Familial Clusters of Coronavirus Disease in 10 Prefectures, Japan, February-May 2020

The overall coronavirus disease secondary attack rate (SAR) in family members was 19.0% in 10 prefectures of Japan during February 22-May 31, 2020. The SAR was lower for primary cases diagnosed early, within 2 days after symptom onset. The SAR of asymptomatic primary cases was 11.8%.

Automated summary

The secondary attack rate of COVID-19 among family members in 10 prefectures of Japan was found to be 19.0% from February 22 to May 31, 2020. Early diagnosis of primary cases within 2 days of symptom onset resulted in lower SAR, with asymptomatic primary cases showing a SAR of 11.8%.