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abstractpubmed· Abstract 2021· item PMID:33622475
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%.