The number of U.S. measles cases has topped 700 already this year, the Centers for Disease Control and Prevention reported Monday, adding that nearly 70 children have been hospitalized.

This year’s surge in measles cases—the worst year for the disease since 1994—has sent 66 children to the hospital, newly released data show. Twenty-four have developed pneumonia.

Health and Human Resources Secretary Alex Azar said the department is very concerned about the rise in cases.

“Vaccine-preventable diseases belong in the history books, not in our emergency rooms,” Azar said on a teleconference with the CDC director, Dr. Robert Redfield, and other CDC and HHS experts.

As of April 26, there have been 704 cases of measles reported to the CDC, from 22 states. Most infections were associated with 13 outbreaks, the largest of which are in New York state. The New York outbreaks—in Brooklyn and in Rockland County, north of Manhattan—account for 67% of the national total so far in 2019.

Washington state, which had an outbreak involving 72 cases, has just declared its epidemic over.

Most of the people who have contracted measles this year have been unvaccinated—503 or 71%. Another 11% had received one or more doses of measles-containing vaccine; two shots are needed to be fully vaccinated. The vaccination status of another 125 people was not known, the CDC said.

Ongoing spread of the measles virus, called endemic measles, was halted in the United States in the late 1990s, leading to a declaration in 2000 that measles had been eliminated in the United States.

Since that time, outbreaks have occurred when someone—either an American who had traveled abroad or a traveler from another country—contracted the virus elsewhere and brought it to the U.S. There have been 44 such introductions of measles so far in 2019, the CDC report said.

The top three countries from which measles importations came were the Ukraine, Israel, and the Philippines.

Much of the spread so far this year has occurred in groups of people who were either not immunized or underimmunized and part of close-knit groups, the CDC reported. In fact, such cases account for 88% of the total thus far this year.

Republished with permission from STAT. This article originally appeared on April 29, 2019