As many as 24 Canadians, including one who died, suffered from a severe allergic reaction after receiving the H1N1 pandemic vaccine.
But health officials stressed Wednesday that the rate of anaphylaxis appears to be low: 0.32 per 100,000 doses. Anaphylaxis is a severe allergic reaction that can be characterized by respiratory distress, swelling of the lips, eyelids, throat or tongue, and low blood pressure, and can be life-threatening.
"As we move through this, we will have numbers or rates that are higher and lower. But at the end of the day, it is the overall safety of the vaccine which is really clear and we have a great deal of confidence in [it]" said David Butler-Jones, Canada's chief public health officer.
All vaccines can induce anaphylaxis but do so at very low rates. The majority of side-effects are minor reactions, such as soreness at the injection site.
The elderly person who died after receiving the H1N1 shot had other health problems, so officials are still investigating the cause of death.
Federal officials said Wednesday that influenza activity appears to be levelling off in some parts of the country, like British Columbia, although it is still as much as seven times higher across the country than normal for this time of year.
Roughly 10 per cent of Canadians have been infected, and another 25 per cent have been immunized. That means the vast majority of people still risk serious infection and possibly death if they don't get the H1N1 shot, Dr. Butler-Jones said.
While many experts agree that the second wave of the virus appears to be peaking, there is talk in some circles of a potential third wave or the virus reappearing next fall. More than 15 million doses of vaccine have been distributed across the country, as flu clinics open their doors to healthy people.
"I'm really concerned about the Christmas period when we do share so many things. Good things usually. But unfortunately also infectious diseases," Dr. Butler-Jones said.
He added: "The more that are immunized, the harder it is for this virus to spread."