatres, were closed until thorough renovation works were carried out as
a condition for a permit to reopen. Chicago had been the first city to adopt ventilation
ordinances in public buildings and conveyances (including street cars) and in workplaces
in 1910. The city reopened within 6 weeks and did not have a second wave of pandemic
(62), although it may have fared better than other cities for a combination of reasons.
However, the understanding of the pathogen and its transmission were limited, and
Chapin’s ideas became firmly established over the next two decades.
In the 1930s, Harvard engineering professor William Wells and physician Mildred
Wells, his wife, started applying more contemporary methods to the investigation of
airborne transmission. Chapin had successfully shifted the paradigm and his theory was
now viewed as scientific progress, while the Wellses were accused of a retrograde
approach to science which sought to bring back the miasma theory (63).
William Wells was the first person to rigorously study the size of spray-borne droplets
vs. airborne aerosols. He conceptualized a dichotomy of sprayborne droplets (100
microns) that reach the ground before they dry, vs. aerosols (100 microns) that dry
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before they reach the ground (thus referred to as "droplet nuclei"). He correctly
understood the connection with meteorology where these facts are common knowledge
(64), stating “A raindrop 2 mm in diameter can fall miles without completely evaporating
under conditions which would cause a 0.2 mm droplet to evaporate before it had fallen
from the height of a man” (65).
The Wellses suspected that tuberculosis and measles were airborne, but both were
already believed to be droplet diseases, and they encountered intense resistance from the
epidemiological community. Measles was described as a droplet disease as late as 1985,
because of ease of transmission in close proximity and cases of lack of infection with
shared air (66). W. Wells had some initial success showing that UV light installed in the
ceiling of classrooms, irradiating upward such that only aerosols rising through thermal
plumes would be exposed to UV, greatly reduced measles infection (67). However,
subsequent attempts to replicate these findings produced mixed results. In retrospect, it is
clear why: in the schools where UV prevented transmission, children were together
indoors only in the school, not elsewhere. Thus, disinfecting the school air was effective.
In subsequent studies at other schools, the children shared other indoor spaces (such as
school buses), for hours. Thus, there were plenty of opportunities for transmission of
measles via shared indoor air that was not subject to UV disinfection. In a 1945 article in a
predecessor journal to Science, W. Wells lamented how our societies had invested and
been successful in eliminating infections through drinking water and food, but no action
had been taken to limit airborne infection, since it was widely accepted that natural
diseases were not airborne (68).
In 1951, Langmuir stated, "It remains to be proved that airborne infection is an
important mode of spread of naturally occurring disease” (69). Langmuir had worked on
preventing infectious disease transmission among US military personnel during World War
II. Substantial resources were dedicated to the effort, given the impact of disease
outbreaks on military readiness, generating knowledge “which would have taken decades
to accumulate under peacetime conditions” and that established the professional leaders
in this area for the next several decades (60). However, Langmuir and collaborators had a
key problem when trying to investigate airborne infection: they viewed the world through
the lens of Chapin's theories. For example, in one study, crowding was reduced in military
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barracks in order to determine if rates of illness decreased, with the reasoning that
increasing distance would reduce close proximity (and thus prevent droplet-based
transmission) Conversely, if transmission were airborne, Langmuir expected that reducing
crowding should have no impact. Reducing crowding reduced disease, thus
“substantiating the role of droplet spread” (60). What Langmuir had not realized is that that
mental model of airborne infection is defective, ignoring the fact that the exhalation of an
infected person is most concentrated in close proximity, with much dilution upon mixing
with room air (13, 70), as illustrated in Figure 1. The impact of Chapin’s views was
profound, leading to the misinterpretation of transmission studies over a century, including
in dominant public health institutions such as the CDC.
Figure 1. Illustration of droplets and aerosols released during talking; these may carry
viruses if the person is infected. The large droplets fall rapidly to the ground in close
proximity. The small aerosols are much more concentrated in close proximity, and they
can remain floating in the air and spread throughout the room, leading to (reduced)
exposure at a distance.
However, Langmuir’s work renewed interest in the physics of airborne infection, as
he concluded that weapons of airborne disease can be created, which became a topic of
intense interest during the cold war (60). Based on studies of occupational exposure, he
learned that aerosols smaller than 5 microns can penetrate deeply into the lung, all the
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way into the alveolar region. Infectious disease aerobiology was extensively developed
during this period as part of the US and Soviet Union bioweapons programs (71).
