Pennsylvania measles outbreak tops 900 cases, spreading from Lancaster to Philadelphia suburbs

The state health department confirmed that measles cases in Pennsylvania have risen to 903, adding 114 new infections in the past week. The largest cluster remains in Lancaster County with 363 cases, but the virus has crossed into Chester County, which now reports 83 cases and is the only county in the Philadelphia area experiencing active spread. Overall, the outbreak spans 39 of the state's 67 counties, largely affecting unvaccinated individuals, and has produced four measles-related deaths among unvaccinated patients, including two infants.

Vaccination rates in Chester County stand at 94.5% for kindergarteners, just below the 95% threshold needed for herd immunity. In response, the county health department has stepped up communication, dispatched nurses to schools and tight-knit communities, and operated pop-up clinics that have delivered 538 MMR shots since the outbreak began in April. Officials stress that two doses of the vaccine provide 97% protection and aim to increase coverage to halt further transmission.