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Pennsylvania’s ongoing measles outbreak has become the largest and deadliest in the United States in more than 30 years. Since the outbreak began in April, the state has reported about 1,300 cases, five deaths associated with measles and a sixth death under review.

Historical data suggest that 1 to 3 people die for every 1,000 unvaccinated people who contract measles. Pennsylvania’s reported toll puts its death rate at least 1.5 times above average and possibly as much as 5 times higher.

The virus itself is not believed to have changed, but the outbreak’s size, probable undercount and the vulnerabilities of the affected community may be shaping its severity. “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 believe the outbreak is larger than official figures indicate, though its true scale remains uncertain. A scientist from the US Centers for Disease Control and Prevention’s Epi-Aid program arrived in Pennsylvania this week to investigate the outbreak’s size and trajectory while assessing local vaccination efforts.

Estimating the spread is difficult because measles data in the United States are outdated and scarce. The country eliminated measles decades ago, leaving researchers without the recent, consistent data they would typically use to track disease patterns.

Dr. Remy Pasco, a research associate at the University of Colorado Boulder, has worked on an analysis estimating the size of last year’s Texas measles outbreak. Some cases are expected to go unreported, he said, but underreporting is unlikely to occur at the same rate across different outbreaks.

Pennsylvania’s outbreak also has several unusual features, including hospitalization rates that are higher than expected. The Amish and broader Plain community, named for its simple style of dress, also interacts with the health care system in a distinctive way.

Public health experts generally view severe outcomes, including hospitalizations and deaths, as the strongest benchmarks for estimating an outbreak’s reach. Not everyone infected will seek testing or medical care, but severe cases are more visible and can help establish a ratio for estimating total infections.

Another striking signal is the share of cases involving adults. About two thirds of Pennsylvania cases have occurred among adults, and all but a few cases involved unvaccinated people, while more than 60% of US cases during the past two years involved children or teenagers.

Ferrari said the large number of unvaccinated adults, particularly among Amish people and other Plain community members, has created an especially dangerous situation. Complications can be more severe among unvaccinated adults than among unvaccinated children.

Low protection among adults may also leave infants without protective maternal antibodies. Women who have neither received vaccination nor experienced measles infection cannot pass those antibodies to their babies, leaving some infants vulnerable before they are old enough for vaccination.

Two Pennsylvania infants infected with measles have died this year, and both were too young to receive the measles mumps rubella vaccine. “Those are absolutely deaths that wouldn’t have been occurring elsewhere in the world,” Ferrari said.

The Pennsylvania Department of Health issued an advisory Monday addressing measles during pregnancy and congenital measles. The department said cases during pregnancy were increasing and congenital measles had been identified for the first time in decades, while knowledge and guidance continued to develop.

Questions surrounding the outbreak have also become politically contentious. Pennsylvania has officially reported five deaths associated with measles, but the CDC dashboard lists only three and says its figure may change as the agency develops a standardized definition for deaths due to measles.

In September, Pennsylvania Department of Health Secretary Dr. Debra Bogen warned CDC Director Dr. Erica Schwartz that the state could rescind its Epi-Aid request if the agency did not communicate transparently and accurately about the deaths. Last week, Bogen accepted the assistance while again asking the CDC to acknowledge all five deaths and update its public dashboard.

Determining the outbreak’s true size will require what Ferrari called “classic boots on the ground epidemiology,” with epidemiologists working throughout the community to examine recent health histories and vaccination coverage. He said standard methods cannot be trusted in populations that have not experienced sustained measles transmission in a long time.