WEST CHESTER, PA — Chester County’s measles outbreak has grown to 83 cases, more than tripling since early August and making the county the Philadelphia region’s only active center of transmission, according to new reporting by The Philadelphia Inquirer.
The outbreak is now the third largest in Pennsylvania, behind Lancaster County with 363 cases and Mifflin County with 106 as of Sept. 28. Pennsylvania’s statewide total reached 903 cases after 114 infections were added during the preceding week.
Chester County cases have been concentrated primarily near the Lancaster County border, where the state’s largest cluster is occurring. Physicians and local health officials believe the official count understates the extent of infections because some cases are never diagnosed or reported.
The rapid increase has prompted the Chester County Health Department to expand vaccination and outreach efforts, including sending nurses into schools and affected communities, working with businesses and community organizations, and operating temporary vaccination clinics.
The county has administered 538 measles, mumps, and rubella vaccinations this year, most since the broader outbreak began in April.
About 94.5% of Chester County kindergarten students are vaccinated against measles, slightly below the roughly 95% threshold considered necessary to prevent sustained community transmission.
That countywide rate can mask lower vaccination levels within individual schools or communities. Measles is highly contagious, with about nine in 10 unvaccinated people who are exposed becoming infected.
Two doses of the MMR vaccine provide about 97% protection against measles. Nearly all Pennsylvania cases this year have involved unvaccinated people, though the state reported four breakthrough infections among vaccinated individuals earlier in September.
The Chester County outbreak has expanded sharply as Pennsylvania confronts its largest measles outbreak in roughly three decades. At least one infection has been reported in 39 of the state’s 67 counties.
Pennsylvania has recorded four measles-associated deaths involving people who were unvaccinated and had confirmed infections. The deaths involved two infants from Amish families in Lancaster County, an 18-year-old in Mifflin County who experienced a serious neurological complication, and a 40-year-old Jefferson County woman with severe respiratory conditions.
Common measles symptoms include fever, cough, runny nose, watery eyes and a red, blotchy rash. Most patients recover, but the disease can cause serious complications and, in rare cases, death.
Read the full Philadelphia Inquirer report at https://www.inquirer.com/health/pennsylvania-measles-outbreak-900-cases-20260928.html
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