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The estimations of the COVID-19 incubation period: a systematic review of the
literature
Nazar Zaki1,2 and Elfadil A. Mohamed3
1Department of Computer Science and Software Engineering, College of Information Technology, UAEU
2Big Data Analytics Center, UAEU
2College of Engineering and Information Technology, Ajman University
Abstract
Objective: to undertake a review and critical appraisal of all published/preprint reports that offer an estimation of
incubation periods for novel coronavirus (COVID-19).
Design: a rapid and systematic review/critical appraisal
Data sources: COVID-19 Open Research Dataset supplied by Georgetown’s Centre for Security and Emerging
Technology as well as PubMed and Embase via Arxiv, medRxiv, and bioRxiv.
Results: screening was undertaken 44,000 articles with a final selection of 25 studies referring to 18 different
experimental projects related to the estimation of the incubation period of COVID-19.
Findings: The majority of extant published estimates offer empirical evidence showing that the incubation period
for the virus is a mean of 7.8 days, with a median of 5.01 days, which falls into the ranges proposed by the WHO (0
to 14 days) and the ECDC (2 to 12 days). Nevertheless, a number of authors proposed that quarantine time should
be a minimum of 14 days and that for estimates of mortality risks a median time delay of 13 days between illness
and mortality should be under consideration. It is unclear as to whether any correlation exists between the age of
patients and the length of time they incubate the virus. Finally, it is generally agreed that robust precautions must
be put in place for the prevention and/or mitigation of asymptomatic transmission to high-risk patients caused by
those incubating the virus.
1. Introduction
At the start of 2020, a new form of coronavirus (COVID-19) was found to be the source of infection responsible for
an epidemic of viral pneumonia in Wuhan, China, a region in which the first patients began to show symptoms in
December 2019. At the time of writing (April 29, 2020) over 3 million people globally have caught the virus, of
whom more than 200,000 have died. The novel virus, causing severe acute respiratory disease, is thought to be
from the same family as Middle East Respiratory Syndrome (MERS) coronavirus and Severe Acute Respiratory
Syndrome (SARS) coronavirus, but it is unique in its own right. This means that central epidemiological parameters,
which includes the incubation period, are being urgently researched in real-time from case reports while the
epidemic is continuing [1]. The incubation period of a virus represents the time span from the probable earliest
contact with a source of transmission and the earliest recognition of the first symptoms. Accurately estimating the
length of incubation period is essential for effective contemporary public health measures to be taken [2]. If health
authorities know what the incubation period is then they will know for how long a healthy individual has to be
monitored and have their movement restricted (quarantine period) [3]. Correctly estimating the incubation period
will also help us to comprehend how infectious COVID-19 is, make estimations of the size of the pandemic, and
decide on the best course of action [4, 5, 6, 7]. With insufficient data available to definitively state what the
incubation period for this virus is, the World Health Organisation (WHO) is working with a broad range of 0-14
days, the European Centre for Disease Prevention and Control (CDCDC) is working with a range of 2-14 days, and a
number of studies have made the assumption that the incubation period is similar to that of the MERS and SARS
coronaviruses [1].
Various infectious/viral diseases have a variety of incubation periods. Nevertheless, for some infectious diseases,
we are relatively certain about the incubation period. For every individual the level of pathogens invading the
body, their ability to reproduce and resist treatment will differ, and so for any specific disease the data related to
Manuscript Click here to access/download;Manuscript;COVID-
19_Incubation_Period.pdf
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is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted May 23, 2020. .https://doi.org/10.1101/2020.05.20.20108340doi: medRxiv preprint
incubation periods should be treated with logarithm normal distribution [8]. Log-normal distribution represents
the continuous probability distribution for a random variable with normal logarithm distribution. Incubation
periods can usually be measured via biological experimentation and physiological observation. Accurately
determining the incubation period will significantly influence controls to prevent transmission of the disease and
official policy regarding it. Nevertheless, determining the incubation period for COVID-19 is no simple matter, due
to the fact that there is no consistency in the quality of the available data. One reason for this is that generally we
can only discover the times when the patient was in contact with persons carrying the virus, and then assume that
the incubation period runs from the earliest date of exposure to the appearance of clinical symptoms or medical
diagnosis. This way of calculating the incubation period may well be responsible for overestimation. This paper
represents a systematic review of the literature in order to answer the essential question of what length the
COVID-19 incubation period is. Due to the fact that no method currently exists that can make an accurate
estimation of the incubation period, the only option is to draw lessons from past experience/practice.
