Content uploaded by Helene-Mari van der Westhuizen
Author content
All content in this area was uploaded by Helene-Mari van der Westhuizen on May 14, 2020
Content may be subject to copyright.
Content uploaded by Helene-Mari van der Westhuizen
Author content
All content in this area was uploaded by Helene-Mari van der Westhuizen on Apr 15, 2020
Content may be subject to copyright.
Content uploaded by Amy Price
Author content
All content in this area was uploaded by Amy Price on Apr 23, 2020
Content may be subject to copyright.
Face Masks Against COVID-19: An Evidence
Review
Jeremy Howarda,c,1, Austin Huangb, Zhiyuan Lik, Zeynep Tufekcim, Vladimir Zdimale, Helene-Mari van der Westhuizenf,g,
Arne von Delfto,g, Amy Pricen, Lex Fridmand, Lei-Han Tangi,j , Viola Tangl, Gregory L. Watsonh, Christina E. Baxs, Reshama
Shaikhq, Frederik Questierr, Danny Hernandezp, Larry F. Chun, Christina M. Ramirezh, and Anne W. Rimoint
afast.ai, 101 Howard St, San Francisco, CA 94105, US; bWarren Alpert School of Medicine, Brown University, 222 Richmond St, Providence, RI 02903; cData Institute,
University of San Francisco, 101 Howard St, San Francisco, CA 94105, US; dDepartment of Electrical Engineering & Computer Science, Massachusetts Institute of
Technology, 77 Massachusetts Ave, Cambridge, MA 02139; eInstitute of Chemical Process Fundamentals, Czech Academy of Sciences, Rozvojová 135, CZ-165 02 Praha 6,
Czech Republic; fDepartment of Primary Health Care Sciences, Woodstock Road, University of Oxford, OX2 6GG, United Kingdom; gTB Proof, Cape Town, South Africa;
hDepartment of Biostatistics, UCLA Fielding School of Public Health, 650 Charles E Young Drive, Los Angeles, CA 90095; iDepartment of Physics, Hong Kong Baptist
University, Kowloon Tong, Hong Kong SAR, China; jComplex Systems Division, Beijing Computational Science Research Center, Haidian, Beijing 100193, China; kCenter
for Quantitative Biology, Peking University, Haidian, Beijing 100871, China; lDepartment of Information Systems, Business Statistics and Operations Management, Hong
Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China; mUniversity of North Carolina at Chapel Hill; nSchool of Medicine
Anesthesia Informatics and Media (AIM) Lab, Stanford University, 300 Pasteur Drive, Grant S268C, Stanford, CA 94305; oSchool of Public Health and Family Medicine,
University of Cape Town, Anzio Road, Observatory, 7925, South Africa; pOpenAI, 3180 18th St, San Francisco, CA 94110; qData Umbrella, 345 West 145th St, New York,
NY 10031; rVrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium; sUniversity of Pennsylvania, 3400 Civic Center Blvd, Philadelphia, PA 19104; tDepartment of
Epidemiology, UCLA Fielding School of Public Health, 650 Charles E Young Drive, Los Angeles, CA 90095
This manuscript was compiled on May 12, 2020
The science around the use of masks by the general public to impede
COVID-19 transmission is advancing rapidly. Policymakers need
guidance on how masks should be used by the general population
to combat the COVID-19 pandemic. Here, we synthesize the relevant
literature to inform multiple areas: 1) transmission characteristics of
COVID-19, 2) filtering characteristics and efficacy of masks, 3) esti-
mated population impacts of widespread community mask use, and
4) sociological considerations for policies concerning mask-wearing.
A primary route of transmission of COVID-19 is likely via respiratory
droplets, and is known to be transmissible from presymptomatic and
asymptomatic individuals. Reducing disease spread requires two
things: first, limit contacts of infected individuals via physical dis-
tancing and other measures, and second, reduce the transmission
probability per contact. The preponderance of evidence indicates
that mask wearing reduces the transmissibility per contact by reduc-
ing transmission of infected droplets in both laboratory and clinical
contexts. Public mask wearing is most effective at reducing spread
of the virus when compliance is high. The decreased transmissibil-
ity could substantially reduce the death toll and economic impact
while the cost of the intervention is low. Given the shortages of med-
ical masks for now we recommend the adoption of public cloth mask
wearing, as an effective form of source control, in conjunction with
existing hygiene, distancing, and contact tracing strategies. We rec-
ommend that public officials and governments strongly encourage
the use of widespread face masks in public, including the use of ap-
propriate regulation.
COVID-19 |SARS-CoV-2 |Masks |Pandemic
Policymakers need urgent guidance on the use of masks by
the general population as a tool in combating SARS-CoV-
2, the respiratory virus that causes COVID-19. Masks have
been recommended as a potential tool to tackle the COVID-
19 pandemic since the initial outbreak in China (1), although
usage during the outbreak varied by time and province (2).
Globally, countries are grappling with translating the evi-
dence of public mask wearing to their contexts. These poli-
cies are being developed in a complex decision-making envi-
ronment, with a novel pandemic, rapid generation of new re-
search, and exponential growth in cases and deaths in many
areas. There is currently a global shortage of N95/FFP2 res-
pirators and surgical masks for use in hospitals. Simple cloth
masks present a pragmatic solution for use by the public. This
has been supported by the United States and European Cen-
tres for Disease Control. We present an interdisciplinary nar-
rative review of the literature on the role of simple cloth masks
and policies in reducing COVID-19 transmission.
1. Components to Evaluate for Public Mask Wearing
In order to identify whether public mask wearing is an appro-
priate policy, we need to consider these questions:
a Do asymptomatic or pre-symptomatic patients pose a
risk of infecting others?
b Would a face mask likely decrease the number of people
infected by an infectious mask wearer?
c Are there face covers that will not disrupt the medical
supply chain, e.g. homemade cloth masks?
d Will wearing a mask impact the probability of the wearer
becoming infected themselves?
e Does mask use reduce compliance with other recom-
mended strategies, such as physical distancing and quar-
antine?
Significance Statement
Governments are evaluating the use of non-medical masks in
the community amidst conflicting guidelines from health orga-
nizations. This review synthesizes available evidence to pro-
vide clarity, and advances the use of the ’precautionary princi-
ple’ as a key consideration in developing policy around use of
non-medical masks in public.
Jeremy Howard prepared the initial literature list; Reshama Shaikh prepared the initial literature
summaries; Frederik Questier conducted additional literature searches and summaries; Zhiyuan Li,
Violet Tang, Lei-Han Tang, and Danny Hernandez did impact modeling; Zeynep Tufekci provided
sociological research and analysis; Helene-Mari van der Westhuizen and Arne von Delft provided
analysis of additional impacts; Christina Bax provided review and feedback; All authors contributed
to the writing.
Anne W. Rimoin is an editor of the British Medical Journal. Larry F. Chu is a member of the editorial
advisory board of the British Medical Journal.
1To whom correspondence should be addressed. E-mail: jphoward@usfca.edu
www.pnas.org/cgi/doi/10.1073/pnas.XXXXXXXXXX PNAS | May 12, 2020 | vol. XXX | no. XX | 1–9
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
© 2020 by the author(s). Distributed under a Creative Commons CC BY license.
f Are there any other sociological considerations that will
lead to unintended benefits or harm?
g What could the overall population-level impact of public
mask wearing be?
