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.

The Centers for Disease Control and Prevention confirmed nearly 900 measles cases in the U.S. on Friday, though much of the measles cases in the country remain centered in Texas, which confirmed earlier this week there have been 624 cases of the highly contagious illness in the state this year.