Early Release - Clusters of #Coronavirus Disease in Communities, Japan, January–April 2020 - Volume 26, Number 9—September 2020 - Emerging Infectious Diseases journal - CDC
▻https://wwwnc.cdc.gov/eid/article/26/9/20-2272_article
The most frequently observed age groups among probable primary cases were 20–29 years (n = 6; u%) and 30–39 years (n = 5, 23%) (Figure 2, panel A). For 16 clusters, we determined the date of transmission from probable primary case-patients to other case-patients in a cluster and found 41% (9/22) of probable primary case-patients were presymptomatic or asymptomatic at the time of #transmission ; only 1 had a cough at the time of transmission (Figure 2, panel B).