We will evaluate each consideration in turn.
2. Transmission Characteristics of COVID-19
A primary route of transmission of SARS-CoV-2 is likely via
respiratory droplets that are ejected when speaking, cough-
ing or sneezing. The most common droplet size threshold has
a minimum at 5µmto 10 µm(3,4). There is much debate
about whether these droplets should sometimes be considered
an aerosol (5). An added complexity is that aerosols are not
consistently defined in the literature. Although earlier studies
assumed that droplets were spread mainly through coughing,
a more recent analysis has found that transmission through
talking may be a key vector, with louder speech creating in-
creasing quantities and sizes of droplets (6).
SARS-CoV-2 is highly transmissible, with a basic reproduc-
tion number estimated to be approximately 2.4(7) although
estimates vary (8) and will likely change as improved mea-
surements of asymptomatic spread become available. Many
COVID-19 patients are asymptomatic, and nearly all have
a pre-symptomatic incubation period ranging from 2 to 15
days, with a median length of 5.1 days (9). Patients are
most infectious during the initial days of infection (10–15),
when symptoms are mildest or not present. This characteris-
tic differentiates SARS-CoV-2 (COVID-19) from SARS-CoV,
as replication is activated early in the upper respiratory tract
(14,16). High viral titers of SARS-CoV-2 are reported in the
saliva of COVID-19 patients. These titers have been highest
at time of patient presentation and viral levels are just as high
in asymptomatic or presymptomatic patients (11,16).
A consequence of these disease characteristics is that any
successful intervention policy must properly address transmis-
sion due to infectious patients that display few or no symp-
toms and may not realize that they are infected.
3. Ingress: Filtering Capability of Masks
Masks can be made of different materials and designs (17)
which influence their filtering capability. There are rigorous
standards evaluating masks used in healthcare settings but
these focus on personal protective equipment (PPE) efficacy,
that is, the ability of the mask to protect the wearer from
infectious particles. Masks can also be used for source con-
trol, which refers to blocking droplets ejected by the wearer.
Although we consider both of these as important, our focus in
this paper is on source control. If everyone is wearing masks
to decrease the chance that they themselves are unknowingly
infecting someone, everyone ends up being more protected.
Multiple studies show the filtration effects of cloth masks
relative to surgical masks. Particle sizes for speech are on the
order of 1µm(18) while typical definitions of droplet size are
5µm-10 µm(5). Generally available household materials had
between a 49% and 86% filtration rate for 0.02 µmexhaled par-
ticles whereas surgical masks filtered 89% of those particles
(19). In a laboratory setting, household materials had 3% to
60% filtration rate for particles in the relevant size range, find-
ing them comparable to some surgical masks (20). In another
laboratory setup, a tea cloth mask was found to filter 60%
of particles between 0.02 µmto 1µm, where surgical masks
filtered 75% (21). Dato et al (22), note that “quality com-
mercial masks are not always accessible.” They designed and
tested a mask made from heavyweight T-shirts, finding that it
“offered substantial protection from the challenge aerosol and
showed good fit with minimal leakage.” Many recommended
cloth mask designs also include a layer of paper towel or coffee
filter, which could increase filter effectiveness for PPE, but it
does not appear to be necessary for blocking droplet emission
(6,23,24).
One of the most frequently mentioned papers evaluating
the benefits and harms of cloth masks has been by MacIntyre
et al (25). Findings have been misinterpreted, and therefore
justify detailed discussion here. The authors “caution against
the use of cloth masks” for healthcare professionals compared
to the use of surgical masks and regular procedures, based on
an analysis of transmission in hospitals in Hanoi. We empha-
size the setting of the study - health workers using masks to
protect themselves against infection. The study compared a
“surgical mask” group which received 2 new masks per day, to
a “cloth mask” group that received 5 masks for the entire 4
week period and were required to wear the masks all day, to
a “control group” which used masks in compliance with exist-
ing hospital protocols, which the authors describe as a “very
high level of mask use”. It is important to note that the au-
thors did not have a “no mask” control group because it was
deemed “unethical to ask participants to not wear a mask.”
The study does not inform policy pertaining to public mask
wearing as compared to the absence of masks in a community
setting, since there was not a “no mask” group. The results of
the study show that the group with a regular supply of new
surgical masks each day had significantly lower infection of
rhinovirus than the group that wore a limited supply of cloth
masks. This study lends support to the use of clean, surgi-
cal masks by medical staff in hospital settings to avoid rhi-
novirus infection by the wearer, and is consistent with other
studies that show surgical masks provide poor filtration for
rhinovirus, compared to seasonal coronaviruses (NL63, OC43,
229E and HKU1) (26). It does not inform the effect of using
cloth masks versus not using masks in a community setting
for source control of SARS-CoV-2.
Guideline development for health worker PPE have focused
on whether surgical masks or N95 respirators should be rec-
ommended. Most of the research in this area focuses on in-
fluenza. At this point, it is not known to what extent findings
from influenza studies apply to COVID-19 filtration. Wilkes
et al (27) found that “filtration performance of pleated hy-
drophobic membrane filters was demonstrated to be markedly
greater than that of electrostatic filters.” However, even sub-
stantial differences in materials and construction do not seem
to impact the transmission of droplet-borne viruses in prac-
tice, such as a meta-analysis of N95 respirators compared to
surgical masks (28) that found “the use of N95 respirators
compared with surgical masks is not associated with a lower
risk of laboratory-confirmed influenza.” Radonovich et al (29)
found in an outpatient setting that “use of N95 respirators,
compared with medical masks in the outpatient setting re-
sulted in no significant difference in the rates of laboratory-
confirmed influenza.”
2| www.pnas.org/cgi/doi/10.1073/pnas.XXXXXXXXXX Howard et al.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
4. Egress: Masks for Source Control
When considering the relevance of studies of ingress (masks
as protection for the wearer), it is important to note that
they are likely to substantially underestimate effectiveness of
masks for source control. When someone is breathing, speak-
ing, or coughing, only a small amount of what is coming out
of their mouths is already in aerosol form. Nearly all of what
is being emitted is droplets. Many of these droplets will then
evaporate and turn into aerosolized particles that are 3 to 5-
fold smaller. (30) Wearing a mask as source control is largely
to stop this process from occurring, since big droplets dehy-
drate to smaller aerosol particles that can float for longer in
air (26).
In a study by Johnson et al (31) on 9 influenza patients,
surgical and N95 masks appeared to be equally effective in
blocking egress droplets, given that no influenza could be de-
tected by RT-PCR on sample plates at 20 cm distance of
the coughing patients, while it was detectable without mask
for 7 of the 9 patients. Milton et al (32) checked whether
exhaled droplets might be large enough prior to evaporation
to be effectively captured by masks used as source control.
They found surgical masks produced a 3.4 (95% CI 1.8 to
6.3) fold reduction in viral copies in exhaled breath by 37
influenza patients. Vanden Driessche et al (33) used an im-
proved sampling method based on a controlled human aerosol
model, allowing longer time for droplets to evaporate and be-
come airborne. By sampling a homogeneous mix of all the air
around the patient, the authors could also detect any aerosol
that might leak around the edges of the mask. Among their
6 cystic fibrosis patients producing infected aerosol particles
while coughing, the airborne Pseudomonas aeruginosa load
was reduced by 88% when wearing a surgical mask compared
with no mask (95% confidence interval [CI], 81-96%; P=0.03).
