Kansai International Airport began vaccinating its workers against measles Thursday following an outbreak that has so far infected 39 people, mostly younger airport employees who were never vaccinated against the disease.

The move comes just over three weeks after the first case was confirmed on Aug. 17, when a female employee in her 20s who handles international flights was diagnosed. Over the following weeks, the outbreak spread, mainly among Kansai airport workers.

As of Thursday, 33 workers had been confirmed as having caught the measles, along with two medical staff who treated them; 31 had recovered, airport officials said.