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While COVID-19 vaccination reduces incidence, severity, and transmission of SARS-CoV-2 infection, more-detailed information is lacking regarding the effects of immunity (whether due to vaccination or previous infection) in households that also include nonimmune family members. Now, a population-level study in Sweden provides further information on family transmission in partially immune households.
Using government health records, Nordström et al. assessed about 1,800,000 individuals from >800,000 families with two to five members for the development of COVID-19 in nonimmune family members. Between April 14 and May 26, 2021, 5.7% of nonimmune family members developed COVID-19. Risk for infection was reduced by 45% to 61% if one family member was immune and up to 97% if four family members were immune. Reductions in infection risk were similar whether immunity was due to previous infection or vaccination.
Nordström P et al. Association between risk of COVID-19 infection in nonimmune individuals and COVID-19 immunity in their family members. JAMA Intern Med 2021 Oct 11; [e-pub]. (https://doi.org/10.1001/jamainternmed.2021.5814)
Comment
As this study was conducted before the widespread circulation of the delta variant, the lower infection risk associated with having immune family members would be expected to be subsequently attenuated by waning immunity or more-transmissible virus. Nonetheless, the study demonstrates the importance of encouraging household contacts of at-risk patients to get vaccinated.