The chances are reduced even further with each additional vaccinated or otherwise immune family member, according to new data.
Lead author Peter Nordström, MD, PhD, with the unit of geriatric medicine, Umeå (Sweden) University, said in an interview the message is important for public health: “When you vaccinate, you do not just protect yourself but also your relatives.”
The findings were published online on Oct. 11, 2021, in JAMA Internal Medicine.
Researchers analyzed data from 1,789,728 individuals from 814,806 families from nationwide registries in Sweden. All individuals had acquired immunity either from previously being infected with SARS-CoV-2 or by being fully vaccinated (that is, having received two doses of the Moderna, Pfizer, or Oxford/AstraZeneca vaccines). Persons were considered for inclusion until May 26, 2021.
Each person with immunity was matched in a 1:1 ratio to a person without immunity from a cohort of individuals with families that had from two to five members. Families with more than five members were excluded because of small sample sizes.
Primarily nonimmune families in which there was one immune family member had a 45%-61% lower risk of contracting COVID-19 (hazard ratio, 0.39-0.55; 95% confidence interval, 0.37-0.61; P < .001).
The risk reduction increased to 75%-86% when two family members were immune (HR, 0.14-0.25; 95% CI, 0.11-0.27; P < .001).
It increased to 91%-94% when three family members were immune (HR, 0.06-0.09; 95% CI, 0.04-0.10; P < .001) and to 97% with four 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,” the authors wrote. They listed as an example that, in three-member families in which two members were immune, the remaining nonimmune family member had an 80% lower risk for hospitalization (HR, 0.20; 95% CI, 0.10-0.43; P < .001).
Dr. Nordström said the team used the family setting because it was more easily identifiable as a cohort with the national registries and because COVID-19 is spread among people in close contact with each other. The findings have implications for other groups that spend large amounts of time together and for herd immunity, he added.
The findings may be particularly welcome in regions of the world where vaccination rates are very low. The authors noted that most of the global population has not yet been vaccinated and that “it is anticipated that most of the population in low-income countries will be unable to receive a vaccine in 2021, with current vaccination rates suggesting that completely inoculating 70%-85% of the global population may take up to 5 years.”
Jill Foster, MD, a pediatric infectious disease specialist at the University of Minnesota, Minneapolis, said in an interview she agrees that the news could encourage countries that have very low vaccination rates.
This study may help motivate areas with few resources to start small, she said: “Even one is better than zero.”
She added that this news could also help ease the minds of families that have immunocompromised members or in which there are children who are too young to be vaccinated.
With these data, she said, people can see there’s something they can do to help protect a family member.
Dr. Foster said that although it’s intuitive to think that the more vaccinated people there are in a family, the safer people are, “it’s really nice to see the data coming out of such a large dataset.”
The authors acknowledged that a limitation of the study is that, at the time the study was conducted, the Delta variant was uncommon in Sweden. It is therefore unclear whether the findings regarding immunity are still relevant in Sweden and elsewhere now that the Delta strain is dominant.
The authors reported no relevant financial relationships. Dr. Foster has received grant support from Moderna.
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