FREDERICTON -- New Brunswick health authorities are advising staff and students at a Saint John high school that they must receive a measles booster shot if they want to continue going to the school.

The directive follows news of a third confirmed measles case in the Saint John area -- the second at Kennebecasis Valley High School.

An immunization clinic has been set up at the school today.

Chief Medical Officer of Health, Dr. Jennifer Russell, says the vaccination is not mandatory, but the message to staff and students is they must receive the booster shot or they can't go to the school.

The province's second measles case was also at the high school.

That person was exposed to the disease when they were in the emergency department of the Saint John Regional Hospital at the same time as the person with the first confirmed case.

Measles is a highly contagious infection and can be prevented with a vaccine. Most people who contract the virus make a full recovery.

According to the Centers for Disease Control and Prevention in the United States, one of out of every 1,000 people infected with measles will develop acute encephalitis, which often results in permanent brain damage.

The agency adds that one or two out of every 1,000 children who are infected with the virus will die from respiratory and neurological complications.