Asymptomatic infection and herd immunity of COVID-19 in Wuhan and Japan

Kurita, J.; Sugawara, T.; Ohkusa, Y.

Background: The COVID-19 outbreak has shown two inconsistent phenomena: its reproduction number is almost two; and it shows earlier and lower peaks for new cases and the total number of patients. Object: To resolve this inconsistency, we constructed a mathematical model to explain these phenomena. Method: To outbreak data from Wuhan, China and Japan, we applied a susceptible-infected-recovery model with the proportion of asymptomatic patients among infected people (q) as a key parameter for estimation, along with the basic reproduction number (R0). Results: The first outbreak peak was recorded in Japan on April 3 for those infected on March 29. Their R0 and q were estimated respectively as 3.19 and 99.32% in Wuhan. In Japan, these were estimated around the peak as 2.96 and 99.99%. Discussion and Conclusion: By introducing a very high proportion of asymptomatic cases, the two inconsistent phenomena might be resolved. Especially in Japan, the asymptomatic cases were 60 times higher than those of China.