Health experts say the virus has spread among school-age children whose parents declined to give them the vaccine, which confers immunity to the disease. A vocal fringe of US parents, some in New York's ultra-Orthodox Jewish communities, cite concerns that the vaccine may cause autism, despite scientific studies that have debunked such claims.

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Although the virus was eliminated from the country in 2000, meaning the disease was no longer a constant presence, outbreaks still happen via travellers coming from countries where measles is still common, according to the CDC.

Experts warn that the outbreak is not over as the number of cases approaches the 1994 total of 958. That was the highest number since 1992, when the CDC recorded 2,126 cases.

More than 40 people in 2019 brought measles to the United States from other countries, most frequently Ukraine, Israel and the Philippines, federal officials said.