Immune Family Members Associated With Lower Risk of COVID-19 Infection for Those Without Immunity

Study indicates COVID-19 vaccination is important for individual protection and for reducing transmission within families.

Individuals without COVID-19 immunity had a significantly lower risk of infection and hospitalization if their family members had immunity from a previous infection or full vaccination, according to a study published by JAMA Internal Medicine. The study, conducted by investigators at Umeå University, Sweden, reviewed data from more than 1.8 million individuals throughout Sweden.

“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,” said Marcel Ballin, doctoral student in geriatric medicine at Umeå University, in a press release. “Consequently, ensuring that many people are vaccinated has implications on a local, national, and global scale.”

To conduct the study, investigators combined registry data from the Public Health Agency of Sweden, the National Board of Health and Welfare, and Statistics Sweden. These data included more than 800,000 families who were analyzed to quantify the association between the number of family members with immunity against COVID-19 and the risk of infection and hospitalization in nonimmune individuals within that same family.

The investigators found a dose-response association between the number of immune individuals in each of the reviewed families and the risk of infection and hospitalization in family members who were not immune. These individuals had between a 45% and 97% lower risk as the number of immune family members increased, according to the results of the study. These findings accounted for differences in age, socioeconomic status, clustering within families, and several diagnoses previously identified as risk factors for COVID-19.

“The results strongly suggest that vaccination is important not only for individual protection, but also for reducing transmission, especially within families, which is a high-risk environment for transmission,” said Peter Nordström, professor of geriatric medicine at Umeå University, in a press release.


Link between COVID vaccination and reduced household transmission, Swedish study finds [news release]. ScienceDaily; October 11, 2021. Accessed October 13, 2021.