What percentage of individuals infected with COVID-19 don't have symptoms?
The proportion of individuals infected by SARS-CoV-2 who remain
asymptomatic throughout the course of infection has not
yet been definitely assessed.


The COVID-19 epidemic, Tropical Medicine &amp; International health (2020)


The asymptomatic ratio is thus estimated at 41.6% (95% confidence interval (CI): 16.7%, 66.7%) among evacuees. Because fourteen days have elapsed since their departure from Wuhan—longer than the 95th percentile estimate of the COVID-19 incubation period (Li et al., 2020; Linton et al., 2020)—there is very little probability that the five virus-positive asymptomatic individuals will develop symptoms. Should one of the five becomes symptomatic in the future, the overall asymptomatic ratio would decrease to 33.3% (95% CI: 8.3%, 58.3%).


Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19), International Journal of Infectious Diseases (2020)


Our estimated asymptomatic proportion is at 17.9% (95%CrI: 15.5–20.2%), which overlaps with a recently derived estimate of 33.3% (95% confidence interval: 8.3–58.3%) from data of Japanese citizens evacuated from Wuhan [13].


Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020, Eurosurveillance (2020)


Table S1. Model parameters. All time periods are given in days.
Parameter                     Baseline              Distribution  Reference/reason
                              (alternative values)  
...
Pr(Asymptomatic | Infection)  0.178 (0.1, 0.5)      Beta          Mizumoto et al. (13) (lower, higher)



Estimating unobserved SARS-CoV-2 infections in the United States (preprint) (2020)