The United States has recorded 3,471 confirmed measles cases in 2026 through Sept. 17, according to the latest Centers for Disease Control and Prevention data, as outbreaks continue in multiple states and Pennsylvania’s case count approaches 800.
The CDC’s latest published national figure represents a substantial increase from the 3,294 cases reported through Sept. 10. The agency said the September data are preliminary and can change as jurisdictions reconcile their reports.
Pennsylvania has emerged as the largest current focus of the outbreak. The state’s health department reported 792 confirmed cases as of Sept. 21, up 25 from the previous Friday, according to Reuters. The state has also reported 155 hospitalizations associated with the outbreak.
The outbreak has also produced a dispute over deaths. Pennsylvania health officials have reported four measles-associated deaths, while those deaths have not been included in the CDC’s national measles death count. The federal agency has been working with the Council of State and Territorial Epidemiologists on a standardized definition for measles-related deaths.
National case count continues to climb
The latest CDC surveillance shows measles transmission occurring across a broad portion of the country. Through Sept. 17, cases had been reported by 47 U.S. jurisdictions, including Pennsylvania, Texas, South Carolina, Utah, New York, California, Wisconsin and numerous other states.
CDC surveillance data for the week ending Sept. 12 also show that measles reporting remains active at the national level. The agency cautions that weekly disease surveillance figures are provisional and can change as states and territories update their records.
The scale of the 2026 outbreak is particularly notable because measles had been declared eliminated from the United States in 2000. Elimination did not mean the virus disappeared worldwide; it meant continuous endemic transmission had been interrupted in the country. Imported infections can still trigger outbreaks when the virus reaches communities with insufficient immunity.
CDC data show that annual U.S. measles cases after elimination generally remained low, although the country experienced a major resurgence in 2019, when more than 1,200 cases were reported.
Pennsylvania outbreak approaches 800 cases
Pennsylvania’s outbreak has continued expanding across the state.
The Pennsylvania Department of Health reported 792 cases in 39 counties as of Sept. 21, according to reporting based on the state’s measles dashboard. The increase included 99 additional cases compared with the Sept. 14 count.
Lancaster County has recorded the largest number of cases, while Mifflin County has also experienced a rapid increase. Mifflin County’s confirmed case count reached 93 by Sept. 21, according to local reporting citing the state health department’s dashboard.
The Pennsylvania Department of Health has been making case, hospitalization, and death information available through its measles response efforts while encouraging residents to review their vaccination status.
Pennsylvania also requested federal assistance from the CDC as the outbreak expanded. The state’s request became entangled with disagreement over the classification of the four reported deaths. Reuters reported that the state sought an emergency CDC response while asking the federal agency to recognize the fatalities reported by Pennsylvania.
Why the death count is disputed
The four Pennsylvania deaths have become an unusual point of disagreement in the national surveillance system.
Pennsylvania health officials have classified four deaths as measles-associated. The CDC has not included them in its national total while federal and state officials work through the criteria used to determine whether a death should be attributed to measles.
Reuters reported Sept. 18 that the CDC and the Council of State and Territorial Epidemiologists were working on a standardized case definition for measles-related deaths. Such a definition is intended to provide a consistent approach across states and territories.
The disagreement means that the number of deaths reported by Pennsylvania and the number appearing in federal surveillance data are not currently the same. The two figures should therefore not be treated as interchangeable.
Measles is highly contagious
Measles spreads through the air when an infected person coughs or sneezes. The virus can remain in the air in an enclosed space for as long as two hours after an infected person has left, according to the CDC.
The disease can cause complications including pneumonia and inflammation of the brain. Children younger than 5, adults older than 20, pregnant women, and people with weakened immune systems face a higher risk of serious complications.
The CDC says vaccination remains the primary protection against measles. Its current guidance says two doses of measles-containing vaccine are about 97% effective at preventing measles following exposure, while one dose is about 93% effective.
The standard U.S. vaccine is the measles, mumps, and rubella, or MMR, vaccine. The CDC recommends two doses for children.
Vaccination coverage has fallen
The national outbreak is occurring as measles vaccination coverage among U.S. kindergarteners has fallen below the level historically associated with community protection.
CDC data show MMR coverage among U.S. kindergarteners declined from 95.2% during the 2019–20 school year to 92.4% during 2025–26. The agency estimated that roughly 280,000 kindergarteners were at risk during the 2025–26 school year.
Because measles spreads so efficiently, outbreaks can grow when the virus reaches communities where vaccination coverage is lower. CDC guidance notes that local vaccination rates can vary considerably, even when statewide or national coverage appears relatively high.
The disease’s contagiousness also means that cases can appear far from the initial outbreak location. Health departments have continued issuing exposure notices when infected people travel through airports, hospitals, sporting venues or other public settings.
On Sept. 23, Missouri health officials warned about possible measles exposure at Busch Stadium and several businesses in the St. Louis area after an infected person visited those locations between Sept. 12 and Sept. 14.
Wisconsin health officials also announced a confirmed measles case in Chippewa County on Sept. 23. The state said there were no known public exposures associated with that case at the time of the announcement.
What happens next
The national case count will continue to change as state and local health departments submit additional reports and reconcile existing cases. CDC surveillance figures are explicitly considered preliminary.
Pennsylvania’s outbreak remains an important part of the national response, with the state seeking federal assistance while continuing its own investigation and vaccination efforts. The disagreement over the classification of the four deaths is also expected to remain relevant until federal and state officials establish a common reporting framework.
For people who believe they may have been exposed, the CDC advises contacting a health care provider before arriving at a medical facility so staff can take precautions against further transmission. People can review their vaccination records and discuss MMR vaccination with a health care provider if their immunity status is uncertain.
The latest confirmed national CDC figure is 3,471 cases through Sept. 17. Because the surveillance data are provisional and the outbreak remains active, later updates may change that total.
This story may be updated as new information becomes available.
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