However, most of the work remained classified even after the weapons were banned, thus
that body of work had little influence on the general medical and infection control
communities. This may have contributed to the maintenance of dominance of Chapin’s
Reluctant acceptance of as little airborne transmission as possible (1962-2020)
Despite the stubborn resistance to the idea that airborne transmission had any
relevance for natural diseases, W. Wells, Robert Riley, and Cretyl Mills succeeded in
demonstrating airborne transmission of tuberculosis (TB) in 1962. They routed the air from
a tuberculosis ward to 150 guinea pigs for 2 years. About three guinea pigs per month
were infected. However, none were infected in a control group where the only difference
was that the air was irradiated with germicidal ultraviolet light, killing the TB bacterium (72,
73). Because of this study, TB was the first natural disease to be accepted as airborne in
modern times.
The standards of evidence were clearly different, as many diseases were accepted
as “droplet” without such proof. The resistance to a larger role for airborne infection
continued, with a pattern of accepting airborne transmission on a case-by-case basis for
each disease only when the evidence was undeniable--that is, only when all other
transmission routes could be ruled out and the evidence was very clear. For example,
there was an obvious case of long-distance airborne transmission of smallpox in Germany
in 1969. A report on the outbreak reflected the ongoing thinking, concluding, after ruling
out all other plausible infection routes: "The only remaining route of transmission
considered reasonable was airborne spread of a virus-containing aerosol, a possibility
against which all of the investigators were initially prejudiced" (74). In addition, this
outbreak was described as an unusual event, “a unique exception,” and droplet
transmission continued to be considered dominant. The success of the program to
eradicate smallpox was taken as vindication of this view (60). However, ease of infection
in close proximity together with some cases of distant infection in shared indoor air with
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low ventilation is a signature of airborne transmission (15), and there is evidence that
airborne transmission of smallpox was much more important than has been accepted so
far (75). The smallpox incubation period was very precise, virtually 100% of infected were
symptomatic, viral shedding transmission only occurred when people were
infected/symptomatic, and the infected were very sick and did not move around very
much. Thus, the track/trace/isolate/quarantine/ring vaccination approach of the eradication
program worked well, despite the potential airborne transmission (75, 76).
The same pattern of scientific inquiry played out for measles and chickenpox, two
extremely contagious diseases, whose airborne character was resisted for seven decades
and only finally widely accepted in the 1980s based on superspreading events with
long-distance transmission (66, 77). Importantly, ease of transmission in close proximity
was observed in all accepted airborne diseases (hence their original classification as
droplet diseases) (66, 78, 79). But despite this overlap, ease of transmission in close
proximity continued to be taken as evidence of droplet-only transmission for other
diseases. Lack of measles transmission with shared indoor air was also used as an
argument against its airborne transmission. The same feature has been observed for
COVID-19, and is now understood to be due to very high variability in viral load and
aerosol shedding among individuals, as well as differences in respiratory intensity and
vocalization between different situations (80–85).
During the last several decades and until the COVID-19 pandemic, with available
antibiotics, vaccines, and no huge pandemics, studies further probing the details of droplet
vs. airborne transmission had not been a major public health priority. The aftermath of the
Oil Crisis and then the Climate Crisis have led to compromises in building standards for
energy saving over ventilation and public health (86). The high standards of ventilation
and filtration adopted for modern hospitals (87–89) means that airborne risks have been
largely mitigated in these settings, where many key infection control scientists work.
Adherents of droplet transmission were in control of all key public health institutions, and
scientists proposing airborne transmission were typically ignored (50). As just one
example, colleagues report that they would write a research proposal to fund a study of
airborne transmission, and the anonymous peer-reviews would come back saying
"airborne transmission is not important, therefore we should not fund this proposal."
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Evidence also points to the importance of airborne transmission for another disease
with high pandemic potential: influenza (90–92), including superspreading in
poorly-ventilated indoor air (93, 94), low transmission in well-ventilated environments (95),
exhaled infectious virus (96, 97) and viral RNA (98), detection (of both infectious virus and
viral RNA) in room air (99–101), 100 times smaller dose by inhalation of aerosols vs.
intranasal inoculation (102–105), and airborne transmission in animal models (106, 107).
However, likely due to the same kinds of resistance as described above for other
diseases, airborne transmission of influenza virus has not been widely accepted, and it is
still described by WHO and CDC as a droplet/fomite disease, with no mention of airborne
transmission (108, 109).
There is also evidence for airborne transmission of rhinovirus (110–114), adenovirus
(115), SARS-CoV-1 (116, 117), MERS-CoV (118, 119), and RSV (120, 121). Limited data
suggests a role of airborne transmission for enteroviruses (122, 123), filovirus (124), and
other pathogens.