Wood et al (34) found for their 14 cystic fibrosis patients
with high viable aerosol production during coughing, a reduc-
tion in aerosol Pseudomonas aeruginosa concentration at 2
meters from the source by using a N95 mask (94% reduction,
P<0.001), surgical mask (94%, P<0.001), or cough etiquette
(53%, P<0.001). Stockwell et al (35) confirmed in a similar
Pseudomonas aeruginosa aerosol cough study that surgical
masks are effective as source control and tolerable after ex-
tended wear. Dharmadhikari et al (36) found surgical masks
to decrease transmission of tuberculosis (an airborne bacterial
infection) by 56% (95% CI, 33-70.5%) when used as source
control and measuring differences in guinea pig tuberculosis
infections.
Anfinrud et al (6) used laser light-scattering to sensitively
detect droplet emission while speaking. Their analysis showed
that virtually no droplets were “expelled” with a homemade
mask consisting of a washcloth attached with two rubber
bands around the head, while significant levels were expelled
when speaking without a mask. The authors stated that
“wearing any kind of cloth mouth cover in public by every per-
son, as well as strict adherence to distancing and handwashing,
could significantly decrease the transmission rate and thereby
contain the pandemic until a vaccine becomes available.”
One of the most relevant papers (26), with important im-
plications for public mask wearing during the COVID-19 out-
break, is one that compares the efficacy of surgical masks for
source control for seasonal coronaviruses (NL63, OC43, 229E
and HKU1), influenza, and rhinovirus. With ten participants,
the masks were effective at blocking coronavirus droplets of all
sizes for every subject. However, masks were far less effective
at blocking rhinovirus droplets of any size, or of blocking small
influenza droplets. The results suggest that masks may have
a significant role in source control for the current coronavirus
outbreak. The study did not use COVID-19 patients, and it
is not yet known whether SARS-CoV-2 behaves the same as
these seasonal coronaviruses; however, they are closely related
viruses, so similar behavior is likely.
In another potentially relevant, but very under-powered
study (37), four patients with COVID-19 were asked to
cough repeatedly, alternating between no mask, surgical mask,
cloth mask and then again without a mask onto a sample
plate placed approximately 20 cm from the coughing per-
son’s mouth. The authors state “The median viral loads after
coughs without a mask, with a surgical mask, and with a cot-
ton mask were 2.56 log copies/mL, 2.42 log copies/mL, and
1.85 log copies/mL, respectively.” In this statement, they ex-
clude Patient 2 who had detectable virus in all experiments
except when she was wearing a cotton mask. If we assume,
conservatively, the limit of detection is 1.4 log copies/mL and
use this value for the ND value for Patient 2, and allow each
patient to serve as their own control (using the fact that the
study design allows for paired comparisons) the median within
patient difference of no mask control versus wearing a cotton
mask results in an approximately 1 log (10 fold) decrease in
virus. Note that we, like Bae et al, exclude Patient 4 in these
calculations as they did not have detectable virus in the first
3 trial conditions. While the study is under-powered, the re-
sults are suggestive that cloth masks are able to reduce the
level of SARS-CoV-2 escaping from an infected person cough-
ing. However, more studies are needed.
A comparison of homemade and surgical masks for bac-
terial and viral aerosols (19) observed that “the median-fit
factor of the homemade masks was one-half that of the sur-
gical masks. Both masks significantly reduced the number of
microorganisms expelled by volunteers, although the surgical
mask was 3 times more effective in blocking transmission than
the homemade mask.” Research focused on aerosol exposure
has found all types of masks are at least somewhat effective
at protecting the wearer. Van der Sande et al (38) found
that “all types of masks reduced aerosol exposure, relatively
stable over time, unaffected by duration of wear or type of
activity,” and concluded that “any type of general mask use
is likely to decrease viral exposure and infection risk on a
population level, despite imperfect fit and imperfect adher-
ence.” However, overall analysis of particle filtration is likely
to underestimate the effectiveness of masks, since the frac-
tion of particles that are emitted as aerosol (vs. droplet) is
quite small (30). Analysis of seasonal coronavirus compared
to rhinovirus (26) suggests that filtration of COVID-19 may
be much more effective, especially for source control.
In summary, there is laboratory-based evidence that house-
hold masks have some filtration capacity in the relevant
droplet size range, as well as efficacy in blocking droplets and
particles from the wearer (26). That is, these masks help
people keep their droplets to themselves.
5. Evaluating masks as intervention
When evaluating the available evidence for the impact of
masks on community transmission, it is critical to clarify the
Howard et al. PNAS | May 12, 2020 | vol. XXX | no. XX | 3
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
setting of the research study (health care facility or commu-
nity), whether masks are evaluated as source control or pro-
tection for the wearer, the respiratory illness being evaluated
and what control group was used. Although no randomized
controlled trials (RCT) on the use of masks as source control
for SARS-CoV-2 have been conducted, a number of studies
have investigated masks during other disease outbreaks. A
Cochrane review (39) on physical interventions to interrupt
or reduce the spread of respiratory viruses included 67 stud-
ies that were randomized controlled trials and observational
studies. It found that "overall masks were the best performing
intervention across populations, settings and threats." The re-
view recommended that “the following effective interventions
should be implemented, preferably in a combined fashion, to
reduce transmission of viral respiratory disease: 1. frequent
handwashing with or without adjunct antiseptics; 2. barrier
measures such as gloves, gowns, and masks with filtration
apparatus; and 3. suspicion diagnosis with the isolation of
likely cases.” However, it cautioned that routine long-term
implementation of some measures assessed might be difficult
without the threat of an epidemic. There is an updated review
available in preprint format by the same lead author (40). In
the update, only studies where mask wearing was tested as
a stand-alone intervention were included, without combining
it with hand hygiene and physical distancing. Observational
studies from previous epidemics were also excluded. The up-
dated review concluded that “there was insufficient evidence
to provide a recommendation on the use of facial barriers with-
out other measures” but this has not been broadened to eval-
uate combinations of interventions as to update the Cochrane
review.
Several other systematic reviews have recently been con-
ducted. MacIntyre (41) published a review evaluating masks
as protective intervention for the community, protection for
health workers, and as source control. The authors conclude
that “community mask use by well people could be benefi-
cial, particularly for COVID-19, where transmission may be
pre-symptomatic. The studies of masks as source control also
suggest a benefit, and may be important during the COVID-
19 pandemic in universal community face mask use as well
as in health care settings.” Two other preprint systematic re-
views by Brainard (42) and (43) concluded against and for
the use of face masks by the public respectively. This con-
flicting interpretation of the literature points to fundamen-
tal disagreements in what is considered to be best available
evidence. Greenhalgh (44) argues that an “interpretive and
discursive synthesis” is needed when analysing the evidence
base for cloth masks instead of “narrowly-defined biomedical
questions”.
Randomised control trial evidence that investigated the im-
pact of masks on household transmission during influenza and
SARS epidemics indicate potential benefit. Suess et al con-
ducted an RCT (45) that suggests household transmission of
influenza can be reduced by the use of non-pharmaceutical in-
terventions, namely the use of face masks and intensified hand
hygiene, when implemented early and used diligently. Con-
cerns about acceptability and tolerability of the interventions
should not be a reason against their recommendation (45).
