It has been hard to determine, since COVID-19 vaccinations first rolled out, the extent that vaccines protect unvaccinated close contacts from infection. Now, an Israeli team has found that parental vaccination confers substantial protection for unvaccinated children in the same household. Through studying households without prior infection, consisting of two parents and unvaccinated children, the team estimated the effect of parental vaccination on the risk of infection for unvaccinated children.

The results are published in Science in the article, “Indirect protection of children from SARS-CoV-2 infection through parental vaccination.

In Israel, the Pfizer-BioNTech mRNA COVID-19 vaccine authorization was extended in May 2021 to children and adolescents aged 12 years or older and in November 2021, it was extended to children aged 5 years or older.

Using integrated data repositories of Israel’s largest healthcare organization, Samah Hayek, PhD, at the Clalit Research Institute in Israel, studied two periods separately—an early period (January 17, 2021–March 28, 2021) and a late period (July 11, 2021–September 30, 2021).

Regardless of household size, Hayek and colleagues found that parental vaccination substantially reduced the risk of children up to age 12 of becoming infected by reducing the probability of contacting an infectious individual and by a reduction in infectiousness of a vaccinated person who suffers a breakthrough infection.

More specifically, they found that “having a single vaccinated parent was associated with a 26.0% and 20.8% decreased risk, and having two vaccinated parents was associated with a 71.7% and 58.1% decreased risk, in the early and late periods, respectively.”

“These results reinforce the importance of increasing vaccine uptake among the vaccine-eligible population to curb the spread of the SARS-CoV-2 pandemic and protect those who cannot be vaccinated,” the authors said.

In a second study based in Israel, findings showed a similar result—that vaccination reduced both the rate of infection with SARS-CoV-2 and household transmission.

This work is published in Science in the article, “Vaccination with BNT162b2 reduces transmission of SARS-CoV-2 to household contacts in Israel.

Many studies have already estimated the impact of the vaccines on disease severity and susceptibility. However, estimates of the vaccines’ impact on transmissibility are more limited.

Israel exclusively adopted the Pfizer-BioNTech mRNA vaccine. To examine this vaccine’s effectiveness against transmission, Ottavia Prunas, PhD, a postdoctoral associate in the Weinberger lab at the Yale School of Public Health, and colleagues, used statistical approaches—a chain binomial model—to estimate the effectiveness of vaccination with the Pfizer-BioNTech vaccine against household transmission of SARS-CoV-2 in Israel before and after the Delta variant emerged.

The authors found that vaccinated, subsequently infected people were less infectious than unvaccinated persons. More specifically, the authors wrote that “vaccination reduced susceptibility to infection by 89.4%, whereas vaccine effectiveness against infectiousness given infection was 23.0% during days 10 to 90 after the second dose before June 1, 2021.” Total vaccine effectiveness was 91.8%.

Moreover, less transmission occurred within households with vaccinated members than in those with unvaccinated individuals.

However, the ability of the vaccine to prevent transmission waned with time and the advent of the Delta variant. “It is highly unlikely that population-level transmission of SARS-CoV-2 can be eliminated through vaccination alone,” the authors added.