The U.S. declared measles eliminated 25 years ago. But the growing outbreak of the disease centered in West Texas poses a threat to this coveted status—and signals the possibility of measles becoming more common.

Among the 646 measles cases in West Texas, state health officials said 618 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.

West Texas measles outbreak reaches 663 cases, with most patients unvaccinated or unknown vaccination status, health officials report.