This study aimed to elucidate the household secondary attack rate (HSAR) of the Delta variant in comparison to the Alpha variant, and evaluate the risk factors among unvaccinated household contacts of patients with coronavirus disease 2019 (COVID-19). We studied household contacts of index cases of COVID-19 infected with Delta (L452R mutation), Alpha (N501Y mutation), and wild strain from December 2020 through November 2021 in Itako, Japan. The HSARs of the entire household contact, and the contact of index case with Delta variant were calculated and compared across the risk factors. We used a generalized estimating equation regression model for the multivariate analysis. We enrolled 1257 unvaccinated contacts from 580 households. The HSAR was higher in household contacts of index patients with Delta (48.5%) than with Alpha variant (21.7%) (aOR = 3.34, p = 0.000). In Delta variants, the HSAR was higher in household contacts with spousal relationships to index patients (63.4%) than contacts with other relationships (45.5%) (aOR 1.94, p = 0.026), and was lower in household contacts of index patients aged ≤19 (33.1%) than for contacts of index cases aged 20–59 years (52.6%) (aOR = 0.50, p = 0.027). The result of our study can be used to devise informed strategy to prevent transmission within households.

We conducted a study to investigate the proportion of patients with asymptomatic coronavirus disease 2019 (COVID-19) infected with the Delta variant compared with those infected with the wild-type strain at the time of confirmation. A total of 504 patients with confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection by viral transmission through household contact in Ibaraki, Japan were included. The proportion of asymptomatic COVID-19 patients at the time of confirmation was compared between patients infected with L452R mutation strain from June to September 2021 and those infected with the wild-type strain from November 2020 to January 2021, and was found to be 14.2% and 28.8%, respectively (relative risk, 0.49; 95% confidence interval, 0.35-0.70). The proportion of asymptomatic COVID-19 patients by viral transmission through household contact was lower among the Delta variant than those among the wild-type strain at the time of confirmation. It might contribute to attenuation of transmission.