MADISON, Wis. — A groundbreaking new study co-authored by associate professor Daniel Sacks of the Wisconsin School of Business at the University of Wisconsin–Madison reveals that COVID-19 vaccines not only protect individuals but also significantly reduce transmission within households, offering critical insights for future public health policy.
Published in the prestigious journal American Economic Journal: Applied Economics, the study, “Direct and Indirect Effects of Vaccines: Evidence from COVID-19,” provides the most rigorous real-world evidence to date on how far vaccine protection extends beyond the vaccinated individual.
Using near-universal health data from Indiana and a unique natural experiment based on age-based vaccine eligibility, the research team found:
- Real-world evidence that vaccination reduces COVID-19 cases by 80% among vaccinated individuals, confirming strong direct protection against the disease.
- Every two vaccinated children prevent nearly one additional case among household members, demonstrating that indirect effects are about 75% as strong as direct effects.
- These protective spillovers do not extend to schoolmates, suggesting that vaccine-induced externalities are highly context-dependent.
“Vaccines don’t just protect the person who gets the shot—they protect the people they live with. That’s a big deal.”
The findings come at a time when public attention to COVID-19 vaccination is waning and recommendations are evolving. Yet the study underscores a critical point: vaccines are not just a personal health decision—they’re a tool for protecting families.
“As recommendations change, a lot of people may wonder whether to get vaccinated, especially as the virus has changed since the initial clinical trials were run,” said Sacks. “Our results show that during the Delta wave, the COVID-19 vaccines offered strong protection. And they don’t just protect the person who gets the shot—they protect the people they live with. That’s a big deal.”
The research challenges assumptions about the reach of vaccine spillovers and offers a more targeted framework for evaluating public health interventions, especially in close-contact environments like households and workplaces.
“The FDA is making vaccine recommendations primarily based on direct protections,” Sacks added. “But a lot of people may get vaccinated not to protect themselves, but to protect vulnerable community members. That protection is important at the household level, but might not extend indefinitely.”
As policymakers and public health leaders consider the next phase of pandemic preparedness, this study offers timely, data-driven insight into how vaccines work in the real world—and why their benefits may be broader than previously understood.
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