Furthermore, airborne transmission of viruses is well accepted in veterinary medicine
including for some coronaviruses and influenza viruses, sometimes over distances of
many kilometers. Examples include the foot and mouth virus (125, 126), porcine
reproductive and respiratory syndrome virus (PRRSV) (127, 128), porcine respiratory
coronavirus (129), avian infectious bronchitis virus (also a coronavirus) (130), and equine
influenza (131, 132).
The COVID-19 pandemic and the uncovering of the historical error
At the start of the COVID-19 pandemic, a literature review concluded that droplet
infection has never been demonstrated directly for any disease (13). Despite a lack of
direct evidence, early in the pandemic public health institutions like WHO concluded that
ease of transmission in close proximity proved that COVID-19 was another droplet (and
fomite) disease (3), continuing Chapin’s 1910 error. Key experts from the WHO IPC
committee wrote that they would recognize an airborne disease given an expected high R0
(23), despite their fields having taken 70 years to recognize measles and chickenpox as
airborne (66, 77), and despite the fact that pulmonary tuberculosis is exclusively airborne
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and yet less contagious than COVID-19 (20). Interestingly, despite publications with the
types of evidence that were sufficient for accepting tuberculosis (animal experiments,
(133)), and measles/chickenpox (superspreading and long distance transmission, (e.g.
134, 135, 136)) as airborne, WHO and other public health agencies continued to resist the
importance of airborne transmission of COVID-19 for almost a year. The public health
establishment remained entrenched in the old droplet paradigm. It considered the
evidence of airborne transmission provided by the aerosol scientists, who were rebuffed
and systematically excluded from key committees, as weak or irrelevant (10, 22). The
same pattern discussed above, i.e. minimizing the role of airborne transmission as much
as possible, was on display, through the use of terms like “situational airborne,” or by
claiming airborne transmission is restricted only to poorly ventilated crowded locations.
The logic error is obvious, since all airborne pathogens are very sensitive to ventilation
(e.g. 137), and if they can infect in shared room air, they must be much more infective in
close proximity where they are much more concentrated (70).
Over the course of a year, accumulating evidence showed unequivocally that
COVID-19 is a predominantly airborne disease, and the fallacy of conflating infection in
close proximity exclusively with droplet transmission became clear (15, 70, 138). Lack of
control of the pandemic through only droplet/fomite measures such as physical distance,
handwashing and surface disinfection became apparent. WHO (9) and CDC (12) finally
partially accepted airborne transmission of SARS-CoV-2 in April / May 2021 as important.
However, the changes as of Aug. 2021 were often expressed quite confusingly and had
received insufficient publicity (14) and changes in the mitigation measures were only
partially reaching most of the world. Some of the emerging SARS-CoV-2
variants-of-concern are more transmissible (139), and for this reason the cases of
airborne superspreading or long-distance transmission have become easier to detect. The
CDC has compared the transmissibility of the delta SARS-CoV-2 variant with that of
chickenpox, a highly contagious airborne disease (16).
It has also become clear that some public health organizations would at times use
the concept of ‘short-range’ or ‘close-contact’ transmission via “droplets” as due to
particles that can be inhaled, which is actually describing an aerosol phenomenon. To be
inhalable, particles need to be smaller than about 100 microns (140). They are thus
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aerosols that can travel beyond close proximity of the infected person (65, 141). Milton (1)
proposed avoiding the potentially ambiguous term “droplet”, and using the terms
“aerosols” for smaller particles that can be inhaled, and “drops” for the larger particles that
fall to the ground, being too heavy to be inhaled.
WHO commissioned a series of systematic reviews on the transmission of
SARS-CoV-2 to a specific group. In what would appear to be a major error in WHO’s
commissioning, no experts on aerosols or airborne transmission participated in the review
on airborne transmission. Airborne transmission was reviewed in an extremely narrow
way, only considering one type of evidence, namely the detection of viable virus in air
(142), despite the fact that this has not been achieved for accepted airborne diseases
such as tuberculosis, measles, and chickenpox (4, 143). The many other types of
evidence that support airborne transmission as predominant for SARS-CoV-2 and that led
to acceptance of tuberculosis, measles, and chickenpox as airborne (15, 66, 67, 72, 77)
were completely ignored in the review. As of this writing the paper had not passed
peer-review, and the public comments from other scientists remained unanswered (e.g.