Cowling et al (46) investigated hand hygiene and face masks
in an RCT that seemed to prevent household transmission
of influenza virus when implemented within 36 hours of in-
dex patient symptom onset. These findings suggest that non-
pharmaceutical interventions are important for mitigation of
pandemic and inter-pandemic influenza.
RCT findings by Aiello et al (47) “suggest that face masks
and hand hygiene may reduce respiratory illnesses in shared
living settings and mitigate the impact of the influenza A
(H1N1) pandemic”. A randomized intervention trial (48)
found that “face masks and hand hygiene combined may re-
duce the rate of ILI [influenza-like illness] and confirmed in-
fluenza in community settings. These non-pharmaceutical
measures should be recommended in crowded settings at the
start of an influenza pandemic.” The authors noted that
their study “demonstrated a significant association between
the combined use of face masks and hand hygiene and a sub-
stantially reduced incidence of ILI during a seasonal influenza
outbreak. If masks and hand hygiene have similar impacts on
primary incidence of infection with other seasonal and pan-
demic strains, particularly in crowded, community settings,
then transmission of viruses between persons may be signifi-
cantly decreased by these interventions.”
An observational study in Hong Kong on SARS (49) found
that “frequent mask use in public venues, frequent hand wash-
ing, and disinfecting the living quarters were significant pro-
tective factors (OR 0.36 to 0.58)”. An important observation
was that “members of the case group [infected with SARS]
were less likely than members of the control group [not in-
fected] to have frequently worn a face mask in public venues
(27.9% vs. 58.7%).”
Although case reports from aeroplanes could have multi-
ple confounders, they provide some contribution to under-
standing SARS-CoV-2 transmission outside of controlled ex-
perimental settings. One case report (50) describes a man
who flew from China to Toronto and then tested positive for
COVID-19. He was wearing a mask during the flight. The 25
people closest to him on the plane and the flight attendants all
tested negative. Nobody from that flight has been reported as
acquiring COVID-19. Another case study involving a masked
influenza patient on an airplane (51) found that “wearing a
face mask was associated with a decreased risk for influenza
acquisition during this long-duration flight.”
6. Sociological Considerations
Some of the concerns about public mask wearing have not
been around primary evidence for the efficacy of source con-
trol, but concerns about how they will be used.
A. Risk compensation behavior. It is difficult to predict the
behavior change that would accompany regulations encour-
aging public mask use. One concern around public health
messaging promoting the use of face-covering has been that
members of the public may use risk compensation behavior.
This involves neglecting other important preventative mea-
sures like physical distancing and hand hygiene based on over-
valuing the protection a surgical mask may offer due to an
exaggerated or false sense of security (52). Similar arguments
have previously been made for HIV prevention strategies (53)
(54) motorcycle helmet laws (55), seat-belts (56) and alpine
skiing helmets (57). However, contrary to predictions, risk
compensation behaviors have not been significant on popu-
lation level, being out-weighed by increased safety in each
case (56,58–60). Risk compensation is unlikely to undo the
4| www.pnas.org/cgi/doi/10.1073/pnas.XXXXXXXXXX Howard et al.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
positive benefits at the population level. (61) These findings
strongly suggest that, instead of withholding a preventative
tool, accompanying it with accurate messaging that combines
different preventative measures would display trust in the gen-
eral public’s ability to act responsibly and empower citizens.
B. Managing the stigma associated with wearing a mask.
Stigma is a powerful force in human societies, and many ill-
nesses come with stigma for the sick as well as fear of them.
Managing the stigma is an important part of the process of
controlling epidemics, as stigma also leads to people avoiding
treatment as well as preventative measures that would “out”
their illness (62). Tuberculosis is an example of an illness
where masks are used as source control, but become a public
label associated with the disease. Many sick people are reluc-
tant to wear a mask if it identifies them as sick, and thus end
up not wearing them at all in an effort to avoid the stigma of
illness (63,64). Some health authorities have recommended
wearing masks for COVID-19 only if people are sick; however,
reports of people wearing masks being attacked, shunned and
stigmatized have also been observed (65). Having masks worn
only by the people with disease also has led to employers in
high-risk environments like grocery stores, hospitals and pris-
ons, banning employees from wearing masks to prevent them
from scaring the customer, patients or inmates. (66,67). In
many countries, minorities suffer additional stigma and as-
sumptions of criminality (68). Black people in the United
States have reportedly been reluctant to wear masks in public
during this pandemic for fear of being mistaken as criminals
(69,70). Even if it were possible to encourage only infected
people to wear masks, given the lack of access to testing in
many countries, it is not possible for many people to know
for sure if they are infected or not (71). Thus, while this pa-
per has shown the importance of masks for source-control –
preventing asymptomatic and pre-symptomatic people from
infecting others – it may not even be possible to have sick peo-
ple wear masks due to stigma, employer restrictions, or sim-
ple lack of knowledge of one’s status without mask-wearing
becoming universal policy.
C. Creating new symbolism around wearing a mask. Ritual
and solidarity are important in human societies and can com-
bine with visible signals to shape new societal behaviors
(72,73). Universal mask wearing could serve as a visible
signal and reminder of the pandemic. Signaling participa-
tion in health behaviors by wearing a mask as well as visible
enforcement (for example, shops asking customers to wear
masks) can increase compliance with public mask wearing,
but also other important preventative behaviors (74). His-
torically epidemics are a time of fear, confusion and help-
lessness (75,76). Mask-wearing, and even mask-making or
distribution, can provide feelings of empowerment and self-
efficacy (77). Health, especially during an epidemic, is a form
of public good in that everyone else’s health behaviors im-
prove the health odds of everyone else, and that it is non-
rivalrous in that one person’s health does not diminish the
health of anyone else (78,79). This can make masks symbols
of altruism and solidarity (80). In Hong Kong, for example, a
community-driven focus on epidemic prevention started in the
early days of COVID-19, and included community activists
acquiring and distributing masks especially to those without
resources and the elderly, even before it was officially declared
a pandemic or before the government had taken strong steps
(81,82). Currently, Hong Kong has not only a relatively con-
tained epidemic compared with many other countries, but
a significant reduction in influenza cases as well which their
health authorities attribute, among other factors, to the near-
universal mask wearing and strong norms around it (83–85).
7. Implementation considerations
Globally, health authorities have followed different trajecto-
ries in recommendations around the use of face masks by the
public. In China, Taiwan, Japan and South Korea, face masks
were utilized from the start of the pandemic (2). Other coun-
tries, like Czechia and Thailand, were early adopters in a
global shift towards recommending cloth masks. We present
considerations for the translation of evidence about public
mask wearing to diverse countries across the globe, outside of
the parameters of a controlled research setting.
A. Supply chain management of N95 respirators and surgi-
cal masks. There has been a global shortage of protective
equipment for health workers, with health workers falling ill
and dying of occupationally acquired COVID-19 disease (86).
