Public health authorities said Tuesday that an outbreak of measles in western Texas has expanded, while a new case was confirmed nearby across state lines in New Mexico.

Two dozen cases of measles have been reported in a small West Texas county that has one of the state’s highest vaccine exemption rates. In Gaines County, which has a population of about 22,000, 24 cases of the airborne infectious disease have been reported since late January. Most of them are unvaccinated, school-aged children.

It's the latest outbreak of a disease that had been virtually eliminated in the U.S., and it comes as vaccination rates are declining — jeopardizing the country's herd immunity from widespread outbreaks.