Four Calgarians have been diagnosed with the measles this year — of those four, three were fully immunized against the virus. 

Alberta Health Services is warning that the strain of measles appears to be unusual. Health officials are investigating the strain, said AHS's Dr. Judy MacDonald on Friday afternoon.

Between 2000 and 2013, there have been 20 confirmed cases of measles in the Calgary area. Of those, only two cases were confirmed where the person had been immunized, said MacDonald.

"When someone who is immunized happens to get measles, the disease is generally milder than it would be in someone that was not immunized," she said, adding that was true in the three cases this year.

The latest case involves a student around the kindergarten or Grade 1 age from Clear Water Academy, said MacDonald. 

Staff and students at the university prep school who have not been immunized will be sent home for a couple of weeks. 

The other cases include a 16-year-old student and a 16-month-old infant, both of whom contracted the virus in the Philippines, said MacDonald. The baby is the one who had not been immunized. 

The fourth person with the virus is 36-years-old, said MacDonald. None of the four cases required hospitalization. 

Vaccine at least 95% effective 

A single dose of vaccine has a 95 per cent effectiveness rate, said University of Calgary microbiologist Glenn Armstrong, who is an expert in infectious diseases. When you have two doses, that effectiveness rate goes up to 99 per cent. 

"[It's a] pretty rare situation. I haven't heard of something like this happening before. But that doesn't rule out the possibility that there's a whole bunch of circumstances that have come together in this particular situation."

Armstrong​ said it "can't be ruled out" that there's a strain that is less responsive to the vaccine. 

"What we do know about measles virus is its a pretty stable virus and it doesn't mutate very easily or very quickly," said Armstrong. 

"It's probably a pretty rare event if this strain has changed significantly from the strains that have been around historically, but you can never say never to something like that happening."

Armstrong said he doesn't think there's cause for major concern. 

"My guess is that this is probably more of a random type situation that's happening here and its not signalling that the virus has changed any and that the vaccine is no longer covering people as effectively as it has been," he said.

"If folks have been immunized properly, and their children have been immunized properly, I think there's still a pretty good chance the vast majority of those individuals are going to be protected."