CHESTER COUNTY, Pa. (WHP) — Oxford Area High School in Chester County is closed Friday after a confirmed measles case within its community, according to a letter sent to families.

The district said students who are unvaccinated or who can’t prove vaccination can’t attend school or school functions.

“If you are not vaccinated for measles or can’t prove vaccination, you CAN NOT attend school or school functions and must be quarantined 21 days from school and all school functions,” Superintendent David A. Woods wrote in the letter.

Woods said the Chester County Health Department has directed the school to verify measles immunity documention for all staff members who work in the high school.

The closure comes after Pennsylvania eclipsed 1,000 confirmed measles cases this year, which is the most of any state in the U.S, according to the Centers for Disease Control and Prevention.

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As of Wednesday, there have been 86 reported cases in Chester County. Roughly 37% of cases have been reported in Lancaster County, which borders Chester County.