144). A review was written for “close contact” (145), which appears to be a conceptual
error since close contact is a measurement of distance and not a mechanism of
transmission. Shockingly, no review has been posted summarizing the evidence
supporting droplet transmission, despite WHO and key coauthors stating that it is the main
mechanism of transmission.
After the 2003 SARS-CoV-1 outbreaks, a new and misleading ‘miasma’ had
appeared, in the form of “aerosol-generating procedures” (AGPs). These are medical
procedures such as bronchoscopy, intubation, suctioning, etc., which were thought to
generate large amounts of aerosols and to have infected some of the medical staff
performing them during the 2003 SARS-COV-1 outbreaks, although the evidence
supporting this association was weak (146, 147). AGPs were the only circumstance in
which WHO clearly accepted airborne transmission as of mid-2020 (7). However, multiple
studies during the COVID-19 pandemic showed that patients produce more aerosols
through simply breathing, talking, and coughing than from many AGPs (148–152). The
continued emphasis on AGPs as a much higher airborne transmission risk than from
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naturally-produced aerosols is misguided and misleading, but has not been widely
corrected as of this writing.
The 5 micron error and its implications
During the COVID-19 pandemic a different error also became apparent. Public
Health documents such as the July 2020 WHO Scientific Brief on COVID-19 transmission
(7) repeat a long-standing error in previous guidance and scientific literature: they place
the separation between droplets that fall to the ground in 1-2 m and aerosols that remain
airborne at 5 microns, instead of the correct value of 100 microns (an error of a factor of
8000 in the mass of the particles). The correct boundary was published by Wells in 1934
(65), and is shown in the CDC webpage (occupational medicine branch (153)), was
confirmed by more recent publications from aerosol scientists (141), and again multiple
times during the COVID-19 pandemic, including a workshop of the US National
Academies of Science, Engineering, and Medicine (138, 154). However, the error has
persisted in the scientific literature and guidance documents and was not corrected by
WHO as of Aug. 2021. Randall et al. (10, 50) have investigated the source of this error,
and traced it to the 1960s, where tuberculosis was only accepted airborne infection, which
led to a confusion between the particle size that penetrates the deep lung (necessary for
TB infection) and that falls to the ground in 1-2 m.
The fact that the 5 micron error was able to persist for so long demonstrates the
overwhelming dominance of Chapin’s paradigm in epidemiology, where droplet infection is
the assumed mode of transmission unless proven extremely conclusively otherwise, and a
lack of interest in the details of the relevant processes and on the input from disciplines
such as aerosol science and even occupational medicine. Because aerosols (up to 100
microns) can follow air currents, the recognition of their complete size spectrum is
important for the selection of PPE that will provide a seal around the airways (e.g.
N95/FFP3). Also, recognition that only small-size aerosols can penetrate into the lower
respiratory tract (< 20 microns, and <5 microns for the alveolar space (140)) has important
implications for infections affecting only the lower respiratory tract, e.g. MERS-CoV, as this
implies that transmission must occur through (small) aerosols (155).
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Outlook for control of respiratory diseases and the next pandemic
This overview of the history illustrates the pervasiveness of “belief perseverance,” the
psychological tendency to maintain a belief despite clear and strong new evidence that
should challenge it (156). In an era of amazing scientific advances, where mRNA vaccines
were designed in a few days following virus sequencing also obtained in a few days, the
very slow acceptance of critical new knowledge reminds us that the human aspects of
science remain as pervasive as they were in past eras.
However, the intense research and debate associated with the COVID-19 pandemic
has finally begun to generate a paradigm shift in the understanding of disease
transmission. Not only are respiratory diseases not transmitted exclusively by droplets, but
also it is likely that many or most respiratory diseases have an important airborne
component of transmission. It is also clearer that for a disease to cause a fast-spreading
pandemic, airborne transmission is likely to be an essential component. This does not
mark a return to past miasmatic ideas, but a more informed understanding of airborne
transmission as more complex and less scary than in the past, and certainly as a tractable
problem (157). This new paradigm has major implications for the regulation and control of
air quality in indoor spaces, by proper ventilation, filtration and other means, as well as for
PPE for workers, masking by the public etc. Finally, the lack of attention to the quality of
shared indoor air pointed out by Wells (68) may start to be remedied in the coming years
(24), potentially leading to a reduction in respiratory disease transmission for decades to
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... However, with a case of COVID-19 in a person that have had not travelled to China, human-to-human transmission was confirmed in January, 23 of 2020 [17,18]. Yet, due to a century-old dogma regarding routes of disease transmission [19], the WHO guidelines for preventing infections during the pandemic were strongly based on the assumption that human-to-human COVID-19 spreading occurs through large (> 100µm) droplets, usually generated in coughs or sneezes, that would then rapidly fall in the air. An infection would then happen by touching contaminated surfaces or objects, followed by touching exposed mucous parts such as eyes and mouth. ...
