On September 13, a traveler with measles passed through Philadelphia International Airport, moving between terminals for five hours. Six days later, the city health department issued an exposure warning.
The warning concerned a single traveler. The public health problem it illustrates is considerably broader.
“Epidemiologically, it’s a buzz thing,” Dr. Matthew Ferrari, director of the Center for Infectious Disease Dynamics at Penn State, told me. But operationally, it’s something else. Airport exposures generate large numbers of people who develop routine fevers after passing through a terminal – contacts who must be traced, tested and authorized, and who turn out, almost always, not to have measles. Each of these costs time for public health staff who are already choosing where they can afford to intervene. “For a response that is already under-resourced,” Ferrari said, “this is a blow they shouldn’t have to take.”
The Association of State and Territorial Health Officials estimates the average cost of responding to a measles outbreak at more than $750,000 – not including broader societal costs such as lost productivity and caregiver time. A Michigan county health department received $100,000 in emergency contact tracing funds this spring and had “already used and allocated the bulk of that money” by April.
Pennsylvania has confirmed 890 cases of measles as of September 26, with four deaths – the first measles deaths in the state in 35 years. Nationally, the United States has recorded 3,659 cases in 47 jurisdictions as of September 24. For most of the quarter century since measles was declared eliminated in 2000, outbreaks have been relatively small and quickly contained. The resurgence of 2025 and 2026 was a game-changer: Each major outbreak – in Texas, Utah, South Carolina and Pennsylvania – was larger and longer-lasting than anything in the post-elimination era.
“The fact that we’ve had several certainly implies that we have the conditions for endemic persistence,” Ferrari said — the conditions, not yet reality, in which measles continually circulates in the United States rather than arriving in imported chains that public health can track.
Measles is the most difficult case to control. Containing it requires about 95% immunity at the population level – higher than almost all other common diseases – and the outcome depends not only on the national average, but also on where the susceptible people are. Among U.S. kindergartners, two-dose MMR coverage increased from 95.2% in 2019-2020 to 92.4% in 2025-2026. This average is already below the threshold and hides another danger: susceptibility is not distributed equally. Where unvaccinated people congregate, outbreaks can take hold even if rates remain high elsewhere.
The question this raises is no longer simply whether agencies can contain individual outbreaks. It’s a question of whether they can hold enough. “It’s qualitatively different,” Ferrari said. For three decades, the U.S. response to the epidemic relied on the assumption that measles could be detected early and eradicated. This hypothesis has not been tested in this way since the elimination.
Who is protected and who is not
Two doses of MMR are about 97% effective; one dose provides 93% protection. People born before 1957 are generally assumed to be immune. Anyone who does not know their vaccination history or who received a single dose under the old pre-1989 schedule should check their status. Infants, pregnant people without immunity, and people with weakened immune systems face particular risks.
The exposure monitoring window at the Philadelphia airport extends until October 5. Symptoms – fever, runny nose, cough, red eyes, followed by a rash – appear seven to fourteen days after exposure. Call a doctor before going there in person.
The airport warning was about an infected traveler one evening. The larger question is what happens when such events stop being exceptional – when a system designed to interrupt isolated chains of transmission must begin to decide which investigations can wait.
“The worst thing about setting priorities,” Ferrari said, “is that not everyone can be first.”
This article was originally published on Forbes.com
Gn Health
