Among the 561 measles cases in West Texas, state health officials said 550 involved individuals who were either unvaccinated or had an unknown vaccination status. The CDC recommends two doses of measles, mumps and rubella vaccine for children 6 and younger and one or two doses for those who are older and don’t have evidence of immunity.

Texas health data released Friday shows an outbreak of measles in the West Texas region has surged to 597 cases.

The Centers for Disease Control and Prevention reported 800 confirmed cases of measles in the U.S. as of Friday, making the announcement as the highly contagious illness has reached half of the country—though most cases remain focused in Texas.