Nordström P, Ballin M, Nordström A, et al. - In this study, a key role of COVID-19 vaccines in decreasing the transmission of the virus within families was evident, which probably has implications for herd immunity and pandemic control.

• In this cohort study of 1,789,728 individuals from 814,806 families in Sweden, the goal was to inquire about the link between risk of COVID-19 in nonimmune people and the number of their family members with known immunity derived from a previous COVID-19 infection or full vaccination (2 vaccine doses).

• A diagnosis of COVID-19 was made in 5.7% of nonimmune family members during a mean (range) follow-up time of 26.3 (1-40) days.

• An inverse dose-response relationship was found between the number of immune members in each family and the risk of incident COVID-19 infection in nonimmune family members.

• A 45% to 97% lower risk of contracting COVID-19 was observed in family members without immunity as the number of immune family members increased.