Table 1.

An 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 0–14 days assumed by the WHO and of 2–12 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, 4.5–5.8 days)	Estimated the incubation time using a previously described parametric accelerated failure time model. Doubly interval-censored data. Data reduction technique	181	Regions, and countries outside Wuhan, Hubei province, China	101 out of every 10,000 cases (99th percentile, 482) develop symptoms 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 0–24 days
[21]	be 4.9 days (95% confidence interval [CI], 4.4–5.4) days, ranging from 0.8–11.1 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 days after symptom onset) 5 not affected	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 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 who 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.