... This then required an update of the recommended non-pharmaceutical interventions 1 that included well-fit high-grade (N95/PFF2) mask usage, adequate indoor ventilation, isolation of infected individuals and air filtering [26]. It is important to notice that this shift in paradigm is still underway, with implications in other diseases such as Influenza and SARS [19], and it took almost two years into the pandemic for the WHO to explicitly state that airborne transmission is possible in their guidelines [27]. ...
Full-text available
The SARS-CoV-2 pandemic represented a great challenge for Public Health around the world. The virus has spread to almost all countries in the world and caused more than 500 million cases and 6 million deaths to this day (2022-04-20). Yet, due to massive investments and strong international collaborations, several COVID-19 vaccines were available by the end of 2020. This sparkled questions on how these vaccines should be allocated in this scenario of high demand and scarce stock. This work contributes to this discussion by developing several models related to COVID-19 vaccination. We first provide a review of COVID-19 in Brazil, followed by an introduction to mathematical modelling to epidemics, with an emphasis on vaccination models. We then show three applications of this theory to vaccination strategies against COVID-19 in Brazil. As most vaccines developed were designed with a two-dose vaccination rollout, one of the questions raised was the optimal interval for each vaccine in this scenario of scarcity of doses. To answer this, we develop a delay differential equations model coupled with an optimization model to allocate between first and second doses without requiring reserves of doses. We conclude that, if the effectivity of the first dose compared to the second one is lower than 50%, the best strategy to reduce deaths is to inoculate the second dose as fast as possible, whereas if the relative effectivity is higher than 50%, the strategy is strongly dependent on the production rates of vaccine over time. If the production rate is low, the best strategy is to delay as much as possible, whereas if the production rate is higher, the optimal strategy is dependent on the relative effectivity. We also show that the optimal time window between doses is not dependent on the effective reproduction number of the disease, but it has a role in the magnitude of deaths averted by vaccination. As the pandemic progressed and the Gamma variant emerged in Brazil, the recommendations needed to be revisited. We developed a discrete-time static vaccination model to assess the best strategy to allocate doses of AZD1222 (AstraZeneca/Oxford/Fiocruz) with the new estimates of vaccine effectiveness. We first found that, following an age-based rollout, at least 80% of an older age group should be vaccinated before making vaccines available to younger individuals, having additional overall deaths otherwise. We then proceed to show that the interval between doses of AZD1222 should be reduced from 12 to 8 weeks in a Gamma dominated epidemic and in an ideal scenario of availability of doses. However, we show that this change in policy would only have a measurable effect if the number of doses increases by at least 50% of the quantity procured by the Brazilian government until the end of 2021. At the end of 2021, child vaccines were approved by the Brazilian National Agency of Sanitary Vigilance (ANVISA) at the same time that the Delta variant was being substituted by the Omicron variant in Brazil. We then developed a two-strain discrete-time dynamic model with reinfection to assess the impact of children (5 to 11 years old) vaccination in a scenario of variant substitution in Brazil. We estimate that around 2.4 thousand hospitalizations and 180 deaths of children could be averted with the number of doses procured by the Brazilian government, with an indirect effect of reducing almost 10 thousand hospitalizations and 1.5 thousand deaths in all age groups, between mid-January and April 2022. We also show that the impact of vaccinating children more than doubles in an optimal (and achievable) rollout of 1 million doses inoculated per day, with 5.4 thousand hospitalizations and 410 deaths averted in children, with an indirect effect of 26.5 thousand hospitalizations and 4.2 thousand deaths by COVID-19 averted in all age groups, evidencing the necessity of increasing the number of doses procured by the Brazilian government. We continue with a basic introduction to Bayesian statistical modelling and an overview of the relatively new Integrated Nested Laplace Approximation (INLA). We then proceed to develop a statistical model using INLA to estimate the early impact of vaccination against COVID-19 in Brazil. By doing a counterfactual analysis of an autoregressive model with explicit vaccination coverage covariates, we estimated that the vaccination directly averted around 167 thousand hospitalizations and 76 thousand deaths by COVID-19 in Brazil in the high risk age group of 60+ years old until the end of August 2021. By shifting the vaccination rollout by 4 and 8 weeks earlier, we estimated that these figures would increase to 219 mil hospitalizations and 101 thousand deaths, and 268 thousand hospitalizations and 124 thousand deaths, respectively, evidencing the impact if proper planning and vaccine procurement were done by the Brazilian government. We end this work with some remarks regarding the usage of mathematical modelling in public health in Brazil.