N95 respirators (the equivalent in Europe is FFP2 respira-
tors) are recommended for health workers conducting aerosol-
generating procedures during clinical care of COVID-19 pa-
tients, while surgical masks are recommended for non-aerosol
generating procedures (87). The importance of masks for
health worker protection was emphasised in the early phases
of the global pandemic in hospitals in China (88). Strategies
to manage this critical shortage of PPE has been to appeal to
the public to reduce their use of medical masks, and explore
options like sterilization and re-use of respirators (89). There
have been major concerns that public messaging encouraging
mask use will deplete critical supplies. Some regions, like
South Korea and Taiwan, have combined recommendations
for the public to use surgical masks with rapidly increasing
production of surgical masks. In other regions where surgical
mask supplies are limited or unreliable due to supply chain
interruptions, cloth masks are promoted as alternative to sur-
gical masks as source control. This has been accompanied by
public messaging to avoid using medical masks. Cloth masks
offer additional sustainability benefits through re-use, thus
limiting costs and reducing environmental waste.
B. Mandatory mask wearing. Ensuring compliance with non-
pharmaceutical interventions can be challenging, but would
likely rapidly increase during a major pandemic (90). Per-
ceptions of risk play an important role in mask use (91).
Telephone surveys during the SARS-CoV-2 outbreak in Hong
Kong reported enhanced adherence to public mask wearing
as the pandemic progressed over three weeks, with 74.5% self
reported mask wearing when going out increasing to 97.5%,
without mandatory requirements (92). Similar surveys re-
ported face mask use in Hong Kong during the SARS out-
break in 2003 as 79% (93), and approximately 10% during
the influenza A(H1N1) pandemic in 2009 (94). This suggests
that the public have enhanced awareness of their risk, and
display higher adherence levels to prevention strategies than
during other epidemics. At the height of the 2009 influenza
epidemic in Mexico City it was found (95) that mandatory
mask requirements increased compliance compared to volun-
tary recommendations. Voluntary compliance was strongly
Howard et al. PNAS | May 12, 2020 | vol. XXX | no. XX | 5
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
influenced by public perception regarding the effectiveness of
the recommended measures. Countries like Czechia and Hong
Kong offer interesting perspectives on the role of citizen ad-
vocacy and on the acceptability of face-covering in public.
Modelling suggests (96) that population level compliance
with public mask wearing of 70% combined with contact trac-
ing would be critical to halt epidemic growth. Population
level uptake of an intervention to benefit the whole popula-
tion is similar to vaccinations. A common policy response to
this conundrum is to ensure compliance by using laws and
regulations, such as widespread state laws in the US which re-
quire vaccinations to attend school. Research shows that the
strength of the mandate to vaccinate greatly influences com-
pliance rates for vaccines and that policies that set a higher
bar for vaccine exemptions result in higher vaccination rates
(97). The same approach is now being used in many juris-
dictions to increase mask wearing compliance, by mandating
mask use in a variety of settings (such as public transporta-
tion or grocery stores or even at all times outside the home).
Early results suggest that these laws are effective at increasing
compliance and slowing the spread of COVID-19 (98).
C. Additional benefits for concurrent epidemics. While the
focus of this article is on preventing the spread of COVID-
19 disease through public mask wearing, many countries face
concurrent epidemics of contagious respiratory diseases like
tuberculosis and influenza. Tuberculosis kills 1.5 million peo-
ple globally per year, and in 2018, 10 million people fell ill
(99). Face covering has been shown to also reduce the trans-
mission of tuberculosis (36). Similarly, influenza transmission
in the community declined by 44% in Hong Kong after the
implementation of changes in population behaviors, includ-
ing social distancing and increased mask wearing, enforced in
most stores, during the COVID-19 outbreak (92).
8. Estimating COVID-19 Impacts
At the national and global scale, effective local interventions
are aggregated into epidemiological parameters of disease
spread. The standard epidemiological measure of spread is
known as the basic reproduction number R0which parameter-
izes the number of cases infected by one case, in a completely
susceptible population. The goal of any related healthcare
policy is to have an aggregate effect of reducing the effective
reproduction number Reto below 1.
Efficacy of face masks within local interventions would
have an aggregate effect on the reproduction number of the
epidemic. What is the possible magnitude of such an effect?
The HKBU COVID-19 Modelling Group developed a trans-
mission model that incorporated mask wearing and mask effi-
cacy as a factor in the model (96). They estimate reductions
in the effective reproduction number Reunder common inter-
vention measures. For wearing masks, they find that wearing
masks reduces Reby a factor (1 −epm)2, where eis the ef-
ficacy of trapping viral particles inside the mask, and pmis
the percentage of the population that wears masks. When
combined with contact tracing, the two effects multiply.
A conservative assessment applied to the COVID-19 esti-
mated R0of 2.4(7) might posit 50% mask usage and a 50%
mask efficacy level, reducing Reto 1.35, an order of magnitude
impact rendering spread comparable to the reproduction num-
ber of seasonal influenza. To put this in perspective, 100 cases
Fig. 1. Impact of public mask wearing under the full range of mask adherence and
efficacy scenarios. The color indicates the resulting reproduction number Refrom
an initial R0of 2.4 (7). Blue area is what is needed to slow the spread of COVID-19.
Each black line represents a specific disease transmission level with the effective
reproduction number Re indicated. An Rebelow 1, if sustained, will lead to the
outbreak ending.
at the start of a month become 31,280 cases by the month’s
end (R0= 2.4) vs. only 584 cases (Re= 1.35). Such a slow-
down in case-load protects healthcare capacity and renders a
local epidemic amenable to contact tracing interventions that
could eliminate the spread entirely.
A full range of efficacy eand adherence pmis shown with
the resulting Rein Figure 1, illustrating regimes in which
growth is dramatically reduced (Re<1) as well as pessimistic
regimes (e.g. due to poor implementation or population com-
pliance) that nonetheless result in a beneficial effect in sup-
pressing the exponential growth of the pandemic.
Yan et al (100) provide an additional example of an incre-
mental impact assessment of respiratory protective devices
using an augmented variant of a traditional SIR model in the
context of influenza with N95 respirators. They showed that
a sufficiently high adherence rate (~ 80% of the population)
resulted in the elimination of the outbreak with most respira-
tory protective devices.
Qualitative comparisons of outcomes between countries
(98,101) are suggestive of policy differences leading to dif-
ferences in disease spread of up to three orders of magni-
tude. Although between-country comparisons do not allow
for causal attribution, they suggest mask wearing to be a low-
risk measure with a potentially large positive impact on num-
ber of cases. In these countries, masks seem to be a part of
a broadly successful suite of interventions and appears not to
have meaningfully reduced compliance with other measures.
Abaluck et al (102) extend the between-country analyses
from a cost perspective, estimating the marginal benefit per
cloth mask worn to range from $3,000-$6,000. They also
found that “the average daily growth rate of confirmed posi-
tives is 18% in countries with no pre-existing mask norms and
10% in countries with such norms” and “that the growth rate
of deaths is 21% in countries with no mask norms and 11%
in countries with such norms.”
6| www.pnas.org/cgi/doi/10.1073/pnas.XXXXXXXXXX Howard et al.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
9. Discussion and Recommendations
Our review of the literature offers evidence in favor of
widespread mask use as source control to reduce community
transmission: non-medical masks use materials that obstruct
droplets of the necessary size; people are most infectious in
the initial period post-infection, where it is common to have
few or no symptoms (10–16); non-medical masks have been
effective in reducing transmission of influenza; and places and
time periods where mask usage is required or widespread have
shown substantially lower community transmission.
