Nearly 900 immigrants in US detention centers had the mumps over the last 12 months, the CDC said Thursday, at a time when the government is detaining a record number of undocumented people.

A total of 898 cases were reported in adult immigrants in 57 detention facilities across 19 states from Sept. 1, 2018, to Aug. 22 this year, according to the agency. An additional 33 facility staffers were also infected.

These 900-plus cases are the first outbreaks identified in detention facilities by the CDC. As of May, an apparent all-time high of more than 52,000 people were being detained by Immigration and Customs Enforcement (ICE).

In June, ICE said there had been more than 330 mumps cases in the detention centers since September. The CDC’s new tally for such cases is the highest reported to date and is based on four ICE-operated facilities, 19 county jails, and 34 privately operated facilities, according to an agency spokesperson.

Those numbers are “striking” and “noteworthy,” said Marc Stern, an affiliate assistant professor in public health at the University of Washington. Based on the CDC’s data, an ICE detainee had roughly at least a 4,000-fold greater risk of getting mumps than a nondetainee in the United States at large, Stern told BuzzFeed News.

“That’s an incredibly huge risk,” he said by email.

The first mumps cases were identified in Texas detention facilities in December by the Texas Department of State Health Services, the CDC said. Since then, the federal health agency has been working with state and local departments, ICE, and other federal agencies to control the outbreaks.

Once largely eliminated in the US, mumps has recently been on the uptick, driven in part by a surging anti-vaccination sentiment, with more than 150 outbreaks reported across the country since 2015. The airborne virus is not usually considered to be fatal, but it is highly contagious, especially in close-contact settings. The two-dose MMR vaccine prevents most, though not all, cases, and patients are supposed to avoid contact with others to keep the disease from spreading.

