  The coronavirus disease 2019 (COVID-19) has quickly grown from a regional
outbreak in Wuhan, China to a global pandemic. Early estimates of the epidemic
growth and incubation period of COVID-19 may have been biased due to sample
selection. Using detailed case reports from 14 locations in and outside
mainland China, we obtained 378 Wuhan-exported cases who left Wuhan before an
abrupt travel quarantine. We developed a generative model we call BETS for four
key epidemiological events---Beginning of exposure, End of exposure, time of
Transmission, and time of Symptom onset (BETS)---and derived explicit formulas
to correct for the sample selection. We gave a detailed illustration of why
some early and highly influential analyses of the COVID-19 pandemic were
severely biased. All our analyses, regardless of which subsample and model were
being used, point to an epidemic doubling time of 2 to 2.5 days during the
early outbreak in Wuhan. A Bayesian nonparametric analysis further suggests
that about 5% of the symptomatic cases may not develop symptoms within 14 days
of infection and that men may be much more likely than women to develop
symptoms within 2 days of infection.
