LOS ANGELES >> More than 900 students and staff members at two Los Angeles universities were quarantined on campus or sent home this week in one of the most sweeping efforts yet by public health authorities to contain the spread of measles in the U.S., where cases have reached a 25-year high.

By this afternoon, two days after Los Angeles County ordered the precautions, about 200 of those affected had been cleared to return after proving their immunity to the disease, through either medical records or tests, school officials said.

The action at the University of California, Los Angeles, and California State University, Los Angeles — which together have more than 65,000 students — reflected the seriousness with which public health officials are taking the nation’s outbreak.

Those under the quarantine were instructed to stay at home and avoid contact with others, though it wasn’t clear how those orders might be enforced or what penalties violators might face.

“Measles actually kills people. So we have to take that really seriously,” said Dr. Armand Dorian, chief medical officer at USC Verdugo Hills Hospital.

The number of measles cases in the U.S. has climbed to nearly 700 this year, including five in Los Angeles County and 38 altogether in California. The surge is blamed largely on parents not getting their children vaccinated because of misinformation about the supposed dangers.

Cal State-LA reported 656 students and staff still under quarantine, while UCLA said it had fewer than 50.

Call State-LA is primarily a commuter school, while many UCLA students live on campus. Some UCLA students were provided a quarantine area to stay in, university officials said, though they gave no details. Only one person remained there today.

Those covered by the quarantine were singled out based on their possible exposure to either an infected UCLA student who had attended classes in two buildings on three days earlier this month, or a person with measles who visited a Cal State-LA library on April 11, officials said.

Given the amount of time a person can remain contagious, officials said the quarantine would end at UCLA on Tuesday and at Cal State-LA on Thursday.

Around the country, lawmakers in California, New York, Washington state and Oregon have responded to the outbreak by moving to crack down on exemptions to vaccinating children. Today, President Donald Trump urged everyone to get vaccinated.

Most of the cases are centered in two ultra-Orthodox Jewish communities in New York — one in Brooklyn, the other in suburban Rockland County.

In Rockland County, officials declared a state of emergency and at one point tried to bar unvaccinated children from schools and other public places, but a judge overturned the order.

Authorities ordered mandatory vaccinations earlier their month in the affected Brooklyn neighborhoods and threatened fines of $1,000. City officials said earlier this week that 12 people had been issued summonses.

Measles usually causes fever and an all-over rash but in a small number of cases can lead to deadly complications such as pneumonia and swelling of the brain.

The Centers for Disease Control and Prevention recommends the vaccine for everyone over a year old, except for people who had the disease as children. Those who have had measles are immune.