One of the two people who recently died of measles in Pennsylvania was an infant, the state Health Department said Wednesday.

The department announced Tuesday that two unvaccinated residents had died of measles, the first measles-related deaths in the U.S. this year and the first in Pennsylvania in 35 years.

On Wednesday, the department said one of those deaths was under investigation by the Lancaster County Coroner’s Office, which had said it was reviewing the death of an infant with a positive measles test.

The Health Department did not specify the ages of either person who died.

Both were residents of Lancaster County, where the share of kids who have gotten the MMR vaccine fell from 95.5% in 2017 to 87.6% last year.

The Health Department has confirmed 424 measles cases this year, 94 of them diagnosed in the past week. So far, 73 people have been hospitalized for measles complications.

The first dose of the measles, mumps and rubella (MMR) vaccine is recommended at 12 to 15 months and the second at 4 to 6 years.

“The real vulnerability is in infants between 6 and 11 months of age, as maternal antibody is waning and they haven’t gotten their first dose of the MMR vaccine,” said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Dallas.

In places where infants have a high risk of measles exposure, health authorities sometimes recommend an early dose of the vaccine for babies starting at 6 months old. That was the case during an outbreak in Texas last year, when two unvaccinated girls died of the disease.

Nationwide, 2,777 measles cases have been confirmed this year, the most in 35 years.

Measles has been considered eliminated in the U.S. since 2000 because it has not continuously spread over a yearlong period. The Pan American Health Organization is expected to decide in November whether the U.S. has lost its elimination status.