2. Methodology
A search was run with the “COVID-19 Open Research Base set” provided by Georgetown’s Centre for Security and
Emerging Technology. This dataset can be found at https://cset.georgetown.edu/covid-19-open-research-dataset-
cord-19/. It comprises more than 55,000 academic articles, and more than 44,000 of them have some reference to
COVID-19, SARS-CoV-2, and other forms of coronavirus. This dataset is openly available to the global research
community to employ using new techniques in Natural Language Processing (NLP) and other forms of Artificial
Intelligence for the generation of novel insights supporting the continuing battle with the pandemic. This dataset is
regularly updated when new research appears in peer-reviewed publications and archive services, e.g.
bioRxiv, medRxiv, et cetera. This search was undertaken on April 25, 2020.
For reviewing every article in the dataset we employed the Bidirectional Encoder Representations from
Transformers for Biomedical Text Mining (BioBERT) model [9], which is a biomedical language representation
model created to assist in mining biomedical texts. The inspiration for the model comes from the pre-trained
language model BERT [10] created by Devlin J.et al. BioBERT resolves the difficulties caused by moving from a
trained corpus for general use to biomedical use and assists in the understanding of complex biomedical texts as
are found with the work on COVID-19. The model was taken from the Github https://github.com/dmis-lab/biobert
. Filtering of the articles was then undertaken using keywords and questions, e.g. “what is the incubation period of
COVID/normal coronavirus/SARS-CoV-2/nCoV”.
3. Results
As illustrated in Figure 1, a systematic approach was used to screen the full “COVID-19 Open Research Dataset”.
Through employing BioBERT, just 25 articles were found that scored highly on confidence rating. An additional
search was run employing Google Scholar to make sure that every relevant article was included. Every paper
previously found was in the top 29 results alongside four other articles. Each of the 29 papers have been read
thoroughly, causing nine articles to be excluded from the list for the following reasons: one was simply a report of
earlier studies into incubation periods, one related to techniques for estimating incubation periods, two were
letters to editors, one was a literature review regarding the genometric features of SARS-CoV-2, one was a
perspective paper, and the other four were either irrelevant or redundant. The ultimate list of 18 articles was split
into five categories, with four articles focusing on the study of incubation periods, seven on characteristics of
transmission, two on clinical characteristics, four case studies, and one last paper related to epidemiological
characteristics.
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is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted May 23, 2020. .https://doi.org/10.1101/2020.05.20.20108340doi: medRxiv preprint
Fig. 1: Flowchart of the study inclusions and exclusions of articles.
3.1. Estimating the incubation period
Backer J.et al [1) employed travel histories and symptom onsets for 88 confirmed cases discovered beyond the
boundaries of Wuhan, China, in the early stage of the coronavirus outbreak. These authors made an estimation
that the incubation period ranged from 2.1 to 11.1 days (2.5th to 97.5th percentile) and that the mean incubation
period was 6.4 days (95% CI: 5.6-7.7). This research offers empirical evidence and falls into the previously
mentioned incubation periods estimated by both the ECDC and the WHO [11]. The researchers employed three
parametric forms related to the distribution of the incubation period: lognormal distribution, gamma distribution,
and the Weibull distribution. They employed uniform prior probability distribution for the exposure interval
related to the point of infection for all 88 individuals. The researchers used posterior distribution samples
employing the RStan package within R software version 3.6.0 (R Foundation, Vienna, Austria) [12].
Natalie ML. Et al [2] examined COVID-19’s epidemiological characteristics and incubation period. The researchers
harvested information relating to confirmed diagnoses of COVID-19 infection beyond the disease epicentre in
Wuhan, China, using official reporting from state institutes and reporting on mortalities both within and outwith
Wuhan. The data used by the authors was either directly harvested from government sources or from news
websites reporting government statements. The data collection process was real-time, so it was added to as
further details emerged. The final data selection represented a selection of reported cases up to January 31, 2020.
