ATLANTA (AP) 鈥� The number of measles cases in the U.S. has 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 . Now, 2026 has easily become the U.S.鈥檚 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.
In November, international health officials will meet to determine if the U.S. and Mexico have . The U.S. eliminated local measles spread in 2000 by maintaining high vaccination rates, which have fallen in recent years to needed to prevent outbreaks.
Measles cases are undercounted
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鈥檛 seek out treatment 鈥� something not unique to measles.
Furthermore, experts say the spread might be greater than what鈥檚 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 鈥� among kindergartners in the 2024-25 school year, from 95.2% in 2019-20 鈥� obscure even lower rates in certain communities.
In Utah, where public health officials have fought measles , the virus has sickened more than 700 people during that time, according to the . 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鈥檚 currently known, state epidemiologist Dr. Leisha Nolen said.
The picture is muddier in South Carolina, which had . There, the 997 total is likely an undercount, the state health department has said, but it doesn鈥檛 know by how much.
Dr. Brannon Traxler, acting director of the state department of health, said the undercount is 鈥渋mpossible to know and may vary during the course of an outbreak.鈥�
Data gaps can make it hard to reliably estimate an outbreak鈥檚 true scope. Experts say hospitalization rates can be a good barometer, but measles hospitalizations are to be reported in every state.
鈥淚f we really had some kind of solid gold standard for the percentage of all cases that have hospitalization or some measure of severity, then that would be another way of estimating the outbreak size that we could see if it鈥檚 consistent with these other methods,鈥� said Damon Toth, an applied mathematician at the University of Utah鈥檚 medical school who is working on one of the Utah outbreak analyses.
Researchers are still analyzing the 2025 Texas outbreak
In Texas, researchers are still working to estimate the size of the state鈥檚 2025 outbreak and early findings show it might have been nearly twice the size of the 762 known cases.
R茅my Pasco, a mathematical epidemiologist at the University of Colorado Boulder who is working on the analysis, said the undercounts were unsurprising. In the absence of an adequate community vaccination rate 鈥� 95% according to health experts 鈥� 鈥渢here鈥檚 just not much you can do about measles once you have it,鈥� he said.
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, 鈥渆ither you missed the previous ones or you鈥檙e going to have a lot more,鈥� he added.
Dr. Varun Shetty, Texas鈥� chief state epidemiologist, said there were instances where people tested positive for measles but health officials couldn鈥檛 confirm who else was in the person鈥檚 household or if they were sick.
鈥淲hat we鈥檙e able to count and report is almost always an underestimate of what actually occurred, because it relies on people being willing to be able to get tested, to share information with healthcare providers and with public health,鈥� Shetty said. 鈥淪ometimes that just doesn鈥檛 happen.鈥�
The state health department previously reported that went unconfirmed in March 2025 among minors in Gaines County 鈥� the epicenter of the outbreak 鈥� due to a lack of information. The confirmed case count for the western Texas county for the whole outbreak is 414.
Shetty expects to keep seeing measles cases and even outbreaks because so many people are unprotected.
鈥淚t鈥檚 really critical, not only for individual people, but for communities, to have high vaccination coverage,鈥� he said.
Vaccination is about protecting others, physician says
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 鈥渋nnocent bystanders.鈥�
Research and by his team shows those who cannot get vaccinated 鈥� young babies, people with weak immune systems and those who rarely don鈥檛 get robust immunity from the vaccine 鈥� 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.
鈥淣ot vaccinating your kid is like saying, 鈥業 don鈥檛 want my car to have brakes,鈥欌�� he said. 鈥淏ecause it鈥檚 other people that you can hit. It鈥檚 not just you in your car. It鈥檚 other people.鈥�
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