Most of the measles cases are still among school-aged children in Gaines County, Texas health officials said Friday.

In the last three weeks, 48 cases of measles have been identified, sending 13 patients to the hospital, according to the Texas Department of State Health Services.

The ongoing measles outbreak in West Texas has doubled in size to 48 cases, mostly in children and teens, making it the state's worst in nearly 30 years. State health officials said Friday in a news release that those who are infected are either unvaccinated or their vaccination status is unknown. Thirteen people have been hospitalized.