Household Transmission of SARS-CoV-2 in Washington and Oregon, United States, 2022-2024

BACKGROUND: Households remain high-risk settings for SARS-CoV-2 transmission. Identifying contemporaneous risk factors for infection among household contacts of an index case can guide public health strategies. METHODS: We conducted a prospective observational household study in Oregon and Washington, USA, during 2022-2024. Participants completed enrollment, weekly, and follow-up symptom surveys, and submitted sera and weekly nasal swabs. Swabs were tested for SARS-CoV-2 by PCR; a subset of positive specimens underwent whole genome sequencing. Sera was tested for SARS-CoV-2 antibodies. We evaluated individual and household-level factors associated with risk of SARS-CoV-2 infection after 1-14 days (likely secondary infection) among household contacts within households with a distinct (i.e. single) index case. RESULTS: Overall, we identified 617 households with a distinct index case, of which 90% were symptomatic and 41% were children. The estimated secondary infection rate was 16% (95% CI 13.6, 18.8%), reflecting 203 secondary infections among 1,267 household contacts. Among household contacts, factors associated with secondary infection included low anti-nucleocapsid antibody titers (RR: 1.88, 95% CI 1.34, 2.64), or residing with an index case who was symptomatic (RR: 1.85; 95% CI 1.04, 3.23), or had viral detection > 1 week (RR: 2.15, 95% CI 1.55, 2.97). Among 136 sequenced index-secondary case pairs, 128 (94%) shared SARS-CoV-2 lineages. CONCLUSIONS: During 2022-2024, 1 in 7 household contacts of index cases were infected within two weeks, with higher rates in contacts with lower antibody levels and when index cases had symptoms or longer viral detection. These findings can inform ongoing strategies to mitigate SARS-CoV-2 community transmission.