The outcomes of this research concluded that the incubation period falls into a range of 2-14 days (95% CI), with
the mean being approximately five days as found by employing best-fit lognormal distribution. Mean time
between onset of symptoms and admission to hospital (either for treatment or isolation) was estimated to be
between three and four days with no truncation and between five and nine days with right truncation. On the
basis of the 95th percentile estimate for the incubation period, the researchers recommended that exposed
individuals should be quarantined for a minimum of 14 days. When making estimates of the risk of fatality in
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COVID-19 cases, the median time delay between illness onset and death of 13 days (17 days with right truncation)
should be taken into account.
Jiang X et al [13] undertook research making a comparison between incubation periods for MERS, SARS, and SARS-
CoV-2. The researchers reported that SARS-CoV-2 has an extended incubation period, which has led to
modifications in official policy for control and screening. To prevent the virus spreading, any individual who may
have been exposed should go into isolation for 14 days, this being the outer limit of predictions for incubation
times. Nevertheless, by analyzing a large dataset for this research, researchers report that no identifiable
difference exists between incubation times for SARS-CoV-2, severe acute respiratory syndrome coronavirus (SARS-
CoV), and the Middle East respiratory syndrome coronavirus (MERS-CoV), which highlights the requirement for
more extensive and better-annotated datasets. This research covered 49 patients with SARS-CoV-2 who had
definite dates for first exposure, end of exposure and beginning of symptoms, 153 patients with SARS-CoV, and 70
MERS-CoV patients; this data was amalgamated from seven separate papers. The results indicated that MERS
incubates on average 5.8 days (95% CI: 5-6.5), SARS-CoV 4.7 days (95% CI: 4.3-5.1), and SARS-CoV2 4.9 days (95%
CI: 4.4-5.5). This demonstrates that the longest incubation period is MERS-CoV, with SARS-CoV2 second longest.
Lauer Stephen et al [14] researched the COVID-19 incubation period by looking at diagnosed cases that have been
publicly reported. The aim of the study was to ascertain COVID-19’s incubation period and to detail its implications
for public health. The researchers examined diagnosed cases of COVID-19 occurring between January 4, 2020, and
February 24, 2020. The research covered 181 subjects diagnosed with SARS-CoV-2 infection outwith Hubei
province, China by examining press releases and news reports from 50 different provinces, regions, and nations.
The researchers harvested information regarding patient demographics, dates/time of possible exposure, onset of
symptoms, onset of fever, and admission to hospital. The researchers estimate that, conservatively, 101/10,000
cases (99th percentile, 482) will experience symptom onset more than 14 days after being quarantined or actively
monitored. Nevertheless, the researchers noted that severe cases could be overrepresented in public reporting; it
is possible that severe and mild COVID 19 infections have different incubation periods. This research adds to the
evidence that COVID-19 is similar to SARS in having a median incubation period of around five days. This
recommendation comes from the research looking at proposed quarantine/active monitoring times for subjects
with potential exposure to the virus.
3.2. Transmission Characteristics
Li Q. et al [15] researched the early data regarding transmission dynamics for the virus in Wuhan, estimating the
mean incubation period at 5.2 days (95% CI: 4.1-7.0), with the distribution’s 95th percentile being 12.5 days. The
researchers fitted a log-normal distribution to data regarding history of exposure and date of onset using only
those cases where detailed information was available to estimate the length of incubation. Their preliminary
estimate of distribution of the incubation period offers strong support for the case that exposed subjects should be
quarantined or put under medical observation for 14 days. Nevertheless, this study’s accuracy may be questioned
as the estimate was made using data from just 10 patients.
Meili L. Et al [16] researched the transmission characteristics of China's COVID-19 outbreak. They took individual
patient histories from COVID-19 subjects in China (not from Hubei Province) for estimating the distribution of the
time for generation, incubation, and the time span between onset of symptoms and isolation/diagnosis. On
average, patients were isolated 3.7 days after the onset of symptoms, and diagnosed after 6.6 days. The average
patient was found to be infectious 3.9 days prior to displaying any notable symptoms. This militates against
effective quarantining or contact tracing. With contact tracing, isolation, and quarantine, the baseline reproduction
number is estimated at 1.54, with the majority of infection attributable to super spreaders. The authors of this
research, on this basis, suggested that 14 day quarantine periods would fail for 6.7% of subjects; they proposed 22-
day quarantine becoming standard. This research gave an estimation of mean incubation period as 7.2 days.
