The World Health Organization and the Centers for Disease Control and Prevention released new data on Thursday that showed that measles cases spiked by more than 30% from 2016 to 2017.

The WHO and CDC say there were 173,330 officially reported cases worldwide last year alone — though they believe that those numbers represent just a fraction of the actual number.

“In general, the number of reported cases reflects a small proportion of the true number of cases occurring in the community,” the WHO has previously explained, saying the organization uses a statistical model to estimate the actual number. “Many cases do not seek health care or, if diagnosed, are not reported. In addition, there is a one to two month lag time in reporting.”