... But in most western countries the aerosol narrative initially fell on deaf policy ears. By July 2020, aerosol scientists were alarmed that official advice was based on oversimplistic and incorrect models of transmission (which had perpetuated for decades in the infection control literature 37 ) and wrote an open letter to WHO offering to help. 5 "Covid is 'situationally' airborne" ...
... 2,3 The small size (aerodynamic diameters <100 μm) enables virus-laden aerosols to suspend in the air for a long duration. 4 SARS-CoV-2 RNA has been detected in aerosols sampled from two hospitals in Wuhan, China during the outbreak of COVID-19. 5 Moreover, SARS-CoV-2 possesses a half-life of 1.1 h in aerosols based on a lab study, which shows the persistence and potential infectivity of coronaviruses within aerosols. ...
Full-text available
The global COVID-19 pandemic has raised great public concern about the airborne transmission of viral pathogens. Virus-laden aerosols with small size could be suspended in the air for a long duration and remain infectious. Among a series of measures implemented to mitigate the airborne spread of infectious diseases, filtration by face masks, respirators, and air filters is a potent nonpharmacologic intervention. Compared with conventional air filtration media, nanofibrous membranes fabricated via electrospinning are promising candidates for controlling airborne viruses due to their desired characteristics, i.e., a reduced pore size (submicrometers to several micrometers), a larger specific surface area and porosity, and retained surface and volume charges. So far, a wide variety of electrospun nanofibrous membranes have been developed for aerosol filtration, and they have shown excellent filtration performance. However, current studies using electrospinning for controlling airborne viruses vary significantly in the practice of aerosol filtration tests, including setup configurations and operations. The discrepancy among various studies makes it difficult, if not impossible, to compare filtration performance. Therefore, there is a pressing need to establish a standardized protocol for evaluating the electrospun nanofibrous membranes' performance for removing viral aerosols. In this perspective, we first reviewed the properties and performance of diverse filter media, including electrospun nanofibrous membranes, for removing viral aerosols. Next, aerosol filtration protocols for electrospun nanofibrous membranes were discussed with respect to the aerosol generation, filtration, collection, and detection. Thereafter, standardizing the aerosol filtration test system for electrospun nanofibrous membranes was proposed. In the end, the future advancement of electrospun nanofibrous membranes for enhanced air filtration was discussed. This perspective provides a comprehensive understanding of status and challenges of electrospinning for air filtration, and it sheds light on future nanomaterial and protocol development for controlling airborne viruses, preventing the spread of infectious diseases, and beyond.
During the COVID-19 pandemic, WHO and CDC suggest people stay 1 m and 1.8 m away from others, respectively. Keeping social distance can avoid close contact and mitigate infection spread. Many researchers suspect that suggested distances are not enough because aerosols can spread up to 7–8 m away. Despite the debate on social distance, these social distances rely on unobstructed respiratory activities such as coughing and sneezing. Differently, in this work, we focused on the most common but less studied aerosol spread from an obstructed cough. The flow dynamics of a cough jet blocked by the backrest and gasper jet in a cabin environment was characterized by the particle image velocimetry (PIV) technique. It was proved that the backrest and the gasper jet can prevent the front passenger from droplet spray in public transportation where maintaining social distance was difficult. A model was developed to describe the cough jet trajectory due to the gasper jet, which matched well with PIV results. It was found that buoyancy and inside droplets almost do not affect the short-range cough jet trajectory. Infection control measures were suggested for public transportation, including using backrest/gasper jet, installing localized exhaust, and surface cleaning of the backrest.
Full-text available
This paper offers a critique of UK government policy based on mode of transmission of SARS-CoV-2 (which in turn followed misleading advice from the World Health Organisation) through the lens of policymaking as narrative. Two flawed narratives—“Covid is droplet- not airborne-spread” and “Covid is situationally airborne” (that is, airborne transmission is unusual but may occur during aerosol-generating medical procedures and severe indoor crowding)—quickly became dominant despite no evidence to support them. Two important counter-narratives—“Covid is unequivocally airborne” and “Everyone generates aerosols; everyone is vulnerable”— were sidelined despite strong evidence to support them. Tragic consequences of the flawed policy narrative unfolded as social dramas. For example, droplet precautions became ritualised; care home residents died in their thousands; public masking became a libertarian lightning rod; and healthcare settings became occupational health battlegrounds. In a discussion, we call for bold action to ensure that the science of SARS-CoV-2 transmission is freed from the shackles of historical errors, scientific vested interests, ideological manipulation and policy satisficing.
