The number of measles cases in the US has surpassed 1,000 amid the worst outbreak of the disease in 25 years, federal health officials said.

The milestone comes shortly after the number of cases reached 971, surpassing a previous high of 963 cases in 1994, the US Centers for Disease Control and Prevention announced.

Health and Human Services Secretary Alex Azar, in a statement confirming the 1,001 cases tallied by the CDC on Wednesday, expressed concern about undervaccinated communities that are contributing to the disease’s spread across the country.

“We cannot say this enough: Vaccines are a safe and highly effective public health tool that can prevent this disease and end the current outbreak. The measles vaccine is among the most-studied medical products we have and is given safely to millions of children and adults each year,” he said. “Measles is an incredibly contagious and dangerous disease. I encourage all Americans to talk to your doctor about what vaccines are recommended to protect you, your family, and your community from measles and other vaccine-preventable diseases.”

The measles epidemic was declared “eliminated” from the US in 2000 following a decadeslong vaccination campaign that saw an entire year without continuous transmission of the disease. With measles cases on the rise now, there’s concern that years of hard work and millions in expenditures are about to go to waste.

“If these outbreaks continue through summer and fall, the United States may lose its measles elimination status. That loss would be a huge blow for the nation and erase the hard work done by all levels of public health,” the CDC said in a statement.

“Before widespread use of the measles vaccine, an estimated 3 to 4 million people got measles each year in the United States, along with an estimated 400 to 500 deaths and 48,000 hospitalisations,” the CDC said.

It costs an estimated $19,000 to $114,286 to control each measles case from spreading in health care settings, the CDC has said.

Not since 1992 has the U.S. seen more measles cases. That’s when 2,200 cases were reported from 36 states and the District of Columbia, down from 9,643 cases reported in 1991 and 27,786 cases reported in 1990.

Health officials have attributed high vaccination rates for that decline in cases in recent years. The usually high number of cases now has been attributed to travelers who contract the disease overseas and then bring it to U.S. communities that have undervaccinated populations.