The growing outbreak in pockets across the country has triggered multiple public health investigations seeking to limit exposure to measles, including quarantines at two California universities.
"They have to get their shots," Trump told reporters at the White House.
Nearly 700 cases have been confirmed by federal health officials so far this year in a resurgence that has so far been concentrated in a handful of states, including New York, Washington, Michigan, New Jersey and California.
On Thursday, county public health officials ordered dozens of people quarantined at the University of California, Los Angeles (UCLA) and California State University, Los Angeles (Cal State LA) following an April 11 exposure to the virus.