Full-text available
Background: Scientific and policy bodies’ failure to acknowledge and act on the evidence base for airborne transmission of SARS-CoV-2 in a timely way is both a mystery and a scandal. In this study, we applied theories from Bourdieu to address the question, “How was a partial and partisan scientific account of SARS-CoV-2 transmission constructed and maintained, leading to widespread imposition of infection control policies which de-emphasised airborne transmission?”. Methods: From one international case study (the World Health Organisation) and four national ones (UK, Canada, USA and Japan), we selected a purposive sample of publicly available texts including scientific evidence summaries, guidelines, policy documents, public announcements, and social media postings. To analyse these, we applied Bourdieusian concepts of field, doxa , scientific capital, illusio, and game-playing. We explored in particular the links between scientific capital, vested interests, and policy influence. Results: Three fields—political, state (policy and regulatory), and scientific—were particularly relevant to our analysis. Political and policy actors at international, national, and regional level aligned—predominantly though not invariably—with medical scientific orthodoxy which promoted the droplet theory of transmission and considered aerosol transmission unproven or of doubtful relevance. This dominant scientific sub-field centred around the clinical discipline of infectious disease control, in which leading actors were hospital clinicians aligned with the evidence-based medicine movement. Aerosol scientists—typically, chemists, and engineers—representing the heterodoxy were systematically excluded from key decision-making networks and committees. Dominant discourses defined these scientists’ ideas and methodologies as weak, their empirical findings as untrustworthy or insignificant, and their contributions to debate as unhelpful. Conclusion: The hegemonic grip of medical infection control discourse remains strong. Exit from the pandemic depends on science and policy finding a way to renegotiate what Bourdieu called the ‘rules of the scientific game’—what counts as evidence, quality, and rigour.
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Background: SARS-CoV-2 RNA has been detected in fomites which suggests the virus could be transmitted via inanimate objects. However, there is uncertainty about the mechanistic pathway for such transmissions. Our objective was to identify, appraise and summarise the evidence from primary studies and systematic reviews assessing the role of fomites in transmission. Methods: This review is part of an Open Evidence Review on Transmission Dynamics of SARS-CoV-2. We conduct ongoing searches using WHO Covid-19 Database, LitCovid, medRxiv, and Google Scholar; assess study quality based on five criteria and report important findings on an ongoing basis. Results: We found 64 studies: 63 primary studies and one systematic review (n=35). The settings for primary studies were predominantly in hospitals (69.8%) including general wards, ICU and SARS-CoV-2 isolation wards. There were variations in the study designs including timing of sample collection, hygiene procedures, ventilation settings and cycle threshold. The overall quality of reporting was low to moderate. The frequency of positive SARS-CoV-2 tests across 51 studies (using RT-PCR) ranged from 0.5% to 75%. Cycle threshold values ranged from 20.8 to 44.1. Viral concentrations were reported in 17 studies; however, discrepancies in the methods for estimation prevented comparison. Eleven studies (17.5%) attempted viral culture, but none found a cytopathic effect. Results of the systematic review showed that healthcare settings were most frequently tested (25/35, 71.4%), but laboratories reported the highest frequency of contaminated surfaces (20.5%, 17/83). Conclusions: The majority of studies report identification of SARS-CoV-2 RNA on inanimate surfaces; however, there is a lack of evidence demonstrating the recovery of viable virus. Lack of positive viral cultures suggests that the risk of transmission of SARS-CoV-2 through fomites is low. Heterogeneity in study designs and methodology prevents comparisons of findings across studies. Standardized guidelines for conducting and reporting research on fomite transmission is warranted.
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Since the start of the COVID-19 pandemic, experts and the broader public have vigorously debated the means by which SARS CoV-2 is spread. And understandably so, for identifying the routes of transmission is crucial for selecting appropriate nonpharmaceutical interventions to control the pandemic. The most controversial question in the debate is the role played by airborne transmission. What is at stake is not just the clinical evidence, but the implications for public health policy, society, and psychology. Interestingly, however, the issue of airborne transmission is not a new controversy. It has reappeared throughout the history of western medicine. This essay traces the notion of airborne infection from its development in ancient medical theories to its manifestation in the modern era and its impact today.
