Adults, not children, account for most of the cases in the largest U.S. measles outbreak in decades. About 67% of Pennsylvania's 1,136 confirmed cases this year have been in adults, according to state data reported Oct. 9 by the University of Minnesota's Center for Infectious Disease Research and Policy, which noted the pattern runs counter to national trends.

Pennsylvania now accounts for more than a quarter of the 4,080 confirmed measles cases the Centers for Disease Control and Prevention counted nationwide as of Oct. 8, the highest national total since 1991. The state added 157 cases in the seven days before its Oct. 9 update.

The age split matters for households because many adults assume their measles protection was settled in childhood. Pennsylvania officials are urging adults with only one documented dose of the measles, mumps and rubella (MMR) vaccine, or no records at all, to talk with a health care provider or pharmacist.

Adults at the Center of the Outbreak

The outbreak has been severe — about 19% of Pennsylvania patients have been hospitalized, compared with roughly 10% nationally, and 99% of the state's cases are in people who were not vaccinated, CIDRAP reported.

The state has confirmed five measles-associated deaths. They include two infants who were too young to be vaccinated, an unvaccinated 18-year-old in Mifflin County, an unvaccinated 40-year-old woman in Jefferson County, and an unvaccinated Lancaster County resident whose age has not been released.

The CDC lists only two measles deaths nationwide this year. The agency says it is working with state epidemiologists on a standard definition for measles deaths. Pennsylvania uses its own definition, posted on the state measles page, which counts a death within 30 days of symptom onset in a lab-confirmed patient when no unrelated cause is found.

State Health Secretary Dr. Debra Bogen has stressed that age offers no protection on its own. "If you are not protected or immune, you are at risk of getting measles," she said in an August statement from the governor's office after the state's first two deaths.

The Department of Health has not explained in its public updates why adults make up such a large share of cases, and NWN found no official analysis of the age pattern.

Lancaster and Mifflin Counties Carry the Heaviest Load

Lancaster County has reported 415 cases and Mifflin County 167, according to CIDRAP's review of the Oct. 9 state data. In its latest statewide release, the department reported cases in 41 counties, 207 total hospitalizations, and four cases among vaccinated people.

That update also carried an important caveat. More than 40 of the 74 cases added that day involved people who had been sick more than two weeks earlier and were only recently reported. "Today's update does not reflect a current surge in new infections," the department said.

Demand for vaccine has climbed. Providers in the state typically give about 25,000 MMR doses a month, but they administered more than 49,000 in August and more than 56,000 in September. Department nurses have given nearly 9,000 doses this year, including more than 5,700 at 198 pop-up clinics since the outbreak began in late April.

The pressure extends beyond Pennsylvania. The CDC has counted 42 new outbreaks in 2026, and 96% of confirmed cases are outbreak-associated. New York declared a state disaster emergency over measles in early October, and Wisconsin, Ohio and Kentucky have each reported more than 100 cases, CIDRAP reported. Nationally, MMR coverage among kindergartners fell from 95.2% in the 2019-2020 school year to 92.4% in 2025-2026, below the 95% level the CDC cites for community protection.

Checking Your Own Measles Protection

The CDC's vaccination guidance recommends that adults have at least one documented MMR dose. Two doses are recommended for higher-risk adults, including college students, health care workers, international travelers, close contacts of people with weakened immune systems and people in groups at increased risk because of an outbreak.

People born before 1957 are generally presumed immune because most were infected as children. Adults who received a measles vaccine between 1963 and 1967 that was either the inactivated type or of unknown type should get at least one dose of the live vaccine, the CDC says, because the inactivated version was not effective. Pennsylvania officials say people with two documented doses do not need another.

Some people should not get MMR or should wait, including those who are or may be pregnant and people with weakened immune systems. They should discuss options with a provider. After an exposure, MMR given within 72 hours may offer some protection, and high-risk people may be offered immune globulin within six days.

Most private insurance plans cover MMR. In Pennsylvania, children 18 and under who are on Medicaid, uninsured, or underinsured can get the vaccine at no cost at the state's 59 health centers.

Symptoms usually appear seven to 14 days after exposure but can take up to 21 days. They begin with high fever that may top 104 degrees Fahrenheit (40 degrees Celsius), cough, runny nose, and red, watery eyes, followed by a rash that starts on the head and spreads downward. People are typically contagious from four days before to four days after the rash appears. Anyone with symptoms after a possible exposure should call a provider before arriving so staff can prevent spread in waiting rooms, or call the state hotline at 877-PA-HEALTH (877-724-3258).

Gaps the Data Cannot Yet Fill

Several important questions remain unanswered — including why adults dominate Pennsylvania's case count and whether late-reported cases are masking a slowdown. Federal and state death counts also differ, and the CDC counts only confirmed cases on a weekly schedule, so its state totals will lag Pennsylvania's own dashboard.

Next Updates From Harrisburg and Atlanta

The CDC updates its national count weekly, and the Pennsylvania Department of Health has been posting statewide updates several times a week, with county, hospitalization and death data on its dashboard. NWN will update this story as new figures are released.

The bottom line: Pennsylvania's outbreak is unusually concentrated among adults and nearly all cases involve unvaccinated people. Adults unsure of their vaccination history should check their records and talk with a provider, and anyone with symptoms should call ahead before seeking care.

What Readers Want to Know

How many measles cases has Pennsylvania reported?

Pennsylvania had 1,136 confirmed measles cases as of its Oct. 9, 2026, update, with about 67% in adults, according to state data reported by CIDRAP.

Which counties are most affected?

Lancaster County has reported 415 cases and Mifflin County 167. The state's Oct. 7 update listed cases in 41 counties.

Do adults need a measles booster?

The CDC recommends at least one documented MMR dose for adults and two doses for higher-risk groups, including college students, health care workers and international travelers. People with two documented doses do not need another, Pennsylvania officials say.

I was vaccinated in the 1960s. Am I protected?

People who received the inactivated vaccine or an unknown type between 1963 and 1967 should get at least one dose of the live vaccine, according to the CDC. People born before 1957 are generally presumed immune.

What are the early warning signs of measles?

High fever, cough, runny nose, and red, watery eyes usually come first, followed by a rash that starts on the head and spreads downward. Symptoms can appear seven to 21 days after exposure.

What should I do if I think I was exposed?

Call your health care provider before visiting so staff can protect others, or call Pennsylvania's hotline at 877-724-3258. MMR given within 72 hours of exposure may offer some protection.