Nishiura et al. (2020) estimated the asymptomatic ratio of COVID-19 was 41.6% (5 out of 12 confirmed cases) among 565 Japanese individuals evacuated from Wuhan, China. Mao et al. (2020) reported that 2 out of 78 confirmed cases are asymptomatic. One of the two cases showed RT-PCT positivity 15th days after first diagnosis. Mizumoto et al. (2020) estimated that the 17.9% of cases on the Diamond Princess Cruise Ship were asymptomatic case during the outbreak in February 2020. However, the transmissibility of asymptomatic case is unclear, and the positive RT-PCT results only imply the potential infectivity.