(Reuters) – The most widespread U.S. measles outbreak in a quarter-century has infected 60 new patients in the last week, raising the total number of confirmed cases to 764, federal health officials said on Monday.

The U.S. Centers for Disease Control and Prevention reported an 8.5 percent increase in the number of measles cases since April 26, also confirming that the worst outbreak of the measles in the United States since 1994 has now reached 23 states.

Pennsylvania is the most recent state to be hit by the outbreak, after the CDC reported cases in 22 states last week.

Federal health officials say a vocal fringe of U.S. parents who refuse to vaccinate their children have propelled the outbreak, which has mostly affected children who have not received the measles-mumps-rubella vaccine, which confers immunity to the disease. These parents believe, contrary to scientific evidence, that ingredients in the vaccine can cause autism.

Although the virus was eliminated from the country in 2000, meaning it was no longer continually present year round, outbreaks still happen via travelers coming from countries where measles is still common, the CDC says.

This outbreak has escalated since 82 people in 2018 and 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 and Manas Mishra in Bengaluru; editing by Scott Malone and Jonathan Oatis)

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