The measles outbreak first reported in Gaines County, Texas, has doubled to 48 cases in the last four days, the Texas Department of State Health Services said in a statement Friday.

The measles outbreak in the South Plains region, in West Texas, has expanded to at least 48 people, most of them children.

Nine of those infected were hospitalized and more cases are likely, health authorities said. U.S. vaccination rates for measles are falling, according to the CDC.