On Jul. 24, 2026, the number of measles cases in the U.S. surpassed the record-breaking tally for all of 2025, with five more months of 2026 still to come. The U.S. Centers for Disease Control and Prevention logged 2,318 cases of measles as of Thursday, according to an agency update on Friday.

The nation had 2,289 cases in 2025, the worst year for measles in more than three decades. Now, 2026 has easily become the U.S.’s worst year for measles since 1991, as major outbreaks in South Carolina, Utah and Arizona have sickened hundreds and smaller clusters of cases sparked in other states.

The number of confirmed measles infections in the United States has passed the number for all of 2025 and is largest outbreak since the disease was declared eradicated. In November, international health officials will meet to determine if the U.S. and Mexico have lost their measles-free status. The U.S. eliminated local measles spread in 2000 by maintaining high vaccination rates, which have fallen in recent years to below the 95% coverage needed to prevent outbreaks.

While measles cases in the U.S. are climbing, it may be impossible to say exactly how far-reaching the spread is. Public health departments and disease researchers have long acknowledged that official counts are incomplete, because some people get sick and don’t seek out treatment — something not unique to measles.

Furthermore, experts say the spread might be greater than what’s known because the virus is one of the most contagious known to medicine and vaccination rates nationwide have declined. National vaccination rates for measles, mumps and rubella — which dropped to 92.5% among kindergartners in the 2024-25 school year, from 95.2% in 2019-20 — obscure even lower rates in certain communities.

In Utah, where public health officials have fought measles for more than a year, the virus has sickened more than 700 people during that time, according to the state health department. Experts relying on mathematical modeling and genetic analyses indicate that a large outbreak on the Utah-Arizona border could be around three to four times bigger than what’s currently known, state epidemiologist Dr. Leisha Nolen said.

The picture is muddier in South Carolina, which had the nation’s largest outbreak in 35 years. There, the 997 total is likely an undercount, the state health department has said, but it doesn’t know by how much. In Texas, researchers are still working to estimate the size of the state’s 2025 outbreak and early findings show it might have been nearly twice the size of the 762 known cases.

The CDC defines an outbreak as three or more linked cases. But by the time three such cases are identified in communities with low vaccination rates and less disease surveillance, “either you missed the previous ones or you’re going to have a lot more,” he added The state health department previously reported that 182 probable measles cases went unconfirmed in March 2025 among minors in Gaines County — the epicenter of the outbreak — due to a lack of information. The confirmed case count for the western Texas county for the whole outbreak is 414.

Dr. Mark Roberts, retired director of the Public Health Dynamics Lab at the University of Pittsburgh School of Public Health, worries most for the people he calls “innocent bystanders.” Research and measles modeling by his team shows those who cannot get vaccinated — young babies, people with weak immune systems and those who rarely don’t get robust immunity from the vaccine — bear the brunt of outbreaks early on. Some people see skipping vaccination as like deciding not to wear a seat belt in a car, Roberts said. “Not vaccinating your kid is like saying, ‘I don’t want my car to have brakes,’” he said. “Because it’s other people that you can hit. It’s not just you in your car. It’s other people.”