Abstract
The association between COVID-19 immunity within families and the risk of infection in nonimmune family members is unknown. To investigate the association between risk of COVID-19 in nonimmune individuals and the number of their family members with known immunity acquired from a previous COVID-19 infection or full vaccination (2 vaccine doses). In this cohort study of data from nationwide registries in Sweden, all individuals who acquired immunity from either previous COVID-19 infection or full vaccination until May 26, 2021, were considered for inclusion. Each person with immunity was matched 1:1 to an individual without immunity from an identified cohort of individuals with families comprising 2 to 5 members. Number of immune family members in each family on April 14, 2021 (index date), who acquired immunity from a previous COVID-19 infection or full vaccination (2 doses of the mRNA-1273, BNT162b2 mRNA, or ChAdOx1 nCoV-19 vaccine). Incident COVID-19 infection in nonimmune family members from April 15 to May 26, 2021. A total of 1 789 728 individuals from 814 806 families were included in the analysis. Each family comprised 2 to 5 family members, with a mean (SD) age at baseline of 51.3 (19.5) years. During a mean (range) follow-up time of 26.3 (1-40) days, 88 797 of 1 549 989 (5.7%) nonimmune family members (mean [SD] age, 51.6 [17.7] years; 790 276 men [51.0%]) were diagnosed with COVID-19. There was an inverse dose-response association between the number of immune members in each family and the risk of incident COVID-19 infection in nonimmune family members. Nonimmune families with 1 immune family member had a 45% to 61% lower risk of contracting COVID-19 (hazard ratio [HR], 0.39-0.55; 95% CI, 0.37-0.61, P < .001). The risk reduction increased to 75% to 86% in families with 2 immune family members (HR, 0.14-0.25; 95% CI, 0.11-0.27; P < .001), 91% to 94% with 3 immune family members (HR, 0.06-0.09; 95% CI, 0.04-0.10; P < .001), and 97% with 4 immune family members (HR, 0.03; 95% CI, 0.02-0.05; P < .001). The results were similar for the outcome of COVID-19 infection that was severe enough to warrant a hospital stay. In this cohort study, family members without immunity had a 45% to 97% lower risk of contracting COVID-19 as the number of immune family members increased. Vaccination is a key strategy for decreasing the transmission of the virus within families.
Highlights
Immune Family Members and the Risk of COVID-19 in Nonimmune Family Members During a total follow-up time of 111 454 years, we found that 88 797 of 1 549 989 nonimmune individuals (5.7%) were diagnosed with COVID-19 during a mean follow-up time of 26.3 (1-40) days
In families with 2 immune family members, nonimmune family members had a 75% to 86% lower risk (HR, 0.14-0.25; 95% CI, 0.11-0.27; P < .001 for all), which was further reduced to 91% to 94% in families with 3 immune family members (HR, 0.06-0.09; 95% CI, 0.04-0.10; P < .001 for all)
The benefits were similar regardless of whether immunity was acquired from a previous infection, a single dose of vaccine, or full vaccination
Summary
We considered for inclusion all individuals who were diagnosed with COVID-19 (n = 1 331 989) and/or vaccinated against COVID-19 (n = 3 640 421) in Sweden until May 26, 2021. Each person with immunity was matched 1:1 to 3 348 248 individuals based on birth year, birth month, and municipality from the total population of Sweden using. 1331989 Individuals diagnosed with COVID-19 in Sweden as of May 26, 2021. 3640421 Individuals vaccinated against COVID-19 in Sweden as of May 26, 2021. 4043275 Excluded 88326 Had a family member who became infected with COVID-19 48331 Died. 461887 Had relatives who were all immune 716270 Had only 1 dose of COVID-19 vaccine 2728461 Had 5 family members.
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