TORONTO — The number of flu cases is continuing to rise across Canada, suggesting the peak of infections with one of the dominant circulating strains could come within a few weeks — or even sooner, say infectious diseases experts, who describe this influenza season as "unusual."
"We really haven't seen a season quite like this in a little while," said Dr. Michelle Murti of Public Health Ontario, referring to the mix of two primary strains making people sick during this year's epidemic.
The dominant influenza A strain is H3N2, a nasty virus that tends to infect the elderly in greater numbers, with concurrent circulation of a B strain, a type that typically causes less severe illness. Influenza B can also affect older people and is the strain that most often infects children.
"Normally in a season, we'll see a peak of influenza A happening some time towards the end of December or through January," Murti said Monday. "And as that is coming down toward the end of February, that's when we start to see that peak of influenza B activity into the spring and later season."
But this year's B strain, known as B/Yamagata, began circulating in the fall, much earlier than is usually the case.
British Columbia, for example, is seeing an atypical 50-50 mix of H3N2 and B/Yamagata, although other regions in Canada may have different ratios of the two strains affecting their populations, Dr. Danuta Skowronski of the BC Centre for Disease Control said from Vancouver.
"The spike in influenza activity that we're experiencing now is not unusual," she said. "In fact, such a sharp increase in activity is a signature feature of influenza that distinguishes it from other respiratory viruses that have a more prolonged, grumbling activity through the winter period."
Skowronski described graphs illustrating flu activity as looking like a church steeple — with a sudden rise, a peak and then a sharp decline.
"We are currently spiking, but whether we have passed the peak or are continuing to rise, it's still too early to tell," she said, adding that peaks may arrive at varying times across the country as regions and communities experience major upticks in cases at different points in the epidemic.
In its weekly FluWatch report, the Public Health Agency of Canada says there were 11,277 laboratory-confirmed cases of flu across the country as of Dec. 30 — about 70 per cent attributed to H3N2 — with more than 1,000 influenza-related hospitalizations and 34 deaths.
However, Murti said those figures are an underestimate of the actual number of cases, as most people don't seek medical attention for flu and, therefore, aren't tested. As well, not all provinces and territories keep track of hospitalizations due to influenza.
"Looking at our numbers over the last couple of weeks and knowing there's a bit of a reporting delay, we're certainly on the upswing right now in terms of increasing activity for flu," she said of Ontario. "So probably in the next few weeks, we're going to continue to see increased activity."
Murti predicts the province's peak — for H3N2 cases, at least — will likely come in the next few weeks.
"But you really don't know the peak until you're coming down the other side," she said.
In the meantime, Skowronski advises that people at risk for influenza complications, including the elderly and those with underlying heart and lung conditions, seek immediate medical attention if they develop flu-like symptoms so they can be treated with an antiviral medication to reduce their risk for hospitalization and death.
To be effective, the drugs must be taken in 12 to 48 hours from onset of symptoms, which include cough, muscle aches and fever.
"Given the expectation of low vaccine effectiveness this season, especially for H3N2, that advice about getting early care for those with high-risk conditions applies to both vaccinated and unvaccinated people," she said.
Canada's flu shot contains the same vaccine components as that used by Australia during the Southern Hemisphere's flu season that ended in August, which was found to have only 10 per cent effectiveness in preventing H3N2.
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Sheryl Ubelacker, The Canadian Press