Measles vaccination rates in most Pennsylvania counties lag below herd immunity
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More than half of Pennsylvania's counties, including Philly, don't meet the herd immunity threshold for measles, per new data collected and analyzed by the Washington Post.
Why it matters: Falling vaccination rates across Pennsylvania may allow measles to spread rapidly in densely populated cities like Philly, which could leave them vulnerable to outbreaks, health experts say.
Driving the news: Pennsylvania recorded 16 measles cases in 2025, including three in Philadelphia, per the state Department of Health.
- The cases mark a rise over the past three years after the state reported no measles infections from 2020 through 2022.
State of play: 38 of Pennsylvania's 67 counties don't meet the herd immunity threshold, data shows.
- They include Philadelphia (94.7%) and Delaware (94.3%) counties, both of which have just below 95% of their students vaccinated — the rate public health authorities say is necessary to contain the virus's spread.
- Philly's remaining collar counties — Bucks, Montgomery and Chester — are all above the threshold.
The big picture: Vaccination rates for school-age children have plunged in hundreds of counties in recent years. Confusion following the changing vaccination schedules could set the stage for a rise in the prevalence of certain diseases.
- The new vaccination figures offer stark evidence of the extent of the backlash against public health mandates that began during the pandemic.
- Plunging vaccination rates contributed to a spike in measles cases and set the U.S. up to lose its coveted measles elimination status for the first time in decades.
The latest: This week, the Trump administration overhauled the federal childhood vaccination schedule by cutting the number of recommended shots to 11, down from 17.
Flashback: In 2024, a measles outbreak in Philly infected nine mostly unvaccinated individuals, seven of whom were hospitalized.
Zoom out: Only 815 of the more than 3,000 counties nationwide have reached the herd immunity threshold, the data show.
- At least 5.2 million kindergarten-age children reside in counties where vaccination rates are below the herd immunity level, according to the Post's data.