(Reuters) – The worst measles outbreak in the United States in 25 years has spread to Oklahoma, federal health officials said on Monday as they reported 41 new cases nationwide, raising the total number sickened this year to 880 people.

The U.S. Centers for Disease Control and Prevention reported a 4.9% increase in the number of measles cases from May 10 to May 17 in an outbreak that has now reached 24 states. The agency has been providing weekly updates every Monday.

The CDC said there had been one confirmed case in Oklahoma.

Most of the new cases were in New York, CDC spokesman Jason McDonald said, with 21 cases recorded in New York City and nine in Rockland County.

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 U.S. 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.

Although the virus was eliminated from the country in 2000, meaning the disease was no longer a constant presence, outbreaks still happen via travelers 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.

(Reporting by Gabriella Borter in New York, Ankur Banerjee in Bengaluru; Editing by Scott Malone and Bill Berkrot)