An ongoing measles outbreak in Pennsylvania has rapidly escalated into the largest and deadliest in the United States in more than 30 years. However, the crisis has revealed an even more alarming dimension: the resurgence of congenital measles, a disease that was once extremely rare in the country.

As of Monday, the Pennsylvania Department of Health confirmed that 66 pregnant women had been diagnosed with measles since the outbreak began in April. Furthermore, the state has reported five measles-associated deaths – with a sixth under review – out of approximately 1,300 total cases.

Nationally, measles cases have surged to 4,410 this year. Pennsylvania alone accounts for 1,286 cases as of Monday, making it the epicenter of the current crisis.

Death Rate Significantly Higher Than Historical Average

The mortality figures coming out of Pennsylvania are deeply concerning to public health officials. Historically, data suggests that 1 to 3 people will die out of every 1,000 unvaccinated people who contract measles. Consequently, the Pennsylvania outbreak puts the measles death rate at least 1.5 times higher than average – and possibly up to 5 times higher.

It is not likely that the measles virus itself has changed. Instead, several factors may be contributing to these elevated mortality rates. One possibility is the size of the outbreak and a probable undercount of cases, making it difficult to know how many people have truly been affected. Additionally, the unique vulnerabilities of the Pennsylvania community where measles is raging may play a significant role.

“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.

Congenital Measles Returns After Decades

The Pennsylvania Department of Health issued an alert Monday to doctors about an increase in measles infections in pregnant women, as well as a concerning rise in congenital measles. Moreover, a department spokesperson emphasized the serious risks involved.

“Getting measles while pregnant increases the risk of miscarriage, early labor, and low birth weight,” a department spokesperson said in an email.

According to the American Academy of Pediatrics, congenital measles occurs in newborns whose mothers have been infected during the two weeks before birth and who test positive or develop measles symptoms within 10 days of delivery. Anecdotally, doctors have said that at least six babies were born in Pennsylvania with congenital measles.

Even before widespread measles vaccination, congenital measles was rare. That is because most women had already been infected as children. It is only now emerging as a threat as measles outbreaks increase while vaccination rates fall.

Severe Outcomes and ICU Admissions

Babies born with congenital measles can have a range of symptoms, from a rash and fever to pneumonia, neurological problems, and low blood platelets. At least two babies developed pneumonia and were admitted to an intensive care unit, doctors said.

Tragically, congenital measles can be deadly. One father in Lancaster County previously told NBC News that his wife was sick with measles in August and went into labor a month early. Their baby died shortly after birth. Of the six deaths reported in the state, at least two have been in infants, but it is not known if they had congenital infections.

It is unclear how many women have miscarried or delivered babies with congenital measles. The department said that state law prohibits it from releasing details of the outcomes of those pregnancies. In addition, there is no national database that tracks cases. True numbers may also be obscured as the outbreak has largely been limited to Amish families who don’t always seek conventional medical assistance.

Limited Data Hampers Response Efforts

Experts say the outbreak is even larger than official data suggests, and it is 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.

“We don’t have denominator data on congenital measles,” said Adam J. Ratner, MD, MPH, a member of the American Academy of Pediatrics Committee on Infectious Diseases, told CIDRAP News.

Ratner explained that most of what is known about the disease comes from individual case studies and series. Because of this, it is impossible to extrapolate how many pregnant women with measles will have an infant with congenital measles.

Challenges in Tracking and Estimation

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. Nevertheless, measles data in the US is quite “outdated,” according to researchers.

“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,” said Dr. Remy Pasco, a research associate at the University of Colorado Boulder.

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 is 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 noted.

Tight Case Definition Creates Additional Complications

Furthermore, the case definition for congenital measles is tight: Only measles infections that occur in the last two weeks of pregnancy or the first 10 days post-delivery are considered congenital. However, Ratner cited a 2021 case series in France that described asymptomatic infants who tested positive for measles more than 20 weeks after maternal infection.

Another case series conducted during the large Canadian measles outbreak of 2025 described 51 pregnant women who contracted the virus, resulting in nine cases of congenital measles. One of the nine case-patients died.

“There is definitely underreporting of prenatal measles, because mild cases of measles in pregnancy may not be picked up,” Ratner said.

Compounding the denominator problem is reporting. Last month, the American Academy of Pediatrics added a frequently asked questions page about congenital measles to its website in response to ongoing and increasing measles outbreaks. Releasing information on infant outcomes in those pregnancies is illegal in Pennsylvania. According to NBC News, there is also no national database on congenital measles, likely because the disease hasn’t been routinely seen in the United States for decades.

In its alert, the Pennsylvania health department urges women to receive vaccinations before pregnancy. As the outbreak continues to unfold, public health officials remain focused on expanding vaccination efforts and better understanding the true scope of this unprecedented crisis.

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