Lauren C.T.et al [17] researched the estimations for transmission intervals for COVID-19. The researchers used
transmission clusters for estimations of serial interval distribution and incubation period. They used information on
outbreaks in Tianjin, China (between January 21 and February 27) and Singapore (between January 19 and
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February 26). Interval censoring was employed (R package icenReg [18]) for making parametric estimations of the
distribution of the incubation period. Mean incubation periods for Tianjin were estimated at nine days (7.92, 10.2)
and for Singapore 7.1 days (6113, 8.25). It was additionally recorded by the researchers that in both datasets cases
that occurred earlier showed shorter incubation periods.
China Chengfeng Q. et al [19] undertook research into the virus' transmission and clinical characteristics. This
research comprised contact investigation involving 104 patients in special hospitals in Hunan province from
January 22, 2020 and February 12, 2020. Collection and analysis was made of information regarding patient
demographics, clinical, laboratory, and radiological findings, medication administered, and patient outcomes.
Confirmation of patient illness was made with the PCR test. The patients had a mean age of 43 (ranging from 8 to
84), with 52.88% female. The researchers found a median incubation period of six (range 1-32) days; eight patients
incubated between 14 and 17 days, and eight patients incubated between 18 and 30 days.
Yan Bei et al [20] undertook research into cases where it was assumed that the virus had been passed by an
asymptomatic carrier. This research looked at a family from Anyang, China, of which five members were suffering
COVID-19 pneumonia who had, prior to developing symptoms, been in contact with an asymptomatic member of
the family who had travelled to see them from Wuhan, the origin of the pandemic. The timeline they uncovered
implies that the coronavirus could have been passed on by this asymptomatic carrier. The first patient to develop
symptoms incubated for 19 days, a long period but one which falls within the reported range (0 to 24 days). This
patient initially produced a negative return for the RT-PCR test; RT-PCR is a common test for diagnostic virology
and does not often return false positives, so her second result from this test was probably not a false positive, and
so it was assumed that she was infected with the coronavirus that is responsible for COVID-19.
Wei Xia et al [21] undertook research into how the coronavirus was transmitted in incubation periods in 2019. The
researchers harvested data on the demographics, possible exposure, and time to symptom onset for confirmed
cases published by local Chinese authorities. They assessed the possibilities of transmission in the course of the
incubation period for 50 clusters of infection; these included 124 cases outwith Wuhan/Hubei province. Every
secondary case examined and been in contact with a first-generation case prior to experiencing symptoms. This
research found that the mean incubation period for COVID-19 was 4.9 days (95% CI, 4.4-5.4), with a range of 0.8 to
11.1 days (2.5th to 97.5th percentile). The infectious curve demonstrated that 73% of secondary cases became
infected prior to symptoms appearing for first-generation cases; this was especially the case in the final three days
of incubation. These findings demonstrated that COVID-19 is transmitted between those who are in close contact
in the course of incubation, which could indicate a weakness in the quarantine system. Robust workable
countermeasures are required for the prevention or mitigation of the virus being transmitted asymptomatically
amongst high-risk populations in the course of the incubation period.
Juanjuan Z. [22] undertook research into the evolution of COVID-19's transmission dynamics and epidemiology
outwith Hubei province. It was hoped that being able to understand the evolution of the transmission dynamics
and epidemiology outwith the center of the epidemic would provide useful information that could be used as
guidance for intervention policies. The researchers harvested data on individuals whose diagnosis was confirmed
by laboratory testing in mainland China (apart from Hubei) that was reported by official public sources between
January 19 and February 17, 2020. The date of the fourth time the case definition was revised (January 27) was
employed as a means of dividing the epidemic into two phases (December 24-January 27 and January 28-February
17) as dates for symptom onset. Trends were estimated in terms of central time-event periods and subject
demographics. This research encompassed 8579 cases, covering 30 provinces. The median age of the subjects was
44 years (range 33 to 56); as the epidemic continued, more cases emerged in the younger age groups and for the
elderly (those aged over 64). Mean time between symptom onset and hospitalization fell from 4.4 days (% 95% CI
0.0-14.0) between December 24 and January 27, down to 2.6 days (0.0-9.0) between January 28 and February 17.
For the whole period, mean incubation period was calculated at 5.2 days (1.8-12.4) with the mean serial interval
being 5.1 days (1.3-11.6).
