President Donald Trump has urged Americans to get vaccinations against measles, as the number of new cases has climbed to its highest level in 25 years.

Key points: New measles cases have reached their highest level in 25 years, attributed to misinformation

New measles cases have reached their highest level in 25 years, attributed to misinformation Donald Trump told Americans to vaccinate, having previously linked vaccinations to autism

Donald Trump told Americans to vaccinate, having previously linked vaccinations to autism Almost 1,000 university students in Los Angeles have been quarantined

In 2019, about 700 cases have been reported, and on Friday, Mr Trump said that vaccinations "are so important".

Measles usually causes fever, runny nose 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.

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The US Centres for Disease Control and Prevention recommends the vaccine for everyone over a year old, except for people who had the disease as children.

The surge is blamed largely on parents not getting their children vaccinated because of misinformation about the supposed dangers.

Mr Trump previously cast doubt on the effectiveness of vaccinations in 2012 and again in 2014 suggesting, incorrectly, that there could be a link between vaccines and autism in children.

'Measles actually kills'

The University of California, Los Angeles, will quarantine students and staff until Tuesday. ( AP: Reed Saxon )

More than 1,000 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 US, where cases have reached a 25-year high.

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By Friday afternoon, two days after Los Angeles County ordered the precautions, about 325 of those affected had been cleared to return after proving their immunity to the disease, through either medical records or tests, health officials said.

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

"Measles actually kills people, so we have to take that really seriously," said Dr Armand Dorian, chief medical officer at the University of Southern California's Verdugo Hills Hospital.

Those under the quarantine were instructed to stay at home and avoid contact with others, and were barred from travelling in vehicles used by the public.

In an emergency, they were told to notify public health officials first.

"This is a legally binding order," the county's public health director, Dr Barbara Ferrer, told reporters.

Anyone who violates it could be prosecuted, she said, but added that it appears everyone is cooperating so far.

She didn't describe what penalties those who don't could face.

'What? Where did that come from?'

An infected student attended UCLA classes over three days earlier this month. ( AP: Jae C Hong )

Students at Cal State-LA were shocked that their campus could be hit by a measles outbreak.

"When they were like measles, I was like, 'What? Where did that come from,'" said Sergio Dula, a communications major.

Eden Guerra, a kinesiology major, was surprised classes weren't cancelled, who noted:

"This is like serious, like it's life, you know."

Cal State-LA reported 875 students, staff, faculty and visitors were placed under quarantine after possibly being exposed to measles earlier this month.

At UCLA, 129 students and faculty were quarantined.

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.

Authorities clamp down on anti-vax ultra-Orthodox Jews

Mandatory vaccinations have begun in some Brooklyn neighbourhoods with ultra-Orthodox Jews. ( AP: Seth Wenig )

Around the country, authorities in California, New York, Washington state and Oregon have responded to the outbreak by moving to crack down on exemptions to vaccinating children.

Most of the cases are centred 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 neighbourhoods and threatened fines of $US1,000 ($1420.80).

City officials said earlier this week that 12 people had been issued summons.

Health officials at a news conference repeatedly urged people who haven't been exposed to measles to get vaccinated, saying the vaccine is safe and effective.

"Ninety per cent of people who are not immunised or haven't had measles before, when they're exposed to measles are likely to get measles," Dr Ferrer said.

ABC/AP