The available evidence suggests that near-universal adop-
tion of non-medical masks when out in public, in combination
with complementary public health measures could successfully
reduce Re(effective-R) to below 1, thereby reducing commu-
nity spread if such measures are sustained. Economic analysis
suggests that the impact of mask wearing could be thousands
of US dollars saved per person per mask (102).
Interventions to reduce COVID-19 spread should be priori-
tized in order of their expected multiple on effective R divided
by their cost. By this criterion, experimentation with and
deployment of universal masks look particularly promising.
When used in conjunction with widespread testing, contact
tracing, quarantining of anyone that may be infected, hand
washing, and physical distancing, face masks are a valuable
tool to reduce community transmission. All of these mea-
sures, through their effect on Re, have the potential to reduce
the number of infections. As governments talk about relax-
ing lockdowns, keeping transmissions low enough to preserve
health care capacity will be critical until a vaccine can be de-
veloped. Mask wearing may be instrumental in preventing a
second wave of infections from overwhelming the health care
system – further research is urgently needed here.
UNESCO states that “when human activities may lead to
morally unacceptable harm that is scientifically plausible but
uncertain, actions shall be taken to avoid or diminish that
harm” (103). This is known as the “precautionary principle.”
The World Charter for Nature, which was adopted by the UN
General Assembly in 1982, was the first international endorse-
ment of the precautionary principle. It was implemented in an
international treaty in the 1987 Montreal Protocol. The loss
of life and economic destruction that has been seen already
from COVID-19 is a “morally unacceptable harm.” The pos-
itive impact of public mask wearing on this is “scientifically
plausible but uncertain”. This notion is reflected in Figure
1 - while researchers may reasonably disagree on the magni-
tude of transmissibility reduction and compliance, seemingly
modest benefits can be massively beneficial in the aggregate
due to the exponential character of the transmission process.
Therefore, the action of ensuring widespread use of masks in
the community should be taken, based on this principle (104).
Models suggest that public mask wearing is most effective
at reducing spread of the virus when compliance is high (96).
We recommend that mask use requirements are implemented
by governments, or when governments do not, by organiza-
tions that provide public-facing services, such as transit ser-
vice providers or stores, as “no mask, no service” rules. Such
mandates must be accompanied by measures to ensure access
to masks, possibly including distribution and rationing mech-
anisms so that they do not become discriminatory, but remain
focused on the public health benefit. Given the value of the
source control principle, especially for presymptomatic peo-
ple, it is not good enough for only employees to wear masks,
customers must wear masks as well.
It is also important for health authorities to provide clear
guidelines for the production, use and sanitization or re-use
of face masks, and consider their distribution as shortages
allow. A number of countries have distributed surgical masks
(South Korea, Taiwan) from early on, while Japan, Singapore
and Belgium are now distributing cloth masks to their entire
populations. Clear and implementable guidelines can help
increase compliance, and bring communities closer to the goal
of reducing and ultimately stopping the spread of COVID-19.
Materials and Methods
A community-driven approach was used for identifying key studies
for this literature review. A multidisciplinary team of researchers
reviewed and identified additional papers to create a narrative re-
view of the effectiveness of public mask wearing as source control.
ACKNOWLEDGMENTS. Thank you to Sylvain Gugger for L
A
T
E
X
help, Cam Woodsum for assistance with preparing bibtex citations,
Jon Schwabish for graph annotation ideas, and Koen Vanden Driess-
che and many others for valuable feedback on the first preprint
version.
References
1. Q Wang, C Yu, Letter to editor: Role of masks/respirator protection against 2019-novel
coronavirus (COVID-19). Infect. Control. & Hosp. Epidemiol., 1–7 (year?).
2. S Feng, et al., Rational use of face masks in the COVID-19 pandemic. The Lancet Respir.
Medicine 0(2020).
3. J Duguid, The size and the duration of air-carriage of respiratory droplets and droplet-nuclei.
Epidemiol. & Infect.44, 471–479 (1946).
4. L Morawska, et al., Size distribution and sites of origin of droplets expelled from the human
respiratory tract during expiratory activities. J. Aerosol Sci.40, 256–269 (2009).
5. L Bourouiba, Turbulent Gas Clouds and Respiratory PathogenEmissions: Potential Implica-
tions for Reducing Transmission of COVID-19. JAMA (2020).
6. P Anfinrud, CE Bax, V Stadnytskyi, A Bax, Could sars-cov-2 be transmitted via speech
droplets? medRxiv (2020).
7. N Ferguson, et al., Report 9: Impact of non-pharmaceutical inter ventions (npis) to reduce
covid19 mortality and healthcare demand (2020).
8. Y Liu, AA Gayle, A Wilder-Smith, J Rocklöv, The reproductive number of covid-19 is higher
compared to sars coronavirus. J. travel medicine (2020).
9. SA Lauer, et al., The Incubation Period of Coronavirus Disease 2019 (COVID-19) From
Publicly Reported Confirmed Cases: Estimation and Application. Annals Intern. Medicine
(2020).
10. KKW To, et al., Temporal profiles of viral load in posterior oropharyngeal saliva samples and
serum antibody responses during infection by SARS-CoV-2: an observational cohort study.
Lancet Infect. Dis. 0(2020).
11. L Zou, et al., SARS-CoV-2 Viral Load in Upper Respiratory Specimens of Infected Patients.
New Engl. J. Medicine 382, 1177–1179 (2020).
12. Y Bai, et al., Presumed asymptomatic carrier transmission of covid-19. Jama (2020).
13. J Zhang, et al., Evolving epidemiology and transmission dynamics of coronavirus disease
2019 outside Hubei province, China: a descriptive and modelling study. The Lancet Infect.
Dis.0(2020).
14. N van Doremalen, et al., Aerosol and Surface Stability of SARS-CoV-2 as Compared with
SARS-CoV-1. New Engl. J. Medicine 0, null (2020).
15. WE Wei, Presymptomatic Transmission of SARS-CoV-2 â Singapore, January 23âMarch
16, 2020. MMWR. Morb. Mor tal. Wkly. Rep.69 (2020).
16. R Wölfel, et al., Virological assessment of hospitalized patients with covid-2019. Nature,
1–10 (2020).
17. Brosseau, N95 Respirators and Surgical Masks | | Blogs | CDC (2009).
18. S Asadi, et al., Aerosol emission and superemission during human speech increase with
voice loudness. Sci. reports 9, 1–10 (2019).
19. A Davies, et al., Testing the Efficacy of Homemade Masks: Would They Protect in an In-
fluenza Pandemic? Disaster Medicine Public Heal. Prep.7, 413–418 (2013).
20. S Rengasamy, B Eimer, RE Shaffer, Simple Respiratory ProtectionEvaluation of the Filtra-
tion Performance of Cloth Masks and Common Fabric Materials Against 201000 nm Size
Particles. The Annals Occup. Hyg.54, 789–798 (2010).
21. Mvd Sande, P Teunis, R Sabel, Professional and Home-Made Face Masks Reduce Expo-
sure to Respiratory Infections among the General Population. PLOS ONE 3, e2618 (2008).
22. VM Dato, D Hostler, ME Hahn, Simple Respiratory Mask. Emerg. Infect. Dis.12, 1033–1034
(2006).
23. Consumer Council Hong Kong, DIY Face Mask – 8 Steps in Making Protective Gear |Con-
sumer Council (2020) [Online; accessed 8. Apr. 2020].