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3.3. Clinical Characteristics
Shaoqing et al [23] undertook research investigating the clinical characteristics and outcomes for patients
submitting to surgery in the COVID-19 incubation period. The researchers undertook analysis of clinical data for 34
subjects submitting to elective surgery during the COVID-19 incubation period at four Chinese hospitals (Renmin,
Tongji, Zhongnan, and Central) in Wuhan between January 1 and February 5, 2020. The patients had a median age
of 55, with 20 of them (58.8%) being female. Every patient exhibited COVID-19 pneumonia symptoms within a
short time of surgery, with abnormalities showing on chest CT scans. Symptoms exhibited by these patients
encompassed fever (31 (91.2%)), fatigue (25 (73.5%)) and a dry cough (18 (52.9%)). 15 of the patients (44.1%) had
to be admitted to the intensive care unit (ICU) as the disease progressed, and seven of these died (20.5%). Every
patient examined in this research had being directly exposed to the environment in Wuhan before being admitted
to hospital; no patient had exhibited any symptoms of COVID-19 prior to surgery. It was notable how swiftly the
COVID-19 symptoms appeared once surgery had been completed, with the infection being confirmed by
laboratory within a short period. The time gap between hospital admission and surgery (median time 2.5 days) is
less than the median incubation period of 5.2 days found in patients with laboratory-confirmed infections in
Wuhan [15]; it is also less than the general incubation time in hospitals in China (median time 4.0 days, from
research into infected patients from 552 Chinese hospitals [19]). Taken together, this evidence is confirmation of
the hypothesis that patients within this research were incubating COVID-19 prior to submitting to surgery.
Guan et al [24] undertook research into coronavirus’ clinical characteristics within China. They reviewed data for
1099 patients who had a COVID-19 diagnosis confirmed by laboratory; data came from 552 hospitals across 32
provinces and municipalities of mainland China up to January 29, 2020. Only data from 291 patients was used to
calculate incubation periods, these being patients who had a clear idea of the date they had been exposed. The
patients had a median age of 47 years, with 58.1% male and 41.9% female. 1.18% of the subjects had been in
direct wildlife contact, 31.30% had visited Wuhan, and 71.80% had been in contact with people from Wuhan.
There was found to be a median incubation period of four days (interquartile range between two and seven days).
3.4. Case studies
Jasper F.C. et al [25] looked at a cluster of pneumonia within a family associated with COVID-19. The researchers
examined clinical, laboratory, epidemiological, microbiological, and radiological outcomes for five patients (aged
between 36 and 66 years) from the same family who all manifested idiopathic pneumonia upon their return to
Shenzen, Guangdong province, having visited Wuhan from December 29, 2019 to January 4, 2020. All patients
exhibited at least one and sometimes more of the symptoms of diarrhoea, upper/lower respiratory tract problems,
or fever, within 3 to 6 days after being exposed. The patients attended hospital between six and ten days from
symptoms appearing. The findings of this research accord with other accounts of COVID-19 transmission in
family/hospital environments, and share features with reports of travellers with the infection in other areas.
Jin-Wei et al [26] undertook research into 102 cases of COVID-19 (51% male, 49% female) in Xiangyang, China, all
of whom tested positive via RT-PCR. The cohort ranged in age from 1.5 years to 90 years, with a mean age of
50.38. The majority of subjects were aged between 50 and 70 years. Of the 71 subjects who had confirmed contact
histories, 7 lived in Wuhan, 37 had travelled there, 4 had contact with patients who had been diagnosed with the
virus, and 23 were members of families where infection clusters appeared. Analysing 44 subjects where it was
obvious where contact occurred, incubation periods were between 1 and 20 days, with the mean being 8.09 days
(4.99). Severe illness/death rates were lower than average, with certain patients incubating the virus for longer
periods than would be predicted. The researchers made a recommendation that quarantine periods should be
extended to 3 weeks where appropriate.
Yuanyuan H. l [27] undertook research into COVID-19 patients who were long-term users of glucocorticoids. Across
the world, clusters of patients having COVID-19 have been reported, with research demonstrating that person-to-
person transmission is the chief route of infection. The research states that average incubation periods are
between 2 and 14 days, with between 3 and 7 days being the most common. The research notes that incubation
periods may be extended in some patients. The research reports on a family cluster of COVID-19; a 47-year-old
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is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted May 23, 2020. .https://doi.org/10.1101/2020.05.20.20108340doi: medRxiv preprint
female member of the family on long-term glucocorticoid therapy had no symptoms in a 14 day quarantine period
but tested positive for COVID-19 antibodies 40 days after leaving Wuhan. These findings imply that long-term
glucocorticoid use could be responsible for long incubation periods, atypical infection, and additional transmissions
of the virus.
