Measles cases hit 3,659 in 47 U.S. jurisdictions by Sept. 24. Pennsylvania had 890 cases and four deaths by Sept. 26—its first measles deaths in 35 years. Penn State's Matthew Ferrari says conditions for endemic spread exist, but measles is not yet circulating continuously.
Measles control requires population immunity of around 95%. Two-dose MMR coverage among U.S. kindergartners fell from 95.2% in 2019–20 to 92.4% in 2025–26. Susceptibility is uneven, and outbreaks can take hold where unvaccinated people cluster, even if rates remain high elsewhere.
A traveler with measles passed through Philadelphia International Airport on Sept. 13; the city health department issued an exposure warning six days later, with monitoring through Oct. 5. Symptoms can appear 7–14 days after exposure. The Association of State and Territorial Health Officials estimates an average outbreak response costs more than $750,000, excluding broader societal costs. One Michigan county had mostly used or allocated its $100,000 in emergency contact-tracing funds by April.
