US President Donald Trump urged Americans to get vaccinated as a measles outbreak spread across the country, reaching the highest number of cases in 25 years.
Medical experts says there is almost 700 cases this year in a resurgence largely attributed to misinformation that is turning parents against vaccines.
"Vaccinations are so important," Mr Trump told reporters at the White House. "They have to get their shots".
The US Centers for Disease Control and Prevention said 695 cases had been reported in 22 states this year as of Wednesday afternoon.
"This is alarming," said Dr. William Schaffner, a Vanderbilt University vaccine expert. "Not only is measles dangerous in itself, but its return could mean other vaccine-preventable diseases seemingly consigned to the past may be coming back as well."
More than 1,000 students and staff members were quarrantined at two Los Angeles universities this week in one of the most sweeping efforts yet by public health authorities.
"Measles actually kills people. So we have to take that really seriously," said Dr Armand Dorian, chief medical officer at USC Verdugo Hills Hospital.
Those under the quarantine were instructed to stay at home and avoid contact with others. They also were barred from travelling by public transportation, including planes, trains, buses or taxis. If they must travel for an emergency, they were told to notify public health officials first.
Across the United States, more than 91 per cent of children have received the first dose of the measles, mumps and rubella (MMR) vaccine by age three.
But anti-vaccine sentiment - often fueled by disinformation - has sent immunisation rates plummeting in pockets.
This year's US caseload - the highest since the disease was declared eliminated almost two decades ago - has been concentrated in three heavily Jewish areas in Brooklyn, Rockland County near New York, and near Detroit, and in a Russian-speaking community in Washington State.
Earlier this month New York's mayor declared a public health emergency in ultraorthodox Jewish parts of Brooklyn, ordering all residents to be vaccinated.
In the past, Mr Trump has given some support to the anti-vaccination movement, claiming, for example, that heavy doses given to young infants may be linked to autism.
"Massive combined inoculations to small children is the cause for big increase in autism," Mr Trump said on Twitter in 2012.
He reiterated that message while running for president in 2015.
"Autism has become an epidemic. Twenty-five years ago, 35 years ago, you look at the statistics, not even close. It has gotten totally out of control. I am totally in favor of vaccines. But I want smaller doses over a longer period of time," Mr Trump said during a presidential primary debate on CNN.
Global spread
The anti-vaxxer movement, based on a scientifically debunked 1988 British report linking the MMR vaccine to autism, has surged in recent years with the rise of online conspiracy theories on social media.
Repeated studies - the most recent involving more than 650,000 children monitored for more than a decade - have shown that there is no such link.
An estimated 169 million children missed out on the vital first dose of the measles vaccine between 2010 and 2017, according to a Unicef report.
The number of cases of the disease had risen 300 percent worldwide in the first three months of 2019 compared to the same period last year, the UN said.
The anti-vaxxer movement has adherents across the Western world but is particularly high profile in the United States.
The US outbreak has been blamed in part on unvaccinated visitors contracting the disease during visits to both Israel and Ukraine.
The New York outbreak has been traced to Orthodox Jews from Brooklyn visiting Israel, then spreading the highly infectious disease through synagogues, schools and apartment blocks to children whose parents had not had them inoculated.
In Clark County, Washington, the disease has spiked among the Russian-speaking community after a child brought the virus back from Ukraine in December and it spread to 74 other people, mostly children, through schools, supermarkets and a bowling alley.
Ukraine, which has experienced five years of simmering conflict with Russia on its eastern border region, has had at least 11 people die from the illness.
Measles is one of the world's most contagious viruses. Spread by coughing or sneezing, the virus can linger in the air long after an infected person leaves a room.
There have been three measles-related deaths reported in the US since 2000, the last one in 2015. The worst year for measles in modern US history was 1958, with more than 763,000 reported cases and 552 deaths.