24. United States CDC, Coronavirus Disease 2019 (COVID-19) (2020) [Online; accessed 8. Apr.
2020].
Howard et al. PNAS | May 12, 2020 | vol. XXX | no. XX | 7
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
25. CR MacIntyre, et al., A cluster randomised trial of cloth masks compared with medical masks
in healthcare workers. BMJ Open 5, e006577 (2015).
26. NH Leung, et al., Respiratory virus shedding in exhaled breath and efficacy of face masks.
Nat. Medicine, 1–5 (2020).
27. AR Wilkes, JE Benbough, SE Speight, M Harmer, The bacterial and viral filtration perfor-
mance of breathing system filters*. Anaesthesia 55, 458–465 (2000).
28. Y Long, et al., Effectiveness of N95 respirators versus surgical masks against influenza: A
systematic review and meta-analysis. J. Evidence-Based Medicine n/a (2020).
29. LJ Radonovich, et al., N95 Respirators vs Medical Masks for Preventing Influenza Among
Health Care Personnel: A Randomized Clinical Trial. JAMA 322, 824–833 (2019).
30. RS Papineni, FS Rosenthal, The size distribution of droplets in the exhaled breath of healthy
human subjects. J. Aerosol Medicine 10, 105–116 (1997).
31. DF Johnson, JD Druce, C Birch, ML Grayson, A quantitative assessment of the efficacy
of surgical and N95 masks to filter influenza virus in patients with acute influenza infection.
Clin. Infect. Dis. An Off. Publ. Infect. Dis. Soc. Am.49, 275–277 (2009).
32. DK Milton, MP Fabian, BJ Cowling, ML Grantham, JJ McDevitt, Influenza Virus Aerosols
in Human Exhaled Breath: Particle Size, Culturability, and Effect of Surgical Masks. PLOS
Pathog.9, e1003205 (2013).
33. KV Driessche, et al., Surgical masks reduce airborne spread of pseudomonas aeruginosa
in colonized patients with cystic fibrosis. Am. J. Respir. Critical Care Medicine 192, 897–899
(2015) PMID: 26426786.
34. ME Wood, et al., Face masks and cough etiquette reduce the cough aerosol concentration
of pseudomonas aeruginosa in people with cystic fibrosis. Am. J. Respir. Critical Care
Medicine 197, 348–355 (2018) PMID: 28930641.
35. RE Stockwell, et al., Face masks reduce the release of pseudomonas aeruginosa cough
aerosols when worn for clinically relevant periods. Am. J. Respir. Critical Care Medicine
198, 1339–1342 (2018) PMID: 30028634.
36. AS Dharmadhikari, et al., Surgical face masks worn by patients with multidrug-resistant
tuberculosis: impact on infectivity of air on a hospital ward. Am. journal respiratory critical
care medicine 185, 1104–1109 (2012).
37. S Bae, et al., Effectiveness of Surgical and Cotton Masks in Blocking SARSCoV-2: A Con-
trolled Comparison in 4 Patients. Annals Intern. Medicine (2020).
38. M van der Sande, P Teunis, R Sabel, Professional and Home-Made Face Masks Reduce
Exposure to Respiratory Infections among the General Population. PLoS ONE 3(2008).
39. T Jefferson, et al., Physical interventions to interrupt or reduce the spread of respiratory
viruses. Cochrane Database Syst. Rev. 7, CD006207 (2011).
40. T Jefferson, et al., Physical interventions to interrupt or reduce the spread of respiratory
viruses. Part 1 - Face masks, eye protection and person distancing: systematic review and
meta-analysis. medRxiv, 2020.03.30.20047217 (2020).
41. CR MacIntyre, AA Chughtai, A rapid systematic review of the efficacy of face masks and res-
pirators against coronaviruses and other respiratory transmissible viruses for the community,
healthcare workers and sick patients. Int. J. Nurs. Stud., 103629 (2020).
42. JS Brainard, N Jones, I Lake, L Hooper, P Hunter, Facemasks and similar barriers to prevent
respiratory illness such as COVID-19: A rapid systematic review. medRxiv (2020).
43. M GUPTA, K Gupta, S Gupta, The use of facemasks by the general population to prevent
transmission of Covid 19 infection: A systematic review. medRxiv (2020).
44. T Greenhalgh, Masks for the public : laying straw men to rest. Authorea Prepr., 1–11 (2020).
45. T Suess, et al., The role of facemasks and hand hygiene in the prevention of influenza
transmission in households: results from a cluster randomised trial; Berlin, Germany, 2009-
2011. BMC infectious diseases 12, 26 (2012).
46. BJ Cowling, et al., Facemasks and hand hygiene to preventinfluenza transmission in house-
holds: a cluster randomized trial. Annals Intern. Medicine 151, 437–446 (2009).
47. AE Aiello, et al., Mask use, hand hygiene, and seasonal influenza-like illness among young
adults: a randomized intervention trial. The J. Infect. Dis.201, 491–498 (2010).
48. AE Aiello, et al., Facemasks, Hand Hygiene, and Influenza among Young Adults: A Ran-
domized Intervention Trial. PLoS ONE 7(2012).
49. JT Lau, H Tsui, M Lau, X Yang, SARS Transmission, Risk Factors, and Prevention in Hong
Kong. Emerg. Infect. Dis.10, 587–592 (2004).
50. KL Schwartz, et al., Lack of COVID-19 Transmission on an International Flight. CMAJ
(2020).
51. L Zhang, et al., Protection by Face Masks against Influenza A(H1N1)pdm09 Virus on Trans-
Pacific Passenger Aircraft, 2009. Emerg. Infect. Dis.19, 1403–1410 (2013).
52. LM Brosseau, ScD, M Sietsema, P| Apr 01, 2020, COMMENTARY:Masks-for-all for COVID-
19 not based on sound data (2020).
53. MM Cassell, DT Halperin, JD Shelton, D Stanton, Risk compensation: the achilles’ heel of
innovations in hiv prevention? Bmj 332, 605–607 (2006).
54. D Rojas Castro, RM Delabre, JM Molina, Give prep a chance: moving on from the risk
compensation concept. J. Int. AIDS Soc.22, e25351 (2019).
55. JV Ouellet, Helmet use and risk compensation in motorcycle accidents. Traffic injury pre-
vention 12, 71–81 (2011).
56. DJ Houston, LE Richardson, Risk compensation or risk reduction? seatbelts, state laws,
and traffic fatalities. Soc. Sci. Q.88, 913–936 (2007).
57. MD Scott, et al., Testing the risk compensation hypothesis forsafety helmets in alpine skiing
and snowboarding. Inj. Prev.13, 173–177 (2007).
58. Y Peng, et al., Universal motorcycle helmet laws to reduce injuries: a community guide
systematic review. Am. journal preventive medicine 52, 820–832 (2017).
59. G Ruedl, M Kopp, M Burtscher, Does risk compensation undo the protection of ski helmet
use? Epidemiology 23, 936–937 (2012).
60. B Pless, Risk compensation: Revisited and rebutted. Safety 2, 16 (2016).
61. A Burgess, M Horii, Risk, ritual and health responsibilisation: Japans safety blanketof surgi-
cal face mask-wearing. Sociol. health & illness 34, 1184–1198 (2012).
62. G Joachim, S Acorn, Stigma of visible and invisible chronic conditions. J. advanced nursing
32, 243–248 (2000).
