The Children's Minnesota health system said one of its clinical workers contracted measles in the Minnesota outbreak despite being vaccinated.
The number of measles cases in the current outbreak ticked up to 51 on Wednesday, the Minnesota health department said, an increase of one case from the day before. The vast majority of cases are in Somali-American children living in Hennepin County.
Patsy Stinchfield, Children's Minnesota's senior director of infection control, said the worker who contracted measles had an "extremely mild" case.
"This health care worker had two doses of MMR but also had intense exposure to measles... intense in two ways one very close-up and one very prolonged," Stinchfield said.
Officials say the measles vaccine with a second booster shot is effective at heading off the disease 97 to 98 percent of the time.
"The employee is doing well, had an extremely mild case of measles [and] in fact barely knew that the little rash that occurred was measles," Stinchfield said.