The United States has already recorded more measles cases in 2026 than it did during all of 2025, setting a new three-decade high with nearly five months still remaining in the year and raising fresh concerns among public health officials about declining vaccination rates and expanding outbreaks.

According to updated figures released Friday by the U.S. Centers for Disease Control and Prevention, 2,318 measles infections had been confirmed nationwide as of Thursday.

That total has already surpassed the 2,289 cases reported during all of 2025, which had previously been the country’s worst measles year in more than 30 years. With large outbreaks continuing in South Carolina, Utah, and Arizona, along with smaller clusters in several other states, 2026 has now become the nation’s worst measles year since 1991.

International health authorities are expected to meet in November to determine whether the United States and Mexico should retain their measles-free status. The U.S. achieved elimination of endemic measles transmission in 2000 through consistently high vaccination rates, but immunization levels have since fallen below the 95% threshold experts say is needed to prevent sustained outbreaks.

Health officials also caution that the true number of infections is likely much higher than official statistics indicate. Public health researchers have long acknowledged that reported case counts underestimate the spread because many infected individuals never seek medical care or receive a confirmed diagnosis.

Experts say the gap between reported and actual infections may be especially significant because measles is among the most contagious viruses known, while vaccination rates have continued to decline nationwide. Although national coverage for the measles, mumps, and rubella (MMR) vaccine among kindergarten students stood at 92.5% during the 2024-25 school year, down from 95.2% in 2019-20, some communities have substantially lower vaccination rates.

Utah offers one example of the challenge. Public health officials there have battled measles for more than a year, with more than 700 confirmed cases reported during that period. State epidemiologist Dr. Leisha Nolen said mathematical models and genetic analyses suggest the outbreak along the Utah-Arizona border could ultimately prove to be three to four times larger than current official figures indicate.

Officials in South Carolina have reached similar conclusions. Although the state has confirmed 997 measles cases during what has become its largest outbreak in 35 years, health authorities believe the true number is likely higher, though they cannot determine by how much.

Dr. Brannon Traxler, acting director of South Carolina’s Department of Health, said the size of the undercount remains uncertain.

“The undercount is ‘impossible to know and may vary during the course of an outbreak.’”

Researchers say incomplete reporting makes it difficult to accurately measure the full scope of an outbreak. While hospitalization rates can provide useful clues, measles-related hospitalizations are not required to be reported uniformly across all states.

“If 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’s consistent with these other methods,” said Damon Toth, an applied mathematician at the University of Utah’s medical school who is working on one of the Utah outbreak analyses.

Scientists are also reexamining last year’s outbreak in Texas. Early analyses suggest the epidemic may have infected nearly twice as many people as the 762 officially confirmed cases.

Rémy Pasco, a mathematical epidemiologist at the University of Colorado Boulder who is helping conduct that research, said such discrepancies are not unexpected. He noted that once vaccination rates fall below the recommended 95% level, containing measles becomes extremely difficult.

In discussing how outbreaks are identified, Pasco noted that the CDC defines an outbreak as three or more connected cases, but warned that by the time those cases are recognized in communities with low vaccination rates and limited surveillance, transmission is often already widespread.

“Either you missed the previous ones or you’re going to have a lot more,” he added.

Texas Chief State Epidemiologist Dr. Varun Shetty said investigators have encountered situations in which infected individuals tested positive for measles but officials were unable to determine who else in the household had been exposed or whether additional family members had also become ill.

“What we’re 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,” Shetty said. “Sometimes that just doesn’t happen.”

Earlier reports from the Texas Department of State Health Services indicated that 182 probable measles infections among minors in Gaines County—the center of the 2025 outbreak—could not be officially confirmed because investigators lacked sufficient information. During the entire outbreak, the county recorded 414 confirmed cases.

Shetty said he expects measles outbreaks to continue as long as large numbers of people remain unvaccinated.

“It’s really critical, not only for individual people, but for communities, to have high vaccination coverage,” he said.

Dr. Mark Roberts, the retired director of the Public Health Dynamics Lab at the University of Pittsburgh School of Public Health, expressed particular concern for individuals who cannot receive the vaccine.

Research conducted by Roberts and his colleagues indicates that infants, immunocompromised individuals, and the small percentage of vaccinated people who do not develop strong immunity often face the greatest risk during outbreaks.

Roberts compared vaccine refusal to removing a car’s most important safety feature, emphasizing that the consequences extend beyond the individual making the decision.

“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.”