63. K Abney, containing tuberculosis, perpetuating stigma: the materiality of n95 respirator
masks. Anthropol. South. Afr.41, 270–283 (2018).
64. E Buregyeya, et al., Acceptability of masking and patient separation to control nosocomial
tuberculosis in uganda: a qualitative study. J. Public Heal.20, 599–606 (2012).
65. DK Li, R Abdelkader, Coronavirus hate attack: Woman in face mask allegedly assaulted by
man who calls her ’diseased’. NBC News (2020).
66. WTSP-TV, Tampa Bay nurses were told not to wear masks in hallways. Now hospitals are
changing the rules. (2020) [Online; accessed 9. Apr. 2020].
67. S Malone, NY Correctional Officers Ordered Not To Wear Masks, Even If They Have Them.
Maven (2020).
68. D Pager, H Shepherd, The sociology of discrimination: Racial discrimination in employment,
housing, credit, and consumer markets. Annu. Rev. Sociol 34, 181–209 (2008).
69. C Fernando Alfonso Iii, Why some people of color say they won’t wear homemade masks
(2020) [Online; accessed 9. Apr. 2020].
70. T Jan, Two black men say they were kicked out of Walmart for wearing protective masks.
Others worry it will happen to them. Wash. Post (2020).
71. K Wells, Why cant I get tested? Atlantic (2020).
72. RE Watson-Jones, CH Legare, The social functions of group rituals. Curr. Dir. Psychol. Sci.
25, 42–46 (2016).
73. R BliegeBird, et al., Signaling theory, strategic interaction, and symbolic capital. Curr. an-
thropology 46, 221–248 (2005).
74. R Van Houten, L Malenfant, B Huitema, R Blomberg, Effects of high-visibility enforcement
on driver compliance with pedestrian yield right-of-way laws. Transp. research record 2393,
41–49 (2013).
75. W Van Damme, W Van Lerberghe, Editorial: Epidemics and fear. Trop. Med. Int. Heal.5,
511–514 (2000).
76. MA Riva, M Benedetti, G Cesana, Pandemic fear and literature: observations from jack
londons the scarlet plague. Emerg. infectious diseases 20, 1753 (2014).
77. E Taal, JJ Rasker, ER Seydel, O Wiegman, Health status, adherence with health recom-
mendations, self-efficacy and social support in patients with rheumatoid arthritis. Patient
education counseling 20, 63–76 (1993).
78. P Illingworth, WE Parmet, Solidarity and health: A public goods justification. Diametros 43,
65–71 (2015).
79. LC Chen, TG Evans, RA Cash, , et al., Health as a global public good. Glob. public goods,
284–304 (1999).
80. KK Cheng, TH Lam, CC Leung, Wearing face masks in the community during the covid-19
pandemic: altruism and solidarity. The Lancet (2020).
81. Z Tufekci, How hong kong did it. The Atl. (2020).
82. Coronavirus can travel twice as far as official ‘safe distance’, study says (2020) [Online;
accessed 10. Apr. 2020].
83. CC Leung, TH Lam, KK Cheng, Mass masking in the COVID-19 epidemic: people need
guidance. The Lancet 395, 945 (2020).
84. J Lyons, To curb the coronavirus, hong kong tells the world masks work; city embraces
widespread use of face coverings alongside other measures to slow spread of disease
(2020).
85. N Liu, Hong kongs coronavirus response leads to sharp drop in flu cases. FT.com (2020)
Name - University of Hong Kong; Chinese University of Hong Kong; Copyright - Copyright
The Financial Times Limited Mar 5, 2020; Last updated - 2020-03-23; SubjectsTermNotLit-
GenreText - China; Hong Kong.
86. The Lancet, COVID-19: protecting health-care workers. The Lancet 395, 922 (2020).
87. JJ Bartoszko, MAM Farooqi, W Alhazzani, M Loeb, Medical masks vs n95 respirators for
preventing covid-19 in health care workers a systematic review and meta-analysis of ran-
domized trials. Influ. Other Respir. Viruses (2020).
88. X Wang, Z Pan, Z Cheng, Association between 2019-nCoV transmission and N95 respirator
use. J. Hosp. Infect.0(2020).
89. P de Man, et al., Sterilization of disposable face masks by means of standardized dry and
steam sterilization processes: an alternative in the fight against mask shortages due to
COVID-19 (2020).
90. EL Larson, et al., Impact of Non-Pharmaceutical Inter ventions on URIs and Influenza in
Crowded, Urban Households. Public Heal. Reports 125, 178–191 (2010).
91. CR MacIntyre, et al., The First Randomized, Controlled Clinical Trial of Mask Use in House-
holds to Prevent Respiratory Virus Transmission. Int. J. Infect. Dis.12, e328 (2008).
92. BJ Cowling, et al., Impact assessment of non-pharmaceutical interventions against COVID-
19 and influenza in Hong Kong: an observational study. medRxiv (2020).
93. GM Leung, et al., A tale of two cities: community psychobehavioral surveillance and related
impact on outbreak control in hong kong and singapore during the severe acute respiratory
syndrome epidemic. Infect. Control. & Hosp. Epidemiol.25, 1033–1041 (2004).
94. BJ Cowling, et al., Community psychological and behavioral responses through the first
wave of the 2009 influenza a (h1n1) pandemic in hong kong. The J. infectious diseases 202,
867–876 (2010).
95. BJ Condon, T Sinha, Who is that masked person: the use of face masks on mexico city
public transportation during the influenza a (h1n1) outbreak. Heal. Policy 95, 50–56 (2010).
96. L Tian, et al., Calibrated intervention and containment of the covid-19 pandemic (2020).
97. WD Bradford, A Mandich, Some state vaccination laws contribute to greater exemptionrates
and disease outbreaks in the united states. Heal. Aff.34, 1383–1390 (2015).
98. C Leffler, E Ing, CA McKeown, D Pratt, A Grzybowski, Country-wide Mor tality from the
Novel Coronavirus (COVID-19) Pandemic and Notes Regarding Mask Usage by the Public,
Technical report (2020).
99. World Health Organization (WHO), Global Tuberculosis Report 2019, (World Health Organi-
zation, Geneva), Technical report (2019).
100. J Yan, S Guha, P Hariharan, M Myers, Modeling the Effectiveness of Respiratory Protective
Devices in Reducing Influenza Outbreak. Risk Analysis 39, 647–661 (2019).
101. C Kenyon, Widespread use of face masks in public may slow the spread of SARS CoV-2:
an ecological study. medRxiv, 2020.03.31.20048652 (2020).
102. J Abaluck, et al., The Case for Universal Cloth Mask Adoption and Policies to Increase
8| www.pnas.org/cgi/doi/10.1073/pnas.XXXXXXXXXX Howard et al.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
Supply of Medical Masks for Health Workers, (Social Science Research Network, Rochester,
NY), SSRN Scholarly Paper ID 3567438 (2020).
103. WC on the Ethics of Scientific Knowledge, Technology, The precautionary principle (2005).
104. T Greenhalgh, MB Schmid, T Czypionka, D Bassler, L Gruer, Face masks for the public
during the covid-19 crisis. BMJ 369 (2020).
Howard et al. PNAS | May 12, 2020 | vol. XXX | no. XX | 9
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 May 2020 doi:10.20944/preprints202004.0203.v2
