Higher levels of COVID-19 vaccination in a population are tied to lower rates of infection in unvaccinated youth younger than 16 years, who were ineligible for the vaccine at the time of the trial, according to an observational, real-world, Israeli study today in Nature Medicine.
Researchers from Technion-Israel Institute of Technology in Haifa led the study, which involved mining vaccination records and COVID-19 test results gathered during a rapid vaccine rollout in 177 communities.
They found that vaccination rates in each community were linked to a large subsequent decline in COVID-19 infections in the unvaccinated youth. For every 20 percentage points of vaccinated people in a population, on average, COVID-19 test positivity declined about twofold.
Unvaccinated may include ineligible, immune-compromised
Participants were 1.37 million members of Israel's second-largest health maintenance organization who received their first dose of the Pfizer/BioNTech coronavirus vaccine from Dec 9, 2020, to Mar 9, 2021, as well as a cohort of the unvaccinated youths.
The researchers evaluated changes in the number of positive COVID-19 tests in each community in fixed time intervals to account for international coronavirus patterns and intracommunity differences. They noted that some people may have chosen not to be vaccinated, been ineligible owing to age, or been vaccinated yet still at risk owing to immunodeficiency.
For each community, the team calculated the average cumulative proportion of people who received their first dose of COVID-19 vaccine during two consecutive 3-week periods for those 16 to 50 years, who were assumed to be most likely to interact with those younger than 16. For each time interval, a period 28 days after the vaccine dose was defined to capture the full effects of immunization and later cross-protection of unvaccinated people.
The authors then calculated the rate of positive COVID-19 tests among the unvaccinated cohort and compared both periods to identify the change in the proportion of positive test results of the unvaccinated group as well as the change in vaccination rate.
To reduce the confounding effect of natural immunity from previous SARS-CoV-2 infection, which could also protect the unvaccinated, the researchers narrowed the results to test records since Mar 1, 2020 and included only communities in which the fraction of the population who tested positive for coronavirus by Mar 9, 2021 was less than 10%.
"This exclusion of communities did not introduce notable biases in the age distributions of neither the vaccinated individuals nor the patients with a confirmed SARS-CoV-2 infection," they wrote.
Cross-protection for unvaccinated
While noting studies that found reduced SARS-CoV-2 viral loads among vaccinated people, the researchers said that vaccination could also lead to undesirable behaviors such as failure to quarantine after exposure to the virus or to continue keeping a distance from others in the community.
Furthermore, they said, because the number of people infected by another person varies by sociobehavioral and environmental factors, even in communities without vaccine availability, and because global infection rates take into account both vaccinated and unvaccinated people, determining the effect of vaccination on community-level SARS-CoV-2 spread has been difficult.
While the study results didn't address the possibility of natural immunity, the researchers said that they hint at possible population-level control of COVID-19, which is crucial for quelling the pandemic.
"These results provide observational evidence that vaccination not only protects individuals who have been vaccinated but also provides cross-protection to unvaccinated individuals in the community," they wrote. "Although the observed vaccine-associated protection of the unvaccinated population is encouraging, further studies are required to understand whether and how vaccination campaigns might support the prospect of herd immunity and disease eradication."