Pennsylvania tallied more than two dozen new cases of measles inthe last week, including several in the Philadelphia region.
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In Chester County, health officials confirmed 13 cases in the suburban Philadelphia county since April, with seven identified since Monday.
The rising case counts come as the Centers for Disease Control and Prevention reported Friday that measles cases nationwide had surpassed all cases reported in 2025, the highest case count in 35 years.
One of the most contagious diseases, measles has made a resurgence as rising numbers of people are refusing the highly effective vaccine that prevents its spread.
In a growing Pennsylvania outbreak centered in Lancaster County, state health officials confirmed 14 new cases in the last week. The county has been at the center of two separate outbreaks this year and has reported 75 cases since April, when the current outbreak began.
So far this year, Pennsylvania has reported 139 cases of measles, more than eight times the cases reported in 2025 and the highest case count in three decades. Twenty-five people have been hospitalized.
Delaware on Wednesday declared a measles outbreak, with four cases reported. New Jersey has seen one case so far in 2026.
Working to track cases
In Chester County, the local health department struggled to track cases in part because some exposed residents have refused to provide contact tracers with detailed information, said Jeanne Franklin, the county health director.
Some residents have not answered calls from the department, or only spoken to health workers about their own family, not other contacts they might have had in the community. Others have simply hung up the phone, Franklin said.
“It does not allow us to paint the true picture. If they’re not talking to us, we’re assuming there’s a lot of cases unreported,” she said.
Public health workers ask residents who are reluctant to reveal information about their contacts to, at least, inform friends and relatives that they may have been exposed to measles, Franklin said.
She said Chester County residents who have tested positive for measles have mostly sought care in Lancaster County, which borders Chester County to the west.
But most measles patients are seeking treatment only when they become seriously ill, she said, and milder cases are likely going unreported.
“People aren’t going to freely get tested. They’re getting tested when their health has declined so much that they need medical assistance,” Franklin said.
Unvaccinated people in Plain communities, Christian denominations like the Amish and Mennonites, have been affected more than others in the county, but the outbreak is not contained to these communities, Franklin said.
Some members of Plain communities oppose vaccination, but opinions vary, and doctors in Lancaster, where Plain communities are heavily concentrated, have stressed cases there are centered among unvaccinated people in general, not Plain residents.
No major religion expressly forbids vaccines. Yet the spread of misinformation about vaccine safety, rising for years, is increasingly amplified in federal policy discussions under Health and Human Services Secretary Robert F. Kennedy Jr., a longtime anti-vaccine activist.
Every measles case in the state this year was diagnosed in people who were not fully vaccinated against the highly contagious disease, which can infect up to 90% of unvaccinated people exposed to it.
The state Department of Health has administered nearly 2,000 measles vaccines in the last year.
Franklin said Chester County health workers are among those also stepping up vaccination efforts. Besides offering vaccinations at the county’s health clinics and working with community groups to host pop-up vaccine clinics, staffers are traveling to residents’ homes to vaccinate them, she said.
The department is seeing renewed interest in measles vaccination as the outbreak continues, Franklin added.
“If you decided or your parents decided a while ago not to get vaccinated for measles, that might have been okay then because the risk was low. In today’s situation, you should reassess your decision,” she said.
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