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COVID-19 vaccine protects vaccinated children but may not prevent transmission to the unvaccinated

For Immediate Release Oct 20, 2022

Note to journalists: This research was published as a working paper on Oct. 16 by the National Bureau of Economic Research. References to these findings should note their pre-publication status and that the findings have not yet been peer reviewed, which means they have not been vetted by a group of peer scientists in the field of study.

BLOOMINGTON, Ind. — New Indiana University research on COVID-19 vaccinations in school-age children suggests one child’s vaccination provides strong protection to themselves, but may offer less protection to other unvaccinated children than expected.

Preliminary findings from a study by researchers at IU and the University of Wisconsin-Madison, published ahead of peer review as a working paper by the National Bureau of Economic Research, show that although the COVID-19 vaccine is effective in preventing illness for the vaccinated, increases in school-wide vaccination rates provided no measurable protection against transmission of the virus to unvaccinated students.

Seth Freedman. Seth Freedman.

Seth Freedman, an associate professor in the IU Paul H. O’Neill School of Public and Environmental Affairs, said this means that if children are unvaccinated, being around vaccinated children does not necessarily protect them.

“It’s really important to know if one person’s vaccination prevents spread of the virus to other unvaccinated peers, especially in a setting like schools,” Freedman said. “Our study suggests that it does not, at least when overall vaccination rates are pretty low. But we also find that school-age children who were vaccinated received really strong protection for themselves.”

Key findings from the paper show that:

  • The effects of the vaccine in the real world were similar to the effects in clinical trials. For children who got the vaccine earlier because they were eligible earlier, the research finds that the vaccine was 80% effective against preventing infection.

  • Increasing the schoolwide vaccination rates from 5% to 25% did not reduce the chances of unvaccinated students getting COVID-19.

Freedman and co-authors Kosali Simon and Coady Wing of IU and Dan Sacks of UW-Madison’s Wisconsin School of Business studied two cohorts of kids over two school years, 2020-21 and 2021-22, using linked data from the Indiana vaccine registry, PCR COVID-19 test registry and a large electronic medical records data system.

The team studied the direct effects of the vaccine, i.e. how much it protects the vaccinated, by following a fifth-grade control group through sixth grade and a sixth-grade treatment group through to seventh grade over a two-year period. While neither group was vaccine eligible in year one because of age requirements, the treatment group became vaccine eligible in year two. The number of COVID-19 cases in year one was the same for both groups, but the treatment group had a lower number of COVID-19 cases in year two.

For children who got the vaccine earlier because they were eligible earlier, the research finds t... For children who got the vaccine earlier because they were eligible earlier, the research finds that the vaccine was 80% effective against preventing infection. Photo by Getty Images

Quantitatively, the researchers found that the direct effect of the vaccine reduces COVID-19 incidence by about 80% — similar to what was found in vaccine clinical trials.

To study the indirect effects of the vaccine, researchers focused on sixth-graders in both years, who were mostly vaccine ineligible in either year. In Indiana, some sixth-graders go to elementary schools (K-6) and others go to middle schools (typically grades 6-8). That means that in year two, the middle school sixth-graders attended school with vaccine-eligible seventh- and eighth-graders; about a quarter of their school was vaccinated. In contrast, the elementary school sixth-graders had vaccine-ineligible schoolmates; less than 5% of their school was vaccinated.

If there were big spillover effects from the vaccine, researchers expected the number of COVID-19 cases to decrease among the middle school sixth-graders who went to school with vaccine-eligible peers. But the study found similar levels of COVID-19 among the middle school and elementary school sixth-graders.

Thus, the indirect effects were small, at least given the scale of vaccine take-up among adolescents in Indiana at that time, which was below 30%.

“One important limitation is that we’re looking at a group where the vaccination rate is low,” Sacks said. “And for a really infectious disease like COVID-19, you might need a much higher vaccination rate before the indirect effects matter.”

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