Family members without immunity had a 45% to 97% lower risk of contracting COVID-19 as the number of immune family members increased, found a large Swedish cohort study.
Non-immune family members had a 45% to 61% lower risk of contracting COVID-19 when they had one immune relative (HR 0.39-0.55, 95% CI 0.37-0.61) and up to a 97% lower risk of contracting the virus when they had four immune family members (HR 0.03, 95% CI 0.02-0.05), reported Dr Peter Nordström of Umeå University in Sweden, and colleagues in JAMA Internal Medicine.
The nationwide cohort study of more than 1.7m people defined “immunity” as either full vaccination with a two-dose COVID-19 vaccine from Pfizer, Moderna, or AstraZeneca or “known immunity from a previous COVID-19 infection”, as confirmed by RT-PCR or genomic sequencing.
“These findings suggest that vaccines play a key role in reducing the transmission of the virus within families, which possibly has implications for herd immunity and pandemic control,” the authors wrote.
Interestingly, while Nordström and colleagues discussed vaccination extensively, one limitation to the study was the fact that there were not enough fully vaccinated participants for a sensitivity analysis on this subgroup alone.
The study was conducted from 15 April to 26 May 2021; each individual with immunity was matched to one without immunity, each from a family of two to five individuals. In total, 1,789,728 individuals were included. Mean age at baseline was about 51. Mean follow-up time was 26 days, during which time 5.7% of non-immune family members were diagnosed with COVID-19 (mean age 51.6, 51% men).
The dose-response relationship between number of immune family members and risk of COVID infection in non-immune family members decreased with the number of immune family members: two immune family members: 75% to 86% reduced risk; three immune family members: 91% to 94% reduced risk.
Nordström and colleagues also noted that the results were comparable for severe COVID-19 infection that required hospitalisation. They performed a sensitivity analysis on people who only received one dose of vaccine, and found similar results. However, they noted caveats to this, namely that the study was done when the Alpha variant was circulating, and it is unknown what effect one dose of vaccine has on other variants.
“Reports have suggested that recipients of a single dose may have less protection against the Beta and Delta variants, and these variants appear to be more transmissible,” they wrote.
Study details
Association Between Risk of COVID-19 Infection in Nonimmune Individuals and COVID-19 Immunity in Their Family Members
Peter Nordström, MD, PhD1; Marcel Ballin, MSc1,2; Anna Nordström, MD, PhD2,3
Published in JAMA on 11 October 2021
Key points
Question How is COVID-19 immunity within families associated with the risk for infection in family members without immunity?
Findings In this cohort study of 1 789 728 individuals from 814 806 families in Sweden, family members without immunity had a 45% to 97% lower risk of contracting COVID-19 as the number of immune family members increased.
Meaning These results suggest that COVID-19 vaccines play a key role in reducing the transmission of the virus within families, which likely has implications for herd immunity and pandemic control.
Abstract
Importance
The association between COVID-19 immunity within families and the risk of infection in nonimmune family members is unknown.
Objective
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).
Design, Setting, and Participants
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.
Exposures
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).
Main Outcomes and Measures
Incident COVID-19 infection in nonimmune family members from April 15 to May 26, 2021.
Results
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.
Conclusions and Relevance
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.
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