Rachael P.F et al [28] undertook an investigation into Singapore’s surveillance/response measures. This research
examined three COVID-19 clusters associated with a church in Singapore, a business conference, and a visiting
Chinese tour group in February 2020. The research employed inpatient medical records and interviews to harvest
clinical and epidemiological data regarding subjects with a confirmed diagnosis of COVID-19; field investigations
were undertaken for assessment of the ways these subjects had interacted with others and the potential ways in
which they had acquired the virus. With overseas cases, open-source reports were used. At the time of the
research (February 15, 2020) there were 36 infections with epidemiological links to the three Singaporean clusters
mentioned above. 425 subjects who had had close contact with people from these clusters were put in quarantine.
Those affected had generally had prolonged direct close contact with others, although it was not possible to
exclude indirect transmission, e.g. through shared food or fomites. This research found a median incubation period
for COVID-19 of four days (IQR 3-6). Serial intervals for transmission pairs were between three and eight days. The
researchers came to the conclusion that the virus can be transmitted within communities and that local clusters of
infection will appear in countries that welcomed high numbers of Chinese visitors prior to Wuhan being locked
down and travel restrictions being imposed. The research further concluded that contact tracing and increased
surveillance is necessary to prevent the virus spreading widely across communities.
3.5. Epidemiological characteristics
China Pei W. et al [29] undertook an investigation into COVID-19's epidemiological characteristics as found in the
Chinese province of Henan, centered on publicly available data related to 1212 patients. The report mentions that
these patients are 55% male and 45% female, with 81% being aged from 21 to 60 years. Statistical analysis
undertaken with 483 patients on this cohort revealed that the estimated average median period was 7.4 days, the
mode 4 days, and the median 7 days. 92% of patients did not have an incubation period in excess of 14 days.
Table 1 summarises all of the articles under review/analysis, including the methodology employed for estimation
of incubation periods.
Table 1: overview of articles investigating and reporting on the incubation period of COVID-19.
Ref.
Incubation Period
Method for estimating the
incubation period.
Number of
cases
Location
Note
Incubation Period Studies
[1]
6.4 days (95% CI: 5.6–
7.7), ranging from 2.1
to 11.1 days (2.5th to
97.5th percentile)
Probability density function
(PDF). Used a doubly interval-
censored likelihood function to
estimate the parameter
values. Used RStan package in
R
88
Outside
Wuhan,
China
- The study provides empirical
evidence and the estimation is within
the range for the incubation period of
0 to 14 days assumed by the WHO
and of 2 to 12 days assumed by the
ECDC [11].
[2]
5 days (2–14 days with
95% confidence)
Best-fit lognormal distribution
Real-time
data
outside of the
Wuhan, China
- Recommend the length of quarantine
to be at least 14 days.
- The median time delay of 13 days
from illness onset to death should be
considered when estimating the
fatality risk.
[13]
- SARS-CoV2 4.9 (95%
CI:4.4.-5.5)
- SARS-CoV 4.7(95% CI:
4.3-5.1)
- MERS-CoV 5.8 (95%
CI: 5-6.5)
Fitted Weibull, lognormal, and
gamma distributions
- SARS-CoV2
49
- SARS-CoV
153
- MERS-CoV
70
China
- long incubation time was reported to
be associated with SARS‐CoV‐2
infection, leading to adjustments in
screening and control policies.
[14]
Median 5.1 days (95%
CI, 4.5 to 5.8 days)
Estimated the incubation time
using a previously described
parametric accelerated failure
181
Regions, and
countries
outside
- 101 out of every 10,000 cases (99th
percentile, 482) develop symptoms
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time model. Doubly interval-
censored data. Data reduction
technique
Wuhan, Hubei
province,
China
after 14 days of active monitoring or
quarantine.
- This work provides additional
evidence for a median incubation
period of approximately 5 days, like
SARS.
Transmission Characteristics
[15]
5.2 days (95%
confidence interval (CI):
4.1-7.0), with the 95th
percentile of the
distribution at 12.5
days
Estimated by fitting a log-
normal distribution to data on
exposure histories and onset
dates in a subset of cases with
detailed information available.
