President Trump has told Americans to “get their shots” as measles cases spread across the country.
“The vaccinations are so important,” he told reporters outside the White House.
Nearly 700 cases have been reported across 22 states amid a resurgence of the highly infectious disease, the US Centers for Disease Control and Prevention (CDC) says.
Mr Trump has previously appeared to link vaccines and autism. Public health experts say there is no link.
Vaccination rates have dropped steadily in the US with many parents objecting for philosophical or religious reasons. Others, known as “anti-vaxxers”, believe discredited information that vaccines cause autism in children.
The CDC says measles cases have reached a level not seen since 2000, when the disease was eliminated from the US.
Mr Trump’s warning comes as hundreds of staff and students at universities in the Californian city of Los Angeles have been quarantined over fears they may have contracted measles.
Those who were at risk of being infected were asked to stay at home unless they could prove their immunity.
As of Thursday, 82 people at the University of California (UCLA) were unable to provide vaccination records, the university says.
At California State University, 156 people – including library employees and students – could not provide records and remain in quarantine, AP news agency reported.