Influenza cases are spiking across the country and infectