Pennsylvania Faces Largest Measles Outbreak in Three Decades as School Vaccination Rates Drop

Lebanon County Outbreak Signals Growing Crisis

As of May 6, 11 Lebanon County residents have been diagnosed with measles, bringing the total number of Pennsylvania residents with measles in 2026 to 23 , marking the state's most severe outbreak in decades. So far in 2026, Pennsylvania has seen 50 cases of measles - 38 linked to an ongoing outbreak in Lancaster and three nearby counties. The state saw 16 cases of measles in all of last year. The crisis began when three people with measles were hospitalized in the central Pennsylvania county, state Department of Health staff conducted contact tracing in the area and uncovered additional cases spreading through the community.

What makes this outbreak particularly alarming is that of the 23 total cases in PA, 22 were unvaccinated, and one had an unverifiable vaccination status . The highly contagious virus has now spread beyond Lebanon County, with Lancaster County had 21 cases, Dauphin County had two, and Berks County had one . Health officials have traced some exposures to public locations, including a Kohl's store in Lancaster on May 21, 23, 25, or 26 could have been exposed to measles. The company confirmed to LancasterOnline that an employee there had been infected.

Measles is highly contagious and can infect 9 in 10 unvaccinated people exposed to the virus. Virus particles can linger in the air for up to two hours after an infected person leaves an area. This extreme transmissibility explains why the outbreak has spread so rapidly across multiple counties in central Pennsylvania.

Vaccination Rates Fall Below Critical Threshold

The outbreak coincides with a troubling decline in childhood vaccination rates across Pennsylvania. Six years ago, 95.5% of kindergarten students across Pennsylvania completed the measles immunization. In the 2024-25 school year, the number slipped to 93.7%, below the 95% needed to claim herd immunity . This drop below the critical threshold means communities no longer have sufficient protection to prevent outbreaks from spreading.

The problem is particularly acute in the counties currently experiencing outbreaks. In Lancaster County last school year, just 88.5% of kindergartners were vaccinated against measles, and in Dauphin County, kindergarten vaccination rates were at 86.8% . Lebanon County's 2024-2025 kindergarten MMR vaccination rate was 93.2% . These rates fall well short of the 95% threshold needed for community protection.

Meanwhile, the exemptions in school districts have doubled in the past five years. In 2024-25, 6,097 non-medical exemptions were submitted on behalf of Pennsylvania kindergartners, compared to 2,993 in 2020-21. Pennsylvania is one of 15 states that permits parents to opt out of vaccinations for philosophical reasons, in addition to medical or religious reasons. Parents and legal guardians are not required to explain their rationale.

National Context and Growing Concerns

Pennsylvania's outbreak reflects a broader national crisis. In 2025, there were 2,288 cases of measles in the United States, according to the Centers for Disease Control and Prevention. That is more than any year in this century. The U.S. is on track to pass that mark this year with 1,814 cases already reported as of April 30. The resurgence threatens America's measles elimination status, which the U.S. received its measles elimination status in 2000 from the Pan American Health Organization. Now it is likely to lose that designation.

Vaccination coverage among U.S. kindergartners has decreased from 95.2% during the 2019–2020 school year to 92.5% in the 2024–2025 school year, leaving approximately 286,000 kindergartners at risk during the 2024–2025 school year. This nationwide decline has created pockets of vulnerability where outbreaks can take hold and spread rapidly.

Computer modeling demonstrates the potential consequences of continued vaccination declines. According to a simulated outbreak using the FRED model, should Allegheny County continue to fall to 93%, an outbreak in the region would be four times the size it would be if 95% of kids were vaccinated. Such projections underscore why public health officials are sounding alarms about the current trajectory.

Public Health Response and Path Forward

Pennsylvania health officials have mobilized extensive resources to contain the outbreak. DOH staff held a measles vaccination clinic in Lebanon yesterday where 26 individuals received vaccinations , and the state continues organizing additional clinics in affected areas. Contact tracing efforts have been crucial in identifying cases and preventing further spread.

The effectiveness of the measles vaccine remains unquestioned by medical experts.