An 18-year-old in Mifflin County, Pennsylvania has died of acute disseminated encephalomyelitis, a rare and severe neurological complication of measles. CIDRAP has the news.

There was also a death in Jefferson County from measles. A 40-year-old woman died on September 12. Not many details have been shared by the county coroners, but Pennsylvania’s Department of Health said both of the patients were unvaccinated. These are the third and fourth measles-associated deaths reported in Pennsylvania this year.

Pennsylvania’s official case count for measles is 693, which is the highest in the U.S. this year. Seventeen new cases have been added to the total in the past week.

Infectious disease physicians caution that the true number of measles infections in Pennsylvania is likely far higher than what has been reported, based on anecdotal reports that many people who are sick don’t get tested. One to three of every 1,000 children who become infected with measles will die from respiratory and neurologic complications.

The disease spikes during the winter and spring historically. The large number of unvaccinated children returning to school this year are a concern, and experts fear there will be a large spike in illness and death from the disease soon. The U.S. will almost certainly lose its measles elimination status later this year for the first time since the disease was declared eliminated in 2000.