Reduced risk of SARS-CoV-2 infection among household contacts with recent vaccination and past COVID-19 infection: results from 2 multisite case-ascertained household transmission studies.

Households are a primary setting for the transmission of SARS-CoV-2. We examined the role of prior SARS-CoV-2 immunity on the risk of infection in household close contacts. Households in the United States with an individual who tested positive for SARS-CoV-2 from September 2021 to May 2023 were enrolled if the index case's illness began ≤ 6 days prior. Household members had daily self-collected nasal swabs tested by reverse-transcriptase polymerase chain reaction for SARS-CoV-2. The effects of prior SARS-CoV-2 immunity (vaccination, prior infection, or hybrid immunity) on SARS-CoV-2 infection risk among household contacts were assessed by robust, clustered multivariable Poisson regression. Of 1532 contacts (905 households), 8% had immunity from prior infection alone, 51% from vaccination alone, 29% hybrid immunity, and 11% had no prior immunity. Sixty percent of contacts tested SARS-CoV-2 positive during follow-up. The adjusted risk of SARS-CoV-2 infection was lowest among contacts with vaccination and prior infection (adjusted relative risk [aRR], 0.81; 95% CI, 0.70-0.93, compared with contacts with no prior immunity) and was lowest when the last immunizing event occurred ≤ 6 months before COVID-19 affected the household (aRR, 0.69; 95% CI, 0.57-0.83). In high-transmission settings like households, immunity from COVID-19 vaccination and prior infection was synergistic in protecting household contacts from SARS-CoV-2 infection.