The Texas health department reported 663 cases of measles in the state on Tuesday, an increase of 17 cases since April 25, as the U.S. battles one of its worst outbreaks of the childhood disease.

Nearly all measles infections seen during the Texas outbreak are among individuals who have not been vaccinated, or whose vaccination status is unknown.

Of those infected across the U.S., 97% of cases came from those who are unvaccinated or whose vaccination status is unknown.