Pennsylvania’s measles outbreak is now the largest in the U.S. in three decades. On Monday, the state surpassed South Carolina’s outbreak from earlier this year, which topped out at 997. That outbreak was contained within six months. Pennsylvania’s outbreak, which began in late April, shows no sign of slowing. “I would bet that this goes on for at least another three to six months,” said Dr. John Goldman, an infectious disease expert at the University of Pittsburgh Medical Center.

“We have seen an increase in vaccinations, but we aren’t seeing enough people get vaccinated to stop the epidemic.”On Monday, the Pennsylvania Department of Health reported 1,004 measles cases, with 198 hospitalizations. Nearly a third of the cases have been in children. Among the pediatric measles cases, 18% have needed hospitalization. Dehydration has become a common problem, particularly among babies and younger children, said Dr. Evan Shirey, a pediatric infectious disease physician at Penn State Health Golisano Children’s Hospital in Hershey.

Doctors are also seeing young patients with liver problems. “One of the more common things that the measles virus can do is cause hepatitis or infection of the liver,” Shirey said. “These children will feel very sick, very nauseous, with abdominal pain and vomiting, and they can’t take a bottle or can’t keep food or liquid down.”Other children are developing severe and irreversible neurological damage because of brain inflammation, or encephalitis, Shirey said. “In some of these cases, it’s a child who can’t recognize their own mother, who’s disoriented and confused,” Shirey said.

Still others have developed secondary bacterial infections. Five people in the state have died, according to the health department. Those people include a teenager with severe brain inflammation and a 40-year-old woman with a history of asthma, as well as two babies. One of those babies was a newborn who died after his mother went into labor a month early.

There are few details about the fifth person who died, other than the person hadn’t been vaccinated and lived in Lancaster County, where the bulk of measles cases are. Lancaster County is home to the nation’s largest Amish population, with approximately 44,000 residents as of 2025. There is nothing in the religion that is inherently anti-vaccine, but historically, their vaccination rates have been lower than those of the general population, Goldman said. According to an NBC News investigation, 88.5% of kindergartners in 2024 had the measles-mumps-rubella (MMR) vaccine.

A vaccination rate of at least 95% is the accepted level of herd immunity needed to protect against a measles outbreak. “There’s nothing special about Lancaster. The exact same thing could happen in any county in the country that has a large percentage of unvaccinated people,” Goldman said. “If it hasn’t happened yet, it’s only because measles hasn’t gotten into that county yet.”The outbreak in South Carolina, which ended in April, was previously the largest in approximately 30 years, according to the Centers for Disease Control and Prevention.

That outbreak was linked to a community of people who emigrated from Ukraine over the past few decades. State health officials credited a massive surge in vaccinations with bringing its outbreak to a halt. In the northwestern corner of the state, where the outbreak was centered, nearly 15,000 MMR vaccinations were given during the South Carolina outbreak, up 82% compared with the previous year, according to the South Carolina Department of Public Health. As of Monday, the Pennsylvania health department said it’s given just 8,843 MMR doses so far in response to the outbreak.

Ohio is also dealing with a growing measles outbreak along the border it shares with Pennsylvania. As of Thursday, Ohio had logged 210 measles cases, with 13 hospitalizations. Most cases are concentrated in the northeastern part of the state, in Geauga, Ashtabula and Wayne counties.