Nishiura H, Kobayashi T, Suzuki A, Jung S-Mok, Hayashi K, Kinoshita R, Yang Y, Yuan B, Akhmetzhanov AR, Linton NM, Miyama T, Estimation of the asymptomatic ratio of novel #coronavirus infections (COVID-19), International Journal of Infectious Diseases (2020), doi: ▻https://doi.org/10.1016/j.ijid.2020.03.020
▻https://www.ncbi.nlm.nih.gov/pubmed/32179137
En passant par sci-hub,
[...] we propose to estimate the asymptomatic ratio by using information on Japanese nationals that were evacuated from Wuhan, China on chartered flights. [...] By 6 February 2020 a total of N=565 citizens were evacuated*. Among them, pN=63 (11.2%) were considered symptomatic upon arrival based on (1) temperature screening before disembarkation, and (2) face-to-face interviews eliciting information on symptoms including fever, cough, and other non-specific symptoms consistent with COVID-19. All passengers additionally undertook reverse transcription polymerase chain reaction (RT-PCR) testing, and m=4 asymptomatic and n=9 symptomatic passengers tested positive for COVID-19.
Employing a Bayes theorem, the asymptomatic ratio is defined as
Pr(asymptomatic | infection) = Pr(infection | asymptomatic) Pr(asymptomatic)/Pr(infection), which can be calculated as m/(n+m), [...]. Using a binomial distribution, the asymptomatic ratio is thus estimated at 30.8% (95% confidence interval (CI): 7.7%, 53.8%) among evacuees. As of 6 March 2020, a total of thirty days have elapsed since their departure from Wuhan, and the length of observation is sufficiently longer than the COVID-19 incubation period (Li et al., 2020; Linton et al., 2020). Thus, there is very little probability that the five virus-positive asymptomatic individuals will develop symptoms.
[...]
However, as the weakness of this study, it should be remembered that (i) age- dependence and (ii) other aspects of heterogeneity were ignored, because our samples relied on Japanese evacuees from Wuhan. Despite a small sample size, our estimation indicates that perhaps less than a half of COVID-19-infected individuals are asymptomatic. This ratio is slightly smaller than that of influenza, which was estimated at 56–80% (Hsieh et al., 2014) using similar definitions for symptomatic individuals. There is great need for further studies on the prevalence of asymptomatic COVID-19 infections to guide epidemic control efforts.