An ongoing measles outbreak in Pennsylvania has quickly become the largest and deadliest in the United States in more than 30 years.

Historical data suggests that 1 to 3 people will die out of every 1,000 unvaccinated people who contract measles. But Pennsylvania has reported five measles-associated deaths — with a sixth under review — out of about 1,300 total reported cases since the start of the outbreak in April.

This puts the measles death rate in Pennsylvania at least 1.5 times higher than average — and possibly up to 5 times higher.

It’s not likely that the measles virus has changed. One factor might be the size of the outbreak and a probable undercount of cases: It’s difficult to know how many people have been affected. It may also be the unique vulnerabilities of the Pennsylvania community where measles is raging.

“There clearly is a shift — not in the biology but a shift in the epidemiology that we’re seeing here in Pennsylvania,” said Dr. Matthew Ferrari, director of the Center for Infectious Disease Dynamics at Penn State University.

Experts say the outbreak is even larger than official data suggests, and it’s not clear how big it is. The US successfully eliminated measles decades ago, making it difficult to estimate how bad the current resurgence has gotten.

A “disease detective” from the US Centers for Disease Control and Prevention arrived in Pennsylvania this week to help better understand the true scale of the outbreak and its trajectory, the state health department said. The scientist, with the CDC’s Epi-Aid program, is also tasked with evaluating the impact of local vaccination efforts.

Scientists and public health workers typically estimate current disease trends by using patterns from recent years, scaling changes against a few variables like birth rates and vaccination rates.

But measles data in the US is quite “outdated,” said Dr. Remy Pasco, a research associate at the University of Colorado Boulder. “It’s not endemic, so we don’t have as much data on it. And because the data has been so scarce, anything we can estimate is with a big grain of salt.”

Pasco has worked on an analysis to estimate the scale of last year’s measles outbreak in Texas. Some underreporting of cases is expected, he says, but it’s unlikely that the rate of underreporting would be the same — even when comparing recent outbreaks in different parts of the US.

There are some “peculiarities” with the outbreak in Pennsylvania, he said. Hospitalization rates with this outbreak are higher than expected. And the Amish and broader Plain community – so named for their simple way of dressing – has a unique way of interacting with the health care system.

Severe outcomes, such as hospitalizations and deaths, are generally the best benchmark for understanding how widespread a measles outbreak is, public health experts say. Not every person who is infected with measles will seek care or testing, but severe cases are more visible in the health care system, and tracking them can help set the ratio to understand the full scale.

A uniquely susceptible group

One signal that this outbreak is different: A disproportionate share of cases are among adults.

About two-thirds of measles cases in Pennsylvania have been among adults, and all but a few were unvaccinated. Nationwide, however, cases skew younger; CDC data shows that more than 60% of cases in the US over the past two years have been among children or teens.

The large population of unvaccinated adults in Pennsylvania, particularly among Amish and other members of the Plain community, has created a uniquely risky outbreak situation, Ferrari said.

Measles complications tend to be more severe for unvaccinated adults than for unvaccinated children, he said. But the low rates of protection among adults in the community also mean vulnerable infants are more likely to be born without protective maternal antibodies.

“We normally think about measles as being a childhood infection,” Ferrari said. “Most people have been exposed to measles or have been vaccinated against measles by the time they reach reproductive age.”

That’s true around the world. Countries typically have high vaccination rates or endemic measles spread, which means people are exposed when they’re younger.

Much of the affected population in Pennsylvania falls into a uniquely susceptible group: both unvaccinated and going decades without exposure because the US had so few cases for so long.