Countywide vaccination rates that appear strong enough to prevent measles outbreaks can conceal much higher risks inside individual schools, according to new Cornell University research that suggests public health officials may need more detailed data to identify vulnerable communities.
The study, published Aug. 18 in Nature Medicine, found that clusters of children who are unvaccinated or haven’t received both recommended doses of the measles, mumps and rubella vaccine can create conditions for sustained transmission even when overall county vaccination rates remain near the roughly 95% level generally associated with community protection.
Researchers said the problem is one of scale: County and statewide averages can look reassuring while individual schools have substantially larger concentrations of students who aren’t fully protected.
That distinction has become increasingly important as measles has returned in larger numbers across the United States. Cornell researchers said the country recorded more than 3,500 cases during 2025 and 2026, decades after measles was declared eliminated in the U.S. in 2000.
Researchers built school-level national database
To better understand where the risk was concentrated, researchers assembled vaccination records covering roughly 50,000 schools, 13,000 school districts and 3,000 counties across 45 states and the District of Columbia.
The records dated to 2013 and focused on kindergarten vaccination coverage.
“There was no single national database of school-level vaccination coverage,” said Siyu Chen, a postdoctoral research associate at Cornell and the study’s first author.
Researchers then used a mathematical model to translate vaccination coverage into the likelihood that measles could spread after being introduced into a school.
The results showed substantially more local variation than county-level statistics suggested.
Nationally, the average share of students considered unprotected roughly doubled following the COVID-19 pandemic, increasing from about 5% to nearly 10%, according to the study.
Researchers attributed the broader decline in vaccination coverage to several factors, including vaccine hesitancy, misinformation and differing vaccine-exemption policies among states. They said the downward trend began before the pandemic but accelerated afterward.
High county rate doesn’t mean every school is protected
Measles is highly contagious, making relatively small pockets of unvaccinated people more consequential than they might be for diseases that spread less easily.
Cornell researchers found that a newly introduced case could spread within a school containing enough children who were unvaccinated or had missed one of the two recommended MMR doses.
Those conditions may not be obvious when public health officials look only at the county average.
“Before this decrease of vaccination, even with an introduction of measles, because vaccination coverage was so high, you could still have one or two cases and the outbreak would fizzle out,” said Ana Bento, an assistant professor in Cornell’s Department of Public and Ecosystem Health and the study’s senior author.
The situation has changed as vaccination coverage has become more uneven.
Bento said areas with particularly low coverage can now experience significant localized outbreaks after a single introduction of the virus.
Researchers pointed to measles outbreaks in Texas, Arizona, New Mexico and South Carolina during 2025 and 2026 as examples of that pattern. Countywide MMR vaccination rates could appear relatively strong while clusters of unvaccinated students at particular schools created enough susceptibility for the virus to spread.
Findings could change how officials measure risk
The research suggests public health agencies may get a clearer picture of measles vulnerability by examining vaccination data at the school or district level rather than relying primarily on countywide averages.
That could be especially relevant when officials are deciding where to direct vaccination outreach, monitoring or other public health resources.
The study doesn’t suggest that every school within a county reporting lower vaccination rates will experience an outbreak. Rather, researchers argue that averages can obscure the concentrations of susceptible people that determine whether an introduced measles case continues spreading.
The findings also highlight the value of consistently collecting and publishing detailed vaccination information.
Chen said building the national dataset required researchers to reconcile more than a decade of records collected independently by states and never intended to function as one national system.
For public health officials in New York and elsewhere, the takeaway is that a county vaccination rate may not tell the entire story.
“We can see the risks now,” Bento said. “We just weren’t watching at the right scale and the window to fix that is closing.”
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