A sixth measles-related death was reported Monday in Pennsylvania, involving a 39-year-old woman in Franklin County, amid the state’s largest outbreak in three decades. In a statement, the county’s coroner office said the woman had underlying medical conditions, though it did not provide specifics. A spokesperson for the Franklin County coroner’s office said the “primary cause of death was a probable acute cardiac event due to possible complications of the measles.”

Cardiac complications from measles can happen, but are uncommon. The woman had not been vaccinated, the spokesperson said. The state’s health department considers a death to be “measles-associated” if it occurred within 30 days of symptom onset and there is clinical evidence of a measles infection and a positive laboratory test. It also takes into account the person’s medical records and whether they were exposed to measles before they died. As of Friday, Pennsylvania had confirmed 1,136 measles cases, with 219 hospitalizations.

Two-thirds of those cases were diagnosed in adults. The Pennsylvania health department said in an email Monday that it was aware of the Franklin County coroner’s announcement, and “is conducting a thorough review of the reported death.”The death announced Monday is the sixth in less than two months in the state, suggesting the outbreak is far greater than reported. Historically, measles infections cause 1 to 2 deaths per 1,000 cases.

“I think we’re probably closer to 6,000 cases,” said Dr. Paul Offit, a pediatrician and director of the Vaccine Education Center at Children’s Hospital of Philadelphia. “They are vastly underestimating the number of cases. It’s much worse.”The first two deaths announced in the state in late August were in infants, both in Lancaster County, which has accounted for the vast majority of cases in the state’s outbreak. One father in Lancaster County 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. The other was a 6-week-old who had a rare genetic condition. Last month, a 40-year-old woman with asthma and severe lung disease died in Jefferson County, as well as an 18-year-old Mifflin County resident who died “due to complications of measles,” specifically a rare and severe neurological condition that causes inflammation in the brain. The Philadelphia Inquirer reported in early October that the fifth death occurred in a 20-year-old woman with a compromised immune system because of leukemia.

The vast majority of people diagnosed with measles are unvaccinated. According to the Pennsylvania Department of Health, less than 1% of cases there occurred in people who were fully vaccinated. The first dose of the measles, mumps and rubella (MMR) vaccine is typically given at around 1 year of age, and then around age 5. Two doses offer 97% protection against measles, usually for life. During outbreaks, babies as young as 6 months old are eligible for a dose ahead of those two recommended shots.

Nationally, there have been 4,080 measles cases this year, according to the Centers for Disease Control and Prevention. It’s the highest number of cases since 1991.