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Among Teens, COVID-19 Vaccine Effective for the Immunized but Shows No Spillover Effect for the Unvaccinated

New study by WSB’s Dan Sacks looks at unique control groups among adolescents

By Leiah Fundell

November 15, 2022

Dan Sacks with chevron graphic

It’s been shown that the COVID-19 vaccine has significant direct effects: If you get vaccinated, your risk of infection is much lower. But new research by Dan Sacks, associate professor of risk and insurance at the Wisconsin School of Business, finds that although the COVID-19 vaccine is effective in preventing infection, among early-adopting adolescents it does not appear to prevent transmission to others.

In other words, your teen’s vaccination doesn’t make other teens safer. And, if your teen is unvaccinated, being around vaccinated peers does not do much to reduce their risk of infection.

“I think everyone’s presumption was that there are really important indirect effects from the COVID vaccine—that vaccines are important not just because they protect me, but also because they reduce transmission to everyone else,” says Sacks. “But our research shows that with early adoption of the vaccine in adolescents, there was great protection for the vaccinated, but there was no measurable benefit to others.”

Key findings from the paper show that:

  • The effects of the vaccine in the real world were very similar to the effects in clinical trials. For teens who got the vaccine earlier because they were eligible earlier, the research finds that the vaccine was 80% effective in preventing infection. 
  • Teens who go to school with 20% more vaccinated peers do not have different COVID-19 infection rates from those who are surrounded by drastically fewer vaccinated peers.

Study design and research summary

Using linked data from the Indiana vaccine registry, PCR COVID-19 test registry, and an extensive electronic medical records data system, Sacks and co-authors Seth Freedman, Kosali Simon, and Coady Wing, all from Indiana University, studied two cohorts of kids over two school years (2020-21 and 2021-22). 

The team studied the direct effects of the vaccine by following a fifth-grade control group through sixth grade and a treatment group in sixth grade through to seventh grade over a two-year period. While neither group was vaccine eligible in year one, the treatment group became vaccine eligible in year two. COVID-19 incidence in year one was the same for both groups, but the treatment group had lower COVID-19 incidence in year two. Quantitatively, they found that the direct effect of the vaccine reduces COVID-19 incidence by about 80%, similar to what was found in vaccine trials. 

To study the indirect effects of the vaccine, researchers focused on sixth graders in both years who were not vaccine eligible 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. In contrast, the elementary school sixth graders had unvaccinated schoolmates. If there were to be big spillover effects from receiving the vaccine, researchers expected COVID-19 incidence to fall among the middle school sixth graders who go to school with vaccine-eligible peers. But the research found no effect like that at all. Thus, the spillover effects appear small, at least given the scale of vaccine takeup among adolescents in Indiana at that time which was only around 30% to 40%.

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

The implications of this research extend beyond schools and teen populations.

“Companies and other entities have a vested interested in the health of their staff for a myriad of reasons,” says Sacks. “Emphasis has been placed on the indirect benefits of a vaccine mandate protecting everyone, even the unvaccinated, but our research shows that that reason may be a bit weaker than we thought.”

Read the working paper, “Direct and Indirect Effects of Vaccines: Evidence from COVID-19 in Schools,” released by the National Bureau of Economic Research.

Dan Sacks is an associate professor of risk and insurance at the Wisconsin School of Business.