The United States is at a tipping point for the return of endemic measles a quarter century after the disease was declared eradicated in the country, researchers warned on Thursday.

At current U.S. childhood vaccination rates, measles could return to spreading regularly at high levels, with an estimated 851,300 cases over the next 25 years, computer models used by the researchers suggest.

If rates of vaccination with the measles-mumps-rubella, or MMR, shot were to decline by 10%, an estimated 11.1 million cases of measles would result over 25 years, according to a report of the study in JAMA.

Measles has not been endemic, or continuously present, in the United States since 2000.

With vaccination rates dropping for MMR shots as well as for other childhood vaccines, outbreaks of preventable infectious diseases are increasing. There have been 10 reported outbreaks and at least 800 measles cases in the U.S. so far in 2025, including 624 cases and two deaths in one Texas outbreak, according to the Centers for Disease Control and Prevention.

Many state and national policies are being debated that may substantially reduce childhood vaccination even further, said study leader Nathan Lo of Stanford Medical School.

The decline in vaccination among U.S. children in recent years has been fueled by promotion of theories - contrary to scientific evidence - that childhood vaccines are a cause of autism and other health risks.

Robert F. Kennedy, Jr., who now heads the U.S. Department of Health and Human Services, has for decades helped sow such doubts, which accelerated during the COVID-19 pandemic with politicizing of vaccines for that virus.

Drawing on state vaccination, birth and death data and historical data on measles infections, researchers simulated a population that mirrors the U.S. population at national and state levels. Then they estimated how measles would spread under various scenarios if imported from a traveling U.S. citizen who gets infected abroad.

If routine childhood vaccinations declined by 50%, the country would see 51.2 million measles cases, 9.9 million rubella cases, and 4.3 million poliomyelitis cases over the next 25 years, Lo said.

Under this scenario, Lo said, there would be 51,200 patients with lasting neurologic side effects of measles, 10,700 birth defects resulting from congenital rubella infections, 5,400 cases of paralysis from polio, 10.3 million hospitalizations, and 159,200 deaths.

Small increases in vaccination rates of around 5% could keep measles from becoming endemic, the researchers’ models suggest.

Under current levels of vaccination, vaccine-preventable diseases other than measles are unlikely to become endemic, Lo said. But if vaccination rates drop by 35%, rubella will likely become endemic, while polio, which has long been eradicated in the U.S., has a 50-50 chance of making a comeback if vaccination drops by 40%.

No one can forecast exact vaccination and infection numbers, but the precise numbers don’t matter, said Dr. Mujeeb Basit, associate director of the Clinical Informatics Center at UT Southwestern Medical Center, who was not involved in the research.

What matters, Basit said, is the trend revealed by the study: as the vaccination rate declines, the rate of increase in measles cases speeds up.

"If vaccination rates go 5% lower, you'll have tens of thousands of infected patients," he said.

"Rates just have to be 15% less and you're at millions of cases," he added. "The trend is what people need to know.”