The characteristics of household transmission during COVID-19 outbreak in Okinawa, Japan from February to May 2020.

From February 14 to May 31, 2020, the Okinawa prefecture confirmed 142 cases of coronavirus disease (COVID-19). Among them, 78 were the first cases of a household, with 174 household contacts. Of the 174 contacts, 21 contracted infection, indicating a secondary attack rate of 12.1% (95% confidence interval (CI) 7.6-17.9%). No significant differences were observed in the demographics and quantitative reverse transcription polymerase chain reaction (qRT-PCR) test results between first cases who became the source of infection to the household members or not. The secondary attack rates per various characteristics of the household members were significantly different: aged > 69 years (40.9% [95% CI 20.7-63.6%]) and those with underlying diseases (36.0% [95% CI 18.0-57.5%]). When the period from the onset to the isolation of the first household case was within 3 days, the secondary attack rate was low (4.5% [95% CI 0.1-22.8%]). Among the 21 secondary cases, 11 (52.4%) developed within 5 days from symptom onset in the first case within the same household. This indicates that secondary infection within the household occurred immediately after symptom onset in the first case. Thus, isolation of a suspected patient is a solution to reduce secondary household infections.