(Reuters) - The United States recorded 41 new measles cases last week, bringing the year’s total number of cases to 981 in the worst outbreak of the disease since 1992, federal health officials said on Monday.

The U.S. Centers for Disease Control and Prevention said the number of cases of the highly contagious and sometimes deadly disease rose 4% in the week ended May 31 from the prior week. The 2019 outbreak, which has spread to 26 states, is the worst since 1992, when 2,126 cases were recorded.

Federal health officials attribute this year’s outbreak to U.S. parents who refuse to vaccinate their children. These parents believe, contrary to scientific evidence, that ingredients in the vaccine can cause autism.

The disease has mostly affected children who have not received the measles-mumps-rubella vaccine, which confers immunity to the disease.

On Thursday, CDC officials said the outbreak had surpassed the total number of cases per year for the past 25 years, topping the 963 cases that were confirmed in 1994.

Measles was declared eliminated in the United States in 2000, meaning there was no continuous transmission of the disease for a year. Still, cases of the virus occur and spread via travelers coming from countries where measles is common.

CDC officials have warned that the country risks losing its measles elimination status if the ongoing outbreak, which began in October 2018 in New York, continues until October 2019.

“Measles is preventable and the way to end this outbreak is to ensure that all children and adults who can get vaccinated, do get vaccinated,” CDC Director Robert Redfield said in a statement on Thursday. “I want to reassure parents that vaccines are safe, they do not cause autism.”

The 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.