Measles is making a comeback. Once considered eliminated from the United States, the disease has been popping up around the country as vaccine coverage declines.
Now, as Florida considers dropping its measles, mumps and rubella (MMR) vaccine mandate for school attendance, a new University of Maryland study predicts that such a policy change could spark the largest measles epidemic in the United States in nearly 500 years. The study will be published in the Proceedings of the National Academy of Sciences on September 28, 2026.
“Measles is one of the most contagious human pathogens, and we’ve seen a striking 650% increase in cases just between 2024 and 2025,” said paper co-author Abba Gumel , a Distinguished University Professor of Mathematics at UMD. “This is an extremely important case study with ramifications well beyond the southeastern United States.”
As vaccine hesitancy and exemptions grow, 3,471 measles cases have been confirmed nationally so far this year—reaching 45 states plus Washington, D.C., and New York City. Recent infection clusters in Texas, New Mexico and South Carolina arose as these states fell below their respective vaccine-induced herd immunity thresholds—the minimum percentage of a population that must be vaccinated to stop the person-to-person spread of a disease.
To assess potential outcomes of Florida’s proposed vaccine-mandate retraction, the UMD team built a transmission model based on a 2025 measles outbreak in Gaines County, Texas, and incorporated Florida-specific demographic structure, MMR vaccination coverage data, and realistic mixing patterns across ages and geographies.
The mathematical model showed that even without removal of the vaccine mandate, Florida’s vaccine-induced herd immunity threshold is already below the level needed to protect against sustained transmission. Simulated declines in MMR vaccination coverage following removal of school-entry requirements further increased large-outbreak potential in the model, with pediatric deaths in numbers not seen since pre-vaccine eras.
“Importantly, we also learned that vaccinating one child is three times more effective at reducing transmission than vaccinating one adult,” said the paper’s first author, Alice Oveson, a Ph.D. student in applied mathematics & statistics, and scientific computation at UMD.
She noted that the difference results mainly from how the age groups interact—with children spending more time face to face than adults.
“Small reductions in child vaccination rates can lead to disproportionately large increases in the scale of outbreaks,” she said.
Oveson compared the situation to wildfire risk.
“It’s like having a very dry forest with a lot of kindling,” she said. “Having a large unvaccinated population including children is the perfect set-up to spark a very big and potentially deadly outbreak.”
Gumel noted that school-entry vaccination mandates for various infectious diseases have long been a cornerstone of U.S. public health policy and an effective tool for preventing disease spread and disease-related mortality.
“What’s most troubling is that we know these mandates effectively prevent erosion of herd immunity,” said Gumel, who has joint appointments at the University of Maryland Institute for Health Computing and the Institute for Physical Science and Technology and holds the Michael and Eugenia Brin Endowed E-Nnovate Chair in Mathematics at UMD. “We also know that strengthening adult immunity through catch-up vaccination could help reduce the severity of outbreaks, and as-needed behavioral interventions like masking and isolating could slow transmission. But neither can compensate for declines in childhood MMR coverage.”
However, Florida has resisted behavioral measures in the past and offers exemptions for people who don’t want to vaccinate for religious, personal or medical reasons. Vaccine exemption requests in Florida are currently at 5.9%, well above the national average of 4.2%.
“Taking it a step further by eliminating the school mandate would suggest that vaccination is unnecessary, which is simply inaccurate and dangerous,” Oveson said, noting that the new paper provides quantitative evidence that such a policy change could make measles endemic in Florida and potentially reignite transmission across the United States.
Health officials would benefit from a better understanding of how people make decisions about vaccination, particularly parents for their children, Oveson said—especially considering recent national survey data showing that some 16% of parents are “not confident” that the MMR vaccine is safe for children, 17% believe that “the risks of the MMR vaccine outweigh its benefits,” and 18% fully oppose school-entry MMR vaccination mandates.
Gumel agrees, noting that targeted outreach is critical.
“Public health officials need to find ways to reach these parents if they are to prevent or contain future measles outbreaks, wherever they occur,” Gumel said. “And they will occur—the disease is out there, vaccination coverage is down, kids are going back to school and the data and science don’t lie.”
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The paper, “Projected effects of removing school-entry vaccination requirements on measles transmission in Florida,” will be published in the Proceedings of the National Academy of Sciences on September 28, 2026.
This work was supported by the U.S. National Science Foundation (Award No. DMS-2052363 transferred to DMS-2330801). This article does not necessarily reflect the views of this organization.
Proceedings of the National Academy of Sciences
Computational simulation/modeling
Not applicable
Projected effects of removing school-entry vaccination requirements on measles transmission in Florida
28-Sep-2026