Probable Longer Incubation Period for Elderly COVID-19 Cases: Analysis of 180 Contact Tracing Data in Hubei Province, China

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.