An analysis of factors contributing to household transmission of COVID-19-using data from active epidemiological investigations performed in the Setagaya ward of Tokyo, Japan.

An active epidemiological investigation of COVID-19 cases in the Setagaya ward of Tokyo revealed that household transmission was the main route of infection spread. This study aimed to identify the factors affecting household transmission in patients diagnosed with COVID-19 and their cohabitants, during the wild type virus (December 2020) and alpha variant epidemic (May 2021). Index case factors significantly associated with household transmission for both wild type (WT) and alpha variant (AV), were at least 3 days from onset to diagnosis (WT: risk ratio [RR] 1.44, 95% confidence interval [CI] 1.16-1.79/AV: RR 1.66, CI 1.32-2.08), and a household size of three or more people (WT: RR 1.37, CI 1.10-1.72/AV: RR 1.29, CI 1.05-1.59). There were also significant differences in age ≥ 65 (RR 2.39, CI 1.26-4.54) and symptomatic at diagnosis (RR 3.05, CI 1.22-7.63) in index cases of WT. Among cohabitants, factors associated with household transmission for both strains were being the spouse/partner of the index case (WT: RR 1.68, CI 1.21-1.82/AV: RR 1.97, CI 1.59-2.43) and at least 3 days from onset to diagnosis of the index case (WT: RR 1.48, CI 1.34-2.10/ AV: RR 1.86, CI1.52-2.28). Early diagnosis and isolation are effective for preventing household transmission.