The flu season in the Greater Toronto Area has been fairly mild so far this year, but one expert says it could get much more severe before it's over, because of two strains of influenza that this season's vaccine doesn't cover.

According to Dr. Naveed Mohammad, vice president of medical affairs at William Osler, a hospital system in Peel region, the week of Dec. 17 to Dec. 23 saw almost twice as many confirmed flu cases in the GTA than in the previous week.

However, the 199 cases which have been reported so far this year are lower than the previous five seasons' average of 281. But Mohammad says he believes this flu season may be a late bloomer.

"We have been preparing for the surge in influenza for a number of months," he told CBC Toronto. "Unfortunately, we feel we may have not peaked yet."

That's because this year's vaccine may not effectively protect against two strains of flu, called H3N2 and the Influenza B virus.  

Mohammad says Ontario is on "pins and needles" waiting for the worst to come because of a "record-breaking surge" that hit Australia during its recent flu season due to the same strains.

Outbreaks cause 'unhappy patients'

So far, the William Osler Health System, which runs three sites, Brampton Civic and Etobicoke General hospitals, as well as the Peel Memorial Centre for Integrated Health and Wellness, has already had three outbreak units within its organization. That means flu has spread among patients and staff shutting down an entire floor to new admissions.

"We lose a significant part of our hospital real estate to admit patients, which is what leads to some of the complications within the surge with wait times and unhappy patients," he explains.

The outbreaks and rise in cases around this time are partly attributed to the holiday season.

"People are often gathering at home in large numbers with family members for the holidays … and that's where they're picking up the illness or transmitting the illness," Mohammad said.

Flu shot misconceptions

Dr. Jerome Leis, the medical director for infection prevention and control at Sunnybrook Health Sciences Centre, says it's too early to tell what kind of flu season Ontario will experience, and whether we are destined to face the same fate as Australia.

Leis adds that the flu shot, which helps protect those who get it as well as "the most vulnerable people in the community" is often misunderstood by the public. 

"People think the flu shot either works or it doesn't," he told CBC Toronto. "Sometimes the flu shot is not as protective as it is in other years, but it's still worth getting."

Flu shot immunization innoculation influenza health syringe needles

Lisa Berger, Toronto Public Health's associate medical officer of health, says it's not too late to get a flu shot. (Chanss Lagaden/CBC)

Leis said the flu this year has three different strains and the vaccine is likely to provide "good protection" for at least two.

Previous vaccines may still provide immunity

Another good reason to get the flu shot every year, says Mohammad, is that you may still carry some immunity to the strains in the previous year's blend.

"Those of us that got the vaccine last year — last year's vaccine may have been more effective against H3N2 ... they still have some immunity to it possibly."

Dr. Lisa Berger, Toronto Public Health's associate medical officer of health, says it's still too early to predict how the rest of the season will go.

"The message is to get vaccinated," she said. "The vaccine takes two weeks for effectiveness and the flu season started later. We still have our season ahead of us."

And she adds the messages about flu prevention are the same as they have been every year.

"Stay home if you're sick," Berger warned. "Cough or sneeze into your sleeve, and practise good hand hygiene."