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Masking out air sharing The effectiveness of masks in preventing the transmission of severe acute respiratory syndrome coronavirus 2 has been debated since the beginning of the COVID-19 pandemic. One important question is whether masks are effective despite the forceful expulsion of respiratory matter during coughing and sneezing. Cheng et al. convincingly show that most people live in conditions in which the airborne virus load is low. The probability of infection changes nonlinearly with the amount of respiratory matter to which a person is exposed. If most people in the wider community wear even simple surgical masks, then the probability of an encounter with a virus particle is even further limited. In indoor settings, it is impossible to avoid breathing in air that someone else has exhaled, and in hospital situations where the virus concentration is the highest, even the best-performing masks used without other protective gear such as hazmat suits will not provide adequate protection. Science , abg6296, this issue p. 1439
The COVID-19 pandemic has exposed major gaps in our understanding of the transmission of viruses through the air. These gaps slowed recognition of airborne transmission of the disease, contributed to muddled public health policies and impeded clear messaging on how best to slow transmission of COVID-19. In particular, current recommendations have been based on four tenets: (i) respiratory disease transmission routes can be viewed mostly in a binary manner of ‘droplets’ versus ‘aerosols’; (ii) this dichotomy depends on droplet size alone; (iii) the cut-off size between these routes of transmission is 5 µm; and (iv) there is a dichotomy in the distance at which transmission by each route is relevant. Yet, a relationship between these assertions is not supported by current scientific knowledge. Here, we revisit the historical foundation of these notions, and how they became entangled from the 1800s to today, with a complex interplay among various fields of science and medicine. This journey into the past highlights potential solutions for better collaboration and integration of scientific results into practice for building a more resilient society with more sound, far-sighted and effective public health policies.
Respirable aerosols (< 5 µm in diameter) present a high risk of SARS-CoV-2 transmission. Guidelines recommend using aerosol precautions during aerosol-generating procedures, and droplet (> 5 µm) precautions at other times. However, emerging evidence indicates respiratory activities may be a more important source of aerosols than clinical procedures such as tracheal intubation. We aimed to measure the size, total number and volume of all human aerosols exhaled during respiratory activities and therapies. We used a novel chamber with an optical particle counter sampling at 100 l.min-1 to count and size-fractionate close to all exhaled particles (0.5-25 µm). We compared emissions from ten healthy subjects during six respiratory activities (quiet breathing; talking; shouting; forced expiratory manoeuvres; exercise; and coughing) with three respiratory therapies (high-flow nasal oxygen and single or dual circuit non-invasive positive pressure ventilation). Activities were repeated while wearing facemasks. When compared with quiet breathing, exertional respiratory activities increased particle counts 34.6-fold during talking and 370.8-fold during coughing (p < 0.001). High-flow nasal oxygen 60 at l.min-1 increased particle counts 2.3-fold (p = 0.031) during quiet breathing. Single and dual circuit non-invasive respiratory therapy at 25/10 cm.H2 O with quiet breathing increased counts by 2.6-fold and 7.8-fold, respectively (both p < 0.001). During exertional activities, respiratory therapies and facemasks reduced emissions compared with activities alone. Respiratory activities (including exertional breathing and coughing) which mimic respiratory patterns during illness generate substantially more aerosols than non-invasive respiratory therapies, which conversely can reduce total emissions. We argue the risk of aerosol exposure is underappreciated and warrants widespread, targeted interventions.
We examine airborne transmission of SARS-CoV-2 potential using a source-to-dose framework beginning with generation of virus-containing droplets and aerosols and ending with virus deposition in the respiratory tract of susceptible individuals. By addressing four critical questions, we identify both gaps in addressing four critical questions with answers having policy implications.
A susceptible person experiences the highest exposure risk of respiratory infection when he or she is in close proximity with an infected person. The large droplet route has been commonly believed to be dominant for most respiratory infections since the early 20th century, and the associated droplet precaution is widely known and practiced in hospitals and in the community. The mechanism of exposure to droplets expired at close contact, however, remains surprisingly unexplored. In this study, the exposure to exhaled droplets during close contact (<2 m) via both the short-range airborne and large droplet sub-routes is studied using a simple mathematical model of expired flows and droplet dispersion/deposition/inhalation, which enables the calculation of exposure due to both deposition and inhalation. The short-range airborne route is found to dominate at most distances studied during both talking and coughing. The large droplet route only dominates when the droplets are larger than 100 μm and when the subjects are within 0.2 m while talking or 0.5 m while coughing. The smaller the exhaled droplets, the more important the short-range airborne route. The large droplet route contributes less than 10% of exposure when the droplets are smaller than 50 μm and when the subjects are more than 0.3 m apart, even while coughing.