Pennsylvania has logged 1,000 measles cases, among the biggest outbreaks in decades

Pennsylvania surpassed 1,000 measles cases Monday, crossing a grim milestone to rank among the largest U.S. measles outbreak in decades.

Pennsylvania reported 106 new measles cases in the past week, totaling 1,004 this year. That includes 992 in the current outbreak that began in April, with Pennsylvania poised to soon surpass an outbreak in South Carolina that sickened 997 this year and last.

Texas also saw a major outbreak in recent years, logging 990 cases in a 2025.

Pennsylvania has reported five measles-associated deaths, the most in the U.S. in more than three decades, and just under 200 hospitalizations this year.

» READ MORE: Chester County has more measles cases than most entire states in the country

The statewide outbreak is now on track to become the largest the country has seen since the 1990s, when the virus whipped through cities across the U.S. Philadelphia alone logged some 1,400 measles cases in the 1990-1991 outbreak.

Yet health leaders believe Pennsylvania’s tally is an undercount, with more cases going unreported, as people are often reluctant to speak with the health department.

Chester County, the only county in the Philadelphia region where the virus is spreading, reported 84 cases as of Monday, one more than last week. Local health officials say they are struggling to identify people exposed to contain the virus actively circulating in the western county.

“People don’t answer the phone. People hang up. People are absolutely not going to stay home for 21 days,” Dale Weiser, the clinical services supervisor for Chester County’s health department, said last week in an interview with The Inquirer about the county’s measles response. “It’s very hard to stop it, especially without real cooperation from the community.”

» READ MORE: A 20-year-old Amish woman with leukemia got measles and died in Pa.’s largest outbreak in three decades

Data released by the Pennsylvania Department of Health suggests that the outbreak has affected an unusually high number of adults, with a higher rate of hospitalizations and deaths than seen in other recent outbreaks.

About two-thirds of measles cases in Pennsylvania have been among adults, and about 20% of cases required hospitalization, according to state data.

In 2025, the CDC recorded 2,289 measles cases across the country, with a hospitalization rate of 11%. The vast majority of cases involved children, who are considered at greater risk because they are less likely to be vaccinated. Adults 20 years and older accounted for a little under a third of cases nationally that year.

An undercount of cases may partially explain these anomalies, said Rémy Pasco, a research associate at the University of Colorado who helped develop a measles outbreak simulator based on school vaccination rates.

While the virus is considered especially dangerous among infants, whose immune systems are still developing and are too young to be vaccinated, older children who contract measles may experience more mild symptoms. Parents may be less likely to seek medical help — and less likely to report the illness — among children with mild symptoms, he said.

Lancaster County has the highest case count in Pennsylvania, with 391 cases as of Monday. The county is home to one of the country’s largest Amish and Mennonite communities, where families have a long tradition of limiting their use of traditional healthcare and often forgo vaccination.

But since April, the virus has quickly spread across communities where significant numbers of residents are undervaccinated. Cases have been reported in 39 of Pennsylvania’s 67 counties.

Measles is so contagious that nine out of 10 unvaccinated people who come into contact with the virus will get sick.

» READ MORE: New vaccination data show hundreds of Philly-area schools vulnerable to a measles outbreak

Chester County and the entire Philadelphia region are pockmarked with vulnerable communities where vaccination rates have dropped below the 95% “herd immunity” threshold scientists say is necessary to keep the virus from spreading.

That’s how, despite an overall vaccination rate of 94.5%, Chester County has become a hot spot for the spreading outbreak.

Measles was declared eradicated in the U.S. in 2000 due to widespread vaccination. The measles vaccine, which is mandatory for school attendance, is 97% effective with two doses.

But families have increasingly been opting out of school-mandated immunization, amid rising politicization of vaccines and widespread misinformation.

Pennsylvania has among the country’s most permissive exemption policies, allowing families to opt out of vaccination for medical, philosophical, or religious reasons, often by simply printing and signing a form.