The measles vaccine is a weakened version of the live virus, and you're not contagious after getting the shot. You may have some mild reactive symptoms like a sore arm, rash at the injection site, or a slight fever, but this is your body building up its immunity.
Kids who get infected with the measles, on the other hand, can shed the virus for several weeks after their infection is over, thereby transmitting the illness to others. Infected people also may not know that they're sick right away, since the initial symptoms are similar to the common cold (runny nose, cough, fever, and pink eyes).
"You can have those for several days before the rash starts, and you're contagious for that period," Melnick said.
The measles can also hang out in the air for two hours after a contagious person has left a room.