Pennsylvania’s Measles Outbreak Becomes Largest in US in Decades

Pennsylvania has recorded more than 1,000 measles cases this year, making its outbreak the largest in a US state in more than three decades as health officials intensify efforts to contain the highly contagious disease.

The Pennsylvania Department of Health reported 1,004 confirmed measles cases across 39 counties as of Monday, October 5. The outbreak has resulted in 198 hospitalizations and five measles-associated deaths, according to state data.

The outbreak began in April in the area west of Philadelphia and has since spread across the state. Lancaster County has recorded the largest number of infections, with more than 390 cases, while several other counties have also reported community transmission.

Pennsylvania’s outbreak has now surpassed South Carolina’s 997 cases, which had been the largest US state outbreak during the recent resurgence. The current Pennsylvania outbreak is the first state-level outbreak to exceed 1,000 cases in more than 30 years.

The wider US measles resurgence has also reached levels not seen in decades. The Centers for Disease Control and Prevention reported 3,887 confirmed measles cases nationwide as of October 1, with 95% of cases associated with outbreaks. The agency said 2026 is shaping up to be the worst year for US measles cases since 1991.

Health officials have linked the resurgence to declining vaccination coverage and pockets of communities with low immunisation rates. The CDC says 92.4% of US kindergarteners were vaccinated against measles, mumps and rubella during the 2025-26 school year, below the roughly 95% coverage generally needed to prevent sustained community transmission.

In Pennsylvania, the state health department said only four of the 1,004 confirmed cases had occurred in vaccinated people, meaning more than 99% of cases were among people who were not fully vaccinated.

The state has responded by expanding vaccination efforts in communities affected by the outbreak. Pennsylvania health officials said more than 53,000 MMR vaccine doses were administered across the state during September, more than twice the number given during a typical month. State health centres and pop-up clinics have also provided thousands of additional doses.

The five deaths reported by Pennsylvania include two infants who were too young to receive routine MMR vaccination and unvaccinated adults. The state reported its fifth measles-associated death on September 30.

Measles is extremely contagious and spreads through the air when an infected person breathes, coughs or sneezes. The virus can remain in the air for up to two hours after an infected person leaves an area, according to the CDC.

Two doses of the MMR vaccine provide about 97% protection against measles, according to the CDC. Health authorities are urging people to check their vaccination status, particularly in communities where cases are being reported.

The resurgence has raised broader concerns about whether the United States can maintain the measles elimination status it achieved in 2000. International health officials are expected to review the country’s measles status in November.