If you haven't caught the flu this year, you can likely give the credit to that flu shot you got earlier this season. The latest data show that the strains used in this year's shot were a good match with the viruses currently circulating.
Data released Friday by the Public Health Agency of Canada show that the vast majority of the flu viruses tested by the agency's National Microbiology Laboratory in Winnipeg have been from H1N1 viruses and have been a good match with this year's vaccine.
The annual flu shot generally protects against viruses of two influenza A subtypes -- H3N2 and H1N1 -- and against one strain of influenza B. All three circulate every winter and cause the vast majority of flu illnesses.
Influenza A viruses are predominating this flu season in both Canada and the U.S. And Canada's National Microbiology Laboratory in Winnipeg reports that all H1N1 viruses they've tested have been excellent matches with the HINI strain chosen for this year's shot, Solomon Islands/3.
The other Influenza A strain used in the shot -- Wisconsin/67/2005 (H3N2)-like -- had been expected to be a bad match, even before the flu season started. CTV medical specialist Avis Favaro reported in October that the Wisconsin strain had already mutated into a different form than the one used for the vaccine. As well, the Influenza B strain chosen -- Malaysia/2506/2004-like antigen -- also showed signs of changing.
Infectious disease experts had been worried that the mismatch could result in more flu illnesses and hospitalizations. But the latest information from PHAC suggests that H3N2 viruses and influenza B haven't been causing much illness in this country so far.
Alberta is the one exception, with the province seeing a lot of influenza B activity. About half of the viruses Alberta has submitted so far to the NML lab for testing have been B viruses -- a much higher proportion than elsewhere in the country.
But while the predominant circulating influenza B virus isn't in this year's vaccine, it has been a component of the flu shot in the recent past, and may still offer protection to those who regularly get the shot.
The story is different in the United States, where H3N2 viruses have made up roughly 46 per cent of the influenza A viruses that the U.S. Centers for Disease Control has tested so far this year.
That's made the flu shot relatively ineffective against the dominant strains circulating there, Dr. Joe Bresee, head of the CDC influenza division's epidemiology and prevention branch, said during a teleconference Friday.
"These data suggest that protection against the H3N2 and B virus strains in the community may not be optimal,'' he said.
But he stressed that the vaccine is still offering protection to those who received it.
"While a less-than-ideal virus match between the viruses in the vaccine and those circulating viruses can reduce vaccine effectiveness, we know from past influenza studies that the vaccine can still protect enough to make illness milder or prevent flu-related complications," Bresee said.
Back in Canada, it appears that in some places, such as Ontario, the flu season has mostly peaked, with flu case reports now starting to fall.
Creating flu shot a long process
The process of creating the annual flu shot is a complicated one and begins almost a year ahead of time.
The World Health Organization monitors flu activity around the world, looking for predominant strains. As flu viruses reproduce, they often trigger slight changes in their genetic code, which scientists call antigenic drift.
The WHO researchers take particular note of what's happening in the southern hemisphere to see what strains are emerging there, since they go through their winter flu season long before we do.
The WHO then selects the strains they think are most likely to predominate in the northern hemisphere. They generally select three -- two subtypes of influenza A viruses and one influenza B virus -- to go into the vaccines to be used the following fall and winter.
Each year, authorities change one or two of the three strains in the vaccine, which is why it is important to get a new flu shot every year to ensure protection against the most recent strains.
