Why the Measles Outbreak in Pennsylvania Poses Risks Beyond US Borders

The United States is facing its largest outbreak of measles since the virus was declared eliminated in 2000. Decades after that declaration, measles has made a resurgence, with the US Centers for Disease Control and Prevention (CDC) announcing the third death of the year from the virus.

Matt Ferrari, director of Pennsylvania State University's Center for Infectious Disease Dynamics, said the US was so successful at wiping out measles that it had 'the luxury to forget' it was a serious problem. 'It's been a long time, and for many of us in the United States, and in fact, throughout the Western Hemisphere, we have been able to forget how nasty measles is,' Ferrari said.

Pennsylvania has been hit especially hard. In September, the state sought emergency support from the CDC to control the spread, even as state and federal health officials quarrelled over the measles-associated death tally — differing over how deaths should be classified. State officials have recorded more than 1,200 cases and over 240 hospitalisations in 42 counties so far this year.

This spike has raised concerns in neighbouring states. On October 5, New York Governor Kathy Hochul declared a state disaster emergency in response to rising cases in rural areas, expanding the number of health professionals authorised to vaccinate and test people for measles. 'No one should get seriously ill or die from a vaccine-preventable disease,' Hochul said.

But experts warn the risks extend far beyond US state borders. The resurgence comes as measles cases increase globally, particularly in communities that are vaccine-sceptical or where there are gaps in healthcare access.

Vaccination coverage among US kindergartners has decreased from 95.2 percent during the 2019-2020 school year to 92.4 percent in the 2025-2026 school year. Experts warn that declining vaccination rates in one location can leave people thousands of miles away vulnerable, making measles prevention a shared global responsibility.

Pennsylvania reported in August that two unvaccinated people died of measles in Lancaster County, in the south-central part of the state. Lancaster is considered the centre of Pennsylvania Dutch Country and home to the nation's oldest Amish and Mennonite communities, both of which have under-vaccinated populations.

In Pennsylvania, public health officials have worked with community leaders to share information and expand access to vaccination. Joshua Good, principal of Ephrata Mennonite School in Lancaster County, where two students contracted measles earlier this year, worked with state officials to organise a pop-up vaccination clinic at the school. Officials also provided communication materials that the school adapted for its community and helped identify at-risk families.

Good, who is vaccinated and vaccinated his children, sees himself as an advocate within the community. He said that if families have questions about measles shots, he will answer them as best he can, engage in respectful, charitable conversations, try to understand their hesitancies — while also respecting their concerns.

Although multiple community members received the measles vaccine at the Ephrata clinic, vaccination rates remain below the threshold for herd immunity. Public health officials advocate investing in robust community outreach before outbreaks happen.

'You can't just show up in a community in an emergency — an emergency in which they might feel like you're there to kind of blame them or shame them,' said Jennifer Nuzzo, director of the Pandemic Center at the Brown University School of Public Health. 'You need to know them so that they see public health as a resource rather than law enforcement.'

Nuzzo said low vaccination coverage and distrust of the government allow measles to spread even when vaccines are available, and that framing the issue around demographics lets governments off the hook for their job of stopping the spread of this dangerous disease.

Experts explain that declining vaccination rates put not only these communities but also immunocompromised people at greater risk. Herd immunity in the wider population formerly helped shield those most vulnerable — in Ferrari's words, higher vaccination rates in surrounding communities offered a barrier.

The continental Americas was the first World Health Organization region to eliminate endemic measles transmission in 2016. Maintaining that status requires high and consistent coverage. The Pan American Health Organization (PAHO) says coverage across the region remains below the target for both doses of the measles vaccine.

PAHO holds an annual conference where an independent technical body reviews whether countries have eliminated measles, rubella and congenital rubella syndrome. A country can lose its 'elimination status' when the same measles virus genotype has circulated continuously for at least 12 months. Last year, Canada became the first country in the continental Americas to lose its status after an outbreak that began in 2024. Some have warned the US may be on track to lose its status this year.

Dr Daniel Salas, executive manager of PAHO's Special Program for Comprehensive Immunisation, said it is not just the US that will be under review: Mexico and Bolivia will also be closely assessed because they could potentially lose status. US Health and Human Services Secretary Robert F Kennedy Jr has repeatedly defended the US response, saying at an August 26 event in Florida, 'We're doing a better job than any country in the world at controlling the measles outbreak.'

But Nuzzo said the repeated outbreaks across the US point to a country struggling to contain transmission. 'We have not seen it this bad,' she said. 'It's like this perpetual game of Whac-A-Mole. As long as there are vulnerable communities, as long as we don't have strategies for early detection and rapid response and containment, measles will just keep on happening over and over and over again.'

Dr Salas said action within the first three days after a suspected case is critical, and health authorities should begin investigating and taking response measures without waiting for laboratory confirmation. Delays of a week or more can allow the highly contagious virus, marked by fever, coughing and a whole-body rash, to spread further.

Scientific journals have found evidence that the virus strain in Canada's latest outbreak was genetically linked to cases reported in US Mennonite communities — the same outbreak that resulted in Canada losing its elimination status. 'The spread of measles in the United States imperils other countries,' said Nuzzo. 'We're all dependent on each other, and it's very hard to protect ourselves when measles is spreading elsewhere.'