HealthDay News — As the number of family members with immunity acquired through either previous infection or full vaccination increases, the risk for contracting COVID-19 among family members without immunity decreases, according to a study published online October 11 in JAMA Internal Medicine.

Peter Nordström, MD, PhD, from Umeå University in Sweden, and colleagues examined the association between COVID-19 risk in nonimmune individuals and the number of their family members with known immunity. Data were included for 1,789,728 individuals from 814,806 families (each with two to five family members).

The researchers found that 5.7% of nonimmune family members were diagnosed with COVID-19 during a mean follow-up of 26.3 days. The number of immune members in each family was inversely associated with the risk for incident COVID-19 infection in nonimmune family members in a dose-response manner. The risk for contracting COVID-19 was 45 to 61% lower for nonimmune families with one immune family member (hazard ratio, 0.39 to 0.55). In families with 2, 3, and 4 immune family members, the risk reduction increased to 75 to 86% (hazard ratio, 0.14 to 0.25), 91 to 94% (hazard ratio, 0.06 to 0.09), and 97% (hazard ratio, 0.03), respectively.

“It seems as if vaccination helps not only to reduce the individual’s risk of becoming infected, but also to reduce transmission, which in turn minimizes not only the risk that more people become critically ill, but also that new problematic variants emerge and start to take over,” a coauthor said in a statement.

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