Chester County's measles outbreak has reached 83 confirmed cases, more than tripling since early August. The county is the only place in the Philadelphia region where the virus is actively spreading.

The count, reported by the Pennsylvania Department of Health as of Monday, Sept. 28, puts Chester County third statewide behind Lancaster County at 363 cases and Mifflin County at 106, The Philadelphia Inquirer reported. Pennsylvania's total reached 903 that day, with 114 new cases added in the prior week.

Two days later, on Wednesday, Sept. 30, state health officials confirmed a fifth measles-related death statewide. The victim was an unvaccinated Lancaster County resident. The statewide total climbed to 943 cases.

Chester County Health Director Jeanne Franklin told the Inquirer the department is pushing outreach into affected communities.

"We are working all the time with organizations, businesses, community leaders — whoever we can get the attention of in the communities impacted," Franklin said.

Chester County's cases have been concentrated near the Lancaster County border, where the state's largest cluster is centered. Physicians and local health officials believe the official count understates actual infections because some cases go undiagnosed or unreported, the Inquirer reported.

The county's kindergarten vaccination rate sits at about 94.5%, just below the roughly 95% threshold needed to prevent sustained community spread. That countywide average can mask lower rates within individual schools or communities.

In a Sept. 16 interview with 6ABC, Franklin described a community-by-community approach: "If they think they want the vaccine, then we're focusing on the vaccine … If nobody has talked to these folks, then, we're going to start at the beginning."

Statewide, nearly one in five people with confirmed measles have been hospitalized. About a third of cases have involved children under 18. Nearly all infections have occurred in unvaccinated people. The state reported just four breakthrough cases among vaccinated individuals in early September.

The Chester County Health Department has administered 538 measles, mumps and rubella (MMR) vaccinations in 2026, most since the outbreak began in April. The department has also sent nurses into schools and operated temporary vaccination clinics.

Two doses of the MMR vaccine provide about 97% protection against measles. The virus is highly contagious: about nine in 10 unvaccinated people exposed to it become infected.

The Chester County Health Department's West Chester clinic offers vaccine appointments on Tuesdays from 8:30 a.m. to 3:30 p.m. and Wednesdays from 10:30 a.m. to 5:30 p.m. Walk-ins are not available. Residents can call 610-344-6225 to schedule. Anyone experiencing symptoms after a possible exposure should contact a health care provider or call the state health hotline at 877-724-3258.