BACKGROUND: Factors associated with the incubation period of COVID-19 are not 
fully known. The aim of this study was to estimate the incubation period of 
COVID-19 using epidemiological contact tracing data, and to explore whether 
there were different incubation periods among different age gr1oups.
METHODS: We collected contact tracing data in a municipality in Hubei province 
during the full outbreak period of COVID-19. The exposure periods were inferred 
from the history of travel in Wuhan and/or history of exposure to confirmed 
cases. The incubation periods were estimated using parametric accelerated 
failure time models accounting for interval censoring of exposures.
RESULTS: The incubation period of COVID-19 follows a Weibull distribution and 
has a median of 5.8 days with a bootstrap 95% CI: 5.4-6.7 days. Of the 
symptomatic cases, 95% showed symptoms by 14.3 days (95% CI: 13.0-15.7), and 99% 
showed symptoms by 18.7 days (95% CI: 16.7-20.9). The incubation periods were 
not found significantly different between male and female. Elderly cases had 
significant longer incubation periods than young age cases (HR 1.49 with 95% CI: 
1.09-2.05). The median incubation period was estimated at 4.0 days (95% CI: 
3.5-4.4) for cases aged under 30, 5.8 days (95% CI: 5.6-6.0) for cases aged 
between 30 and 59, and 7.7 days (95% CI: 6.9-8.4) for cases aged greater than or 
equal to 60.
CONCLUSION: The current practice of a 14-day quarantine period in many regions 
is reasonable for any age. Older people infected with SARS-CoV2 have longer 
incubation period than that of younger people. Thus, more attention should be 
paid to asymptomatic elderly people who had a history of exposure.