Childhood vaccination rates are declining across the United States, and the consequences are already becoming visible. In 2025, the nation recorded more than 1,300 measles cases, the highest annual total since 1992. Public health experts warn that measles, which was declared eliminated in the United States in 2000, could once again become endemic.
A major driver of this trend is the growing number of families opting out of school-required vaccinations. Between 1991 and 2023, the rate of nonmedical vaccine exemptions among U.S. kindergartners tripled. In response, a handful of states took a more aggressive approach by eliminating nonmedical exemptions altogether.
A new study led by Tony Yang, ScD, LLM, MPH, Program Leader of the GW Cancer Center's Cancer Control and Health Equity Program, examined whether these policy changes increased vaccination rates. The findings suggest they do, and that effective vaccine policies may play an important role in protecting public health, including preventing certain cancers.
Why Are Vaccination Rates Declining?
School vaccination requirements have been a cornerstone of American public health since the mid-19th century. Yet in recent years, increasing numbers of parents have sought exemptions based on personal, philosophical, or religious beliefs. As vaccination coverage falls, communities become more vulnerable to outbreaks of preventable diseases.
To better understand the impact of exemption policies, Yang and colleagues from Harvard and Cornell examined what happened when states eliminated nonmedical exemptions for school-entry vaccines.
Previous research had shown that states with stricter exemption requirements generally have lower exemption rates. However, no study had comprehensively evaluated the recent wave of exemption repeals using statistical methods designed to account for policies implemented at different times across multiple states.
A Multi-Institutional Collaboration
The study brought together researchers from three institutions. Anthony Bald, PhD, an economist affiliated with Harvard Kennedy School and California State University, Fullerton, led the data analysis and manuscript preparation. Samantha Gold, BS, from Cornell University's Jeb E. Brooks School of Public Policy, contributed to data analysis and interpretation.Yang oversaw the project's design and execution. All authors participated in reviewing and refining the final manuscript.
What the Researchers Found
The team analyzed publicly available kindergarten vaccination and exemption data collected by the Centers for Disease Control and Prevention between the 2011 and 2023 school years.
The study focused on four states that fully repealed nonmedical exemptions during that period:
- California (2015)
- New York (2019)
- Maine (2019)
- Connecticut (2021)
Together, these policy changes affected approximately 2.8 million kindergarten students.
The results were clear. Vaccination rates increased significantly after nonmedical exemptions were repealed.
Within three years of implementation, vaccination rates for DTaP, hepatitis B, measles-mumps-rubella (MMR), and polio vaccines increased by approximately 3 to 4 percentage points compared with states that retained exemption policies. During the same period, overall exemption rates declined by more than 3 percentage points.
One frequently cited concern is that parents might simply seek medical exemptions instead. The researchers found little evidence of this. Medical exemptions increased by less than half a percentage point, suggesting that state oversight mechanisms largely prevented widespread substitution.
The study also examined states that adopted more limited reforms. Vermont eliminated only philosophical exemptions in 2015, and Washington removed nonmedical exemptions only for the MMR vaccine in 2019. Those partial measures produced smaller and less durable improvements, suggesting that comprehensive policies may be more effective than targeted ones.
Why This Matters for Cancer Prevention
Although many vaccine discussions focus on infectious diseases such as measles, vaccination is also a powerful cancer prevention tool.
The most prominent example is human papillomavirus (HPV) vaccination. HPV is one of the most common viral infections in the United States, affecting an estimated one in eight people at any given time. Persistent infection with certain HPV types can cause cancers of the cervix, anus, penis, vulva, vagina, and oropharynx.
When administered as recommended, the HPV vaccine can prevent more than 90 percent of HPV-related cancers. Vaccination can begin as early as age 9, with routine vaccination recommended during adolescence and catch-up vaccination available through age 26.
Despite extensive evidence demonstrating the vaccine's safety and effectiveness, vaccine misinformation and growing public skepticism continue to influence vaccination decisions. Public health experts worry that declining confidence in vaccines could undermine progress against both infectious diseases and vaccine-preventable cancers.
Why the Findings Matter
Yang's study highlights an important lesson: vaccine policy can influence vaccination behavior at the population level.
Importantly, the benefits of repeal were not immediate. The largest increases in vaccination rates appeared three to four years after states enacted their laws, likely reflecting phased implementation and gradual adaptation by schools, families, and healthcare providers.
As vaccination policies continue to evolve across the United States, differences between states may become increasingly pronounced. States with stronger immunization requirements may maintain higher levels of protection, while those with more permissive policies could experience lower vaccination coverage and greater vulnerability to outbreaks.
As Yang explains:
"When states stopped allowing personal or religious reasons to skip required vaccines, more children got vaccinated. The increase wasn't trivial. It was large enough to strengthen protection across entire school communities. And the concern that parents would simply seek medical exemptions did not materialize on a significant scale."
Looking Ahead
The researchers emphasize that the study does not capture every possible response to stricter vaccination requirements. Some families may choose alternatives such as homeschooling, although previous research suggests those numbers remain relatively small. In addition, state-level data cannot reveal whether policy impacts differ across racial, socioeconomic, or geographic groups.
Future research will examine how changes in vaccine policy, public attitudes, and political environments influence vaccination rates across states. Yang is also interested in studying vaccination patterns at the local level.
"State averages can mask pockets of very low vaccination coverage," he says. "Future research should examine how exemption policies affect geographic clustering of unvaccinated children, because that's often where outbreaks begin."
As debates over vaccination continue, this research provides evidence that policy design matters. The repeal of nonmedical exemptions was associated with sustained increases in childhood vaccination rates, helping protect communities from preventable infectious diseases. More broadly, the findings underscore the importance of maintaining public confidence in vaccination as a cornerstone of disease prevention, including vaccines that protect against cancer-causing infections such as HPV.
