A fourth person in Pennsylvania has died of measles, the state health department confirmed Tuesday. The latest death was in an unvaccinated 18-year-old resident of Mifflin County. The state also confirmed the death of an unvaccinated 40-year-old woman in Jefferson County.

The state had previously reported two deaths in Lancaster County; both were infants—a newborn boy and a 6-week-old girl, both too young to be vaccinated. According to the Mifflin County coroner, Andrea L. Alcalde, the 18-year-old's cause of death was determined to be acute disseminated encephalomyelitis (ADEM), a rare but known severe complication of measles.

So far, the state has reported 693 measles cases in 38 counties this year. There have been 99 cases reported in the last seven days. In all, 133 people have been hospitalized.

"My thoughts are with the two families who lost loved ones to this highly contagious virus, as Pennsylvania faces the worst measles outbreak in more than three decades," Pennsylvania Secretary of Health Debra Bogen said in a statement.