Measles cases are most likely being underreported in the United States as public health officials scramble to find resources to address a ballooning outbreak in the Southwest, according to a senior scientist at the Centers for Disease Control and Prevention.

The Texas health department reported 561 cases of measles in the state on Tuesday, an increase of 20 from April 11, as the U.S. government said it was sending seven people to the state to help battle the outbreak of the childhood disease.

Vaccine critics have downplayed the role of measles in two Texas girls’ deaths, peddled unproven measles treatments and exaggerated the dangers of the vaccine.