Four fully vaccinated Pennsylvania residents — three adults and one child — have contracted measles this year, the first two-dose MMR breakthrough cases identified in the state outbreak; none were hospitalized.
Lancaster County logged 49 new cases in seven days, pushing Pennsylvania's total to 676, including 295 in Lancaster, as health officials said rising community transmission makes rare breakthrough infections more likely.
The MMR vaccine is 97% effective after two doses, and doctors said vaccinated people who do get measles usually have milder illness and face lower risk of severe complications.
Two Lancaster County infants with measles died in August, and the latest surge may put Pennsylvania ahead of South Carolina's 671 cases, potentially making it the nation's largest outbreak.