Clinical trials focus on the modeled (direct) effects of vaccines on health outcomes (for example, reduction in disease, hospitalization, emergency department use, and death). Typically, however, clinical trials do not test whether vaccines reduce transmission. The FDA does not require this because estimating indirect effects of vaccination within a clinical trial would be logistically challenging and expensive.
New one AEJ Applied Economics The paper by Friedman, Sachs, Simon, and Wing (2026) aims to estimate both direct and indirect effects. Use a natural experiment framework to roll-out differential vaccine eligibility by age:
Our center state, Indiana, expanded eligibility to health care workers, first responders and nursing home residents on December 14, 2020. Unlike most states, however, subsequent eligibility groups in Indiana were determined based only on age…Most age groups became eligible in short succession, with a delay of 22 days or less between age groups, until 12–16 year olds became eligible on May 12, 2021. Children aged 5-11 became eligible. Will not be eligible until November 3, 2021.
The authors then examine the impact of vaccinating 11 versus 12-year-olds given delayed vaccine access. They use two data sources: (i) the Indiana Network for Patient Care (INPC) database that includes medical records from health care providers throughout the state, and (ii) a registry of nearly all COVID lab (PCR) tests and COVID vaccinations conducted in Indiana from the Regenstrief Institute. An individual’s home location is taken from the INPC database, which includes all medical encounters (not just COVID-related encounters). Addresses are also used to input school assignments.
The brief direct effects of vaccination use 11 versus 12 years of age as a differential approach to examine the direct effects of early access vaccination. Indirect effects are measured by looking at the health outcomes of individuals ages 2-10 or 30 and older who live with the vaccine versus 11-year-olds (who were not initially eligible for vaccines) versus 12-year-olds (who were eligible for early vaccines).
The authors use a clever methodology to identify school-based indirect effects. Use the fact that some sixth graders go to middle school and others to elementary school.
Our empirical strategy to identify the effects of school-wide vaccination rates on individual COVID risk is based on a comparison of sixth-grade students in middle schools and elementary schools. The idea behind this comparison is that middle school students are mostly eligible for the COVID vaccine in the fall of 2021, as are all students in seventh grade and up. In contrast, almost all primary school students are ineligible. Since some sixth graders go to middle school and others go to elementary school, this difference in eligibility leads to large differences in school-wide vaccination rates
Using these approaches, the authors found that:
Vaccination reduces cases by 80 percent, direct effect. This protection extends to close contacts, making the indirect effect at the household level about three-quarters larger than the direct effect. However, the indirect effect does not extend to classmates. Our results suggest that it is important to consider vaccine access when designing policy for infectious disease.
You can read the full paper Here,
