A record year for cases
Pennsylvania has passed 1,000 measles cases in 2026, state health officials said, making this the largest outbreak in any US state in more than three decades. The department's Monday update counted 1,004 cases, including 198 hospitalisations and five deaths. Later tallies have pushed the total higher as results from older tests arrive.
The outbreak began in April in an area west of Philadelphia. It has since spread to 39 counties. Lancaster County has reported the most cases, followed by Mifflin and Chester counties. Officials say only a handful of cases have occurred in people who were fully vaccinated.
Why measles spreads so fast
Measles is one of the most contagious viruses known. It can stay in the air for up to two hours after an infected person leaves a room, and one case can lead to many more where vaccination rates are low. Symptoms include high fever, cough, red eyes and a rash. Severe cases can cause pneumonia, swelling of the brain and death.
Health officials say falling vaccination rates are behind the return of the virus. Nationwide, cases are nearing 3,900 this year, the highest count since 1991. The virus was declared eliminated in the United States in 2000, when coverage was high enough to stop it spreading.
What officials advise
State health officials say two doses of the measles, mumps and rubella vaccine prevent measles about 97 per cent of the time and give long-lasting protection. They are urging families in affected counties to check their records, and clinics have run extra vaccination sessions.
The outbreak has also strained testing. Officials say doctors have been slow to order tests in some cases, which delays counts and contact tracing. Schools and hospitals in the hardest-hit counties have tightened rules on exclusion after exposure. Officials add that the outbreak is not yet slowing, with new cases still reported each week.