The decision lands hardest in Pennsylvania, which has recorded more than 1,500 measles infections so far this year, with nearly all cases occurring in unvaccinated adults and children, according to state data. State officials have reported seven measles-related deaths as of Nov. 13, including two infants. Cases have surfaced in 46 of the state's 67 counties.

Many infections trace back to an outbreak that began in Lancaster County in April. Public health teams have responded by offering measles, mumps and rubella shots at people's homes, at community pop-up clinics and inside recreational vehicles, while doctors, nurses, community health workers, epidemiologists and virologists have worked to test symptomatic residents, trace contacts, treat hospitalized patients and investigate deaths.

"I think measles is endemic in this country," said Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia. "I think it has gained a foothold. I think we're going to be seeing measles again, not only this year, but next year and the year after and until we get on top of this with vaccination."

The measles vaccine first became available in the United States in 1963 and was later refined and combined with mumps and rubella vaccines into today's MMR shot. By 2000, domestic cases had plummeted, and the few reported each year were mostly tied to international travel or isolated incidents. Offit said that success carried an unintended cost.

"We largely eliminated the memory of measles," he said. "I think people don't remember how sick or dead that virus can make you." He noted that many adults in their 20s, 30s and 40s grew up when the virus was not circulating, were never naturally infected and, for various reasons, were not vaccinated, leaving a sizable susceptible population.

Immunization rates among school-aged children in Pennsylvania and nationwide have declined steadily in recent years, driven by growing mistrust of medical providers and mainstream vaccination guidance, misconceptions on social media and false claims about vaccine dangers. In Pennsylvania, pockets of unvaccinated residents in Amish and Mennonite communities that do not use traditional health care systems are also bearing the brunt of the outbreak, officials say.

About one in five measles patients in the state has been hospitalized, according to state data. Lisa McHugh, state epidemiologist at the Pennsylvania Department of Health, said adults tend to delay care until they are severely ill. Providers are also seeing more infections among pregnant women, linked to congenital measles passed from mother to baby; an October state health advisory warned that babies born with measles infections could face complications including delayed neurological conditions.

McHugh said transmission appears to be slowing as more people get vaccinated and others develop immunity after infection. "You run out of individuals who are susceptible to the infection," she said. "That's the natural course of what we would see from an epidemiologic perspective, is we would see a decreasing trend in the number of cases. Thankfully, we are starting to see that here in Pennsylvania. We're hoping that that continues."

She cautioned, however, that the response has strained a public health system underfunded for years. "The investments that we have made in this measles response will ultimately help us in the future," McHugh said. "But that is part of the challenge, that public health has been underfunded for many, many years. And we need to maintain this strong public health workforce to be able to continue to contain measles as we move forward."