This year's U.S. measles epidemic just surpassed a 25-year-old record, and experts say it's not clear when the wave of illnesses will stop.

U.S. health officials on Thursday reported 971 cases so far this year. That eclipses the 963 measles illnesses reported for all of 1994.

It's been 27 years since the nation saw this many measles cases — 2,237 cases were reported in 1992.

The Centers for Disease Control and Prevention updated the numbers Thursday.

Measles was once common in the U.S. but gradually became rare after vaccination campaigns that started in the 1960s.

The vast majority of this year's cases have been in New York City. But illnesses have been reported in at least 26 states.