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 spread. This study aimed to identify factors associated with household transmission in patients diagnosed with COVID-19 and cohabitants, during the wild-type (WT) (December 2020) and Alpha-variant (AV) (May 2021) epidemics. Factors in index cases significantly associated with household transmission of both WT and AV virus, included more than 3 days from onset to diagnosis (WT: risk ratio [RR] 1.44, 95% confidence interval [CI] 1.16-1.79; AV: RR 1.66, 95% CI 1.32-2.08), and a household size of three or more (WT: RR 1.37, 95% CI 1.10-1.72; AV: RR 1.29, 95% CI 1.05-1.59). Household transmission of WT was also significantly associated with the index case being aged ≥65 years (RR 2.39, 95% CI 1.26-4.54) and symptomatic at diagnosis (RR 3.05, 95% CI 1.22-7.63). Among cohabitants, factors associated with household transmission included being the spouse/partner of the index case (WT: RR 1.68, 95% CI 1.34-2.10; AV: RR 1.97, 95% CI 1.59-2.43) and at least 3 days from onset to diagnosis of the index case (WT: RR 1.48, 95% CI 1.21-1.82; AV: RR 1.86, 95% CI 1.52-2.28). Early diagnosis and isolation are effective for preventing household transmission.