10
Wuhan, China
- The estimation was based on
information from only 10 cases and is
somewhat imprecise.
- The incubation period reported is
Within the WHO limit.
[16]
Mean of 7.2 days
Gamma distribution and a log-
normal distribution
-
Chinese
provinces
excluding
Hubei
- The authors stated that the
recommended 14-day quarantine
period may lead to a 6.7% failure for
quarantine and they suggested a 22-
day quarantine period.
[17]
Mean incubation
periods as
- 7.1 (6.13, 8.25) days
for Singapore and
- 9 (7.92, 10.2) days
for Tianjin
Used interval censoring R
package icenReg [18] to make
parametric estimates of the
incubation period distribution
- 93
Singapore
- 135 Tianjin
Singapore,
Tianjin (China)
- Both datasets had shorter incubation
periods for earlier-occurring cases.
[19]
Median incubation
period was
- 6 (rang, 1-32) days,
of 8 patients ranged
from 18 to 32 days.
Recorded Observation
104
Hunan,
outside-
Wuhan
- The incubation period of 8 patients
exceeded 14 days.
[20]
1-19 days
Observation
5
Anyang, China
- The incubation period for patient 1
was 19 days, which is long but within
the reported range of 0 to 24 days
[21]
be 4.9 days (95%
confidence interval [CI],
4.4 to 5.4) days,
ranging from 0.8 to
11.1 days (2.5th to
97.5th percentile)
Used a Weibull distribution-
based survival analysis model
with the extension of the
Kaplan-Meier estimator to fit
the curve of the incubation
period for COVID-19 cases.
124
Outside
Wuhan city
and Hubei
province in
China
- 73.0% of the secondary cases, their
date of getting infected was before
symptom onset of the first-
generation cases, particularly in the
last 3 days of the incubation period.
- Strong and effective
countermeasures should be
implemented to prevent or mitigate
asymptomatic transmission during
the incubation period in populations
at high risk.
[22]
5·2 days (1·8–12·4) and
the mean serial interval
at 5·1 days (1·3–11·6).
Lognormal distribution
8579 cases
from 30
provinces
outside Hubei
in mainland
China
- The mean time from symptom onset
to hospital admission decreased from
4·4 days (95% CI 0·0–14·0) from Dec
24 to Jan 27, to 2·6 days (0·0–9·0) for
the period of Jan 28 to Feb 17.
Clinical Characteristics
[23]
Median incubation
period of 2.5
Lognormal distribution
34
Wuhan
- The length of time from hospital
admission to surgery (median time,
2.5 days is shorter than the median
incubation time of 5.2 days
[24]
Median incubation
period of 4
Median, Observation (2-7
days)
291
Mainland
China
- The median age was 47 years, and
41.90% were females. Only 1.18% of
patients had direct contact with
wildlife, whereas 31.30% had been to
Wuhan and 71.80% had contacted
people from Wuhan.
Case Study
[25]
- Patient 3 – (3-6 days
after exposure)
- Patients 1-4
symptomatic (6-10
Observation
5
Wuhan
- 5 patients (aged 36–66 years)
- The authors' findings are consistent
with person-to-person transmission
of the novel coronavirus in hospital
and family settings, and the reports
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days after symptom
onset)
- 5 not affected
of infected travelers in other
geographical regions.
[26]
Mean of 8.09 (4.99)
days
Mean, Observation
44
Xiangyang,
China
- Most cases fell into the age group of
50-70 years old.
- The rate of severe illness and death
was low, whereas some patients had
a longer incubation period.
[27]
2–14 days, and mostly
3–7 days.
Observation
3
Hunan, China
- A 47-year-old woman with long-term
use of glucocorticoids did not develop
any symptoms within the 14- day
quarantine period.
- The results suggest that the long-
term use of glucocorticoids might
cause atypical infections, a long
incubation period, and extra
transmission of COVID-19.
- Case 1 is a Wuhan-settled 47-year-old
female. She has a more than 16-year
history of systemic lupus
erythematosus (SLE)
- Case 2 is an 81-year-old male lives in
Yiyang. He has a history of prostate
cancer and coronary heart disease.
- Case 3 is a 44-year-old female from
Yiyang. She also had close contact
with her elder sister (Case 1).
[28]
The Median incubation
period is 4 days.
