Utah has emerged as a major center of measles infections in the US, as an outbreak that has been building for some time continues to expand.

State officials reported a total of 602 measles cases on Wednesday tied to an outbreak that started last year and is still ongoing, including 19 newly identified infections, according to the Center for Infectious Disease Research and Policy (Cidrap). Recent exposures have been reported at several preschools and elementary schools.

About one-third of those infected have experienced symptoms severe enough to require visits to emergency rooms, reported the New York Times, largely due to intense dehydration from vomiting and diarrhea.

Out of the 602 total cases, 405 have occurred since the beginning of this year, with 75 reported in just the last three weeks. So far, 49 individuals have needed hospitalization. A significant majority of those infected – 513 people, or 85% – were not vaccinated against measles.