The U.S. Centers for Disease Control and Prevention has logged 2,318 measles cases so far in 2026, surpassing the 2,289 cases recorded for all of 2025. With five months remaining in the year at the time of the report, 2026 has become the nation’s worst year for measles since 1991. Major outbreaks in South Carolina, Utah and Arizona have contributed large numbers of cases, along with smaller clusters in additional states.
The United States originally eliminated endemic measles transmission in 2000 through sustained high vaccination coverage. That achievement is now threatened by declining uptake of the measles-mumps-rubella (MMR) vaccine and by resurgent chains of transmission.
Public health officials and disease researchers note that reported case counts are likely underestimates. Several factors contribute to incomplete totals: some infected people do not seek medical care or testing; contact tracing can be incomplete; and surveillance intensity varies across jurisdictions. The source emphasizes that undercounting is a longstanding issue for infectious disease surveillance and is not unique to measles.
Experts point out that measles is among the most contagious viruses known, so even small gaps in detection or vaccination can produce larger-than-expected outbreaks. In addition, hospitalization records, while potentially useful as a severity-based gauge, are not uniformly reported — measles hospitalizations are not required to be reported in every state — limiting their use for estimating true outbreak size.
Utah has reported more than 700 cases during an outbreak that has been ongoing for over a year, according to the state health department. Researchers using mathematical modeling and genetic analysis have suggested the Utah–Arizona border outbreak could be roughly three to four times larger than currently known counts, as stated by the Utah state epidemiologist.
South Carolina experienced what the source describes as the nation’s largest measles outbreak in 35 years, with 997 reported cases. The state health department acknowledged that this total is likely an undercount but said it cannot determine by how much. Arizona has also had significant transmission linked to the regional outbreaks.
Researchers are still analyzing the 2025 Texas outbreak. Early findings indicate the outbreak may have been substantially larger than the 762 known cases reported. Investigators at academic institutions are evaluating surveillance and modeling data to better estimate the true scale.
Texas state officials reported specific limitations in case confirmation. For example, in Gaines County — identified as an epicenter — the health department confirmed 414 cases but noted 182 probable cases among minors went unconfirmed in March 2025 because of insufficient information. Texas’s chief state epidemiologist observed that instances occurred where people tested positive but public health teams could not confirm household contacts or additional exposures.
The CDC’s operational definition of an outbreak — three or more linked cases — can lag behind actual transmission in communities with low vaccination coverage and limited surveillance. By the time linked cases are identified, earlier, undetected spread is often already underway.
State and local health departments and researchers described multiple surveillance challenges that hamper accurate counting. These include variability in testing and reporting behavior, limited contact follow-up, and gaps in demographic or exposure information. One public health official quoted in the source called the degree of undercounting “impossible to know” and said it may change during an outbreak.
Analysts noted that if a robust standard existed linking hospitalizations or another severity measure to a known fraction of all infections, it could serve as an alternate estimator of outbreak size. Absent that “solid gold standard,” mathematical modeling and genomic analyses are being used to triangulate likely true case counts.
National kindergarten MMR coverage declined to 92.5% in the 2024–25 school year, down from 95.2% in 2019–20, according to cited vaccination data. Public health experts point out that 95% coverage is the commonly cited threshold to achieve herd protection against measles; rates below that threshold leave communities vulnerable to sustained transmission.
Lower overall coverage obscures much lower rates in specific communities, where outbreaks can ignite and spread rapidly. Mathematical epidemiologists working on outbreak analyses emphasize that in settings with inadequate vaccination coverage, there is “just not much you can do about measles once you have it.”
Public health leaders and researchers stress that vaccination is primarily protective of the community as well as individuals. Retired public health researchers referenced in the source warned that the most at-risk people in outbreaks are those who cannot be vaccinated — very young infants, people with immune suppression, and those who fail to mount a robust immune response after vaccination.
A physician quoted in the report characterized not vaccinating as a decision that endangers others, calling attention to the concept of innocent bystanders who bear the brunt of outbreaks. State and national officials, and modeling teams, underscore that maintaining high MMR vaccination coverage is critical to preventing further spread and protecting those who cannot be vaccinated.
(Reported by the Associated Press and summarized in the source. Specific dates of the CDC update and the precise timing of counts were reported in the source; no additional timing details were provided beyond those statements.)