Observations
36
Tour group
from China, in
Singapore
- Interpretation SARS-CoV-2 is
transmissible in community settings,
and local clusters of COVID-19 are
expected in countries with high travel
volume from China.
- Enhanced surveillance and contact
tracing are essential to minimize the
risk of widespread transmission in the
community.
Epidemiological Characteristics
[29]
Estimated average,
mode and median
incubation periods are
7.4, 4 and 7 days,
respectively
log-normal distribution
483
Henan, China
- COVID-19 patients show gender (55%
vs 45%) and age (81% aged between
21 and 60) preferences, possible
causes were explored.
- The incubation periods of 92% of the
patients did not exceed 14 days.
4. Discussion/conclusion
Throughout human history epidemics have been a disrupter of human civilizations and caused staggering amounts
of mortality and illness for both humans and animals [30]. A novel form of coronavirus (COVID-19) appeared in
Wuhan, China, in December 2019; the virus was unexpected and swiftly spread worldwide. Many leading research
laboratories and the WHO are striving to create a vaccine to protect against the disease. Nevertheless, as this is a
new form of virus, there is much still to understand about it. It is essential that we gain an understanding of the
way the disease behaves. This research has looked at a single element of the virus behavior, viz. its incubation
period. While it is essential that health authorities should know how they can accurately estimate the virus'
incubation period in order to direct the most effective public health interventions, select the best course of action
and make estimations of the size of the epidemic, there is still no definitive answer as to what the incubation
period is.
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On the basis of this research, it appears that the only means of estimating the incubation period for COVID-19 at
present is logarithm normal distribution. Novel ways of effecting improvements to the estimation's accuracy in
terms of estimating incubation periods should be investigated. This is an important issue because date of infection
and/or onset of symptoms are difficult to precisely identify. Nicholas G.R.et al [31] have made a comparison of two
means of making such estimates. One method uses doubly interval-censored data, and the other employs data
reduction techniques making the calculation more tractable. The researchers used both methods on historical data
relating to the incubation periods for respiratory syncytial virus and influenza A. The outcomes demonstrate that
these methods reduce the demands for computational power to analyze the reduced data and make good
estimates of median incubation times in many different experimental conditions. Nevertheless, the researchers do
recommend that doubly interval-censored analysis should be used to estimate the distribution tails.
The systematic review undertaken by this research has demonstrated that the current published estimations have
offered empirical evidence that the virus’ incubation period is approximately a median of 5.01 and a mean of 7.8
days, which falls into the 2 to 12 days assumption of the ECDC and the 0 to 14 days of the WHO [11].
Researchers like [2] propose a recommended quarantine length of a minimum of 14 days, suggesting that fatality
risks should be estimated using a median time of 13 days between first symptoms and mortality. [14] offered
evidence that it is possible for symptoms to appear once patients have left a 14 day period of quarantine or active
monitoring; the authors suggest that the median incubation time is around five days, similar to SARS. In [16], the
authors stated that 14-day quarantines may be insufficient to protect the public in 6.7% of cases; they therefore
proposed that quarantine should be a minimum of 22 days. In [19] it was found that eight patients developed
symptoms after more than 14 days from infection. In [20] the authors referred to a patient who incubated the
virus for 19 days, a substantial period but one which falls into the reported range (0-24 days).
It has yet to be conclusively demonstrated whether age group as any impact on the time a patient incubates the
virus. Out of 291 patients [24] with an average age of 47, the incubation period was 4.0 days, for five patients [25]
with an average age of 49.5 years it was 4.5 days, for 44 patients [26] with an average age of 60 years it was 4.99
days, and for two patients [27] with an average age of 47 years, it was 4.5 days.
The authors of [27] suggested that long-term glucocorticoid use could cause a delay in the onset of symptoms, so
that they would not appear until after a 14 day quarantine period had been completed. Long-term glucocorticoid
use could be responsible for atypical infection, longer incubation periods, and additional COVID-19 transmission.
[13] states that COVID-19 has a mean incubation period lower than MERS coronavirus but higher than SARS
coronavirus. [21] offers a final insight that robust and workable countermeasures must be put in place for the
prevention and/or mitigation of asymptomatic transmission in the course of incubation amongst high-risk
populations.
Funding
None
Competing interests
The authors declare that they have no competing interests.
Ethical approval
Not required.
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