COVID Herd Immunity: A Family Affair?

There was a dose-response association between the number of family members “immune” to COVID-19 and the chance of “non-immune” family members contracting the virus, a Swedish study found.

Non-immune family members had a 45% to 61% lower risk of contracting COVID-19 when they had one immune family member (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 Peter Nordström, MD, PhD, of Umeå University in Sweden, and colleagues in JAMA Internal Medicine.

This nationwide cohort study of over 1.7 million individuals 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 likely 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 April 15 to May 26, 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 (HR 0.14-0.25, 95% CI 0.11-0.27)
• Three immune family members: 91% to 94% reduced risk (HR 0.06-0.09, 95% CI 0.04-0.10)

Nordström and colleagues also noted that the results were comparable for severe COVID-19 infection that required hospitalization.

They performed a sensitivity analysis on people who only received one dose of vaccine, and found similar results, noting that in families with three members, if two had one dose of vaccine, the remaining non-immune family member had a 66% lower risk of COVID-19 infection (HR 0.34, 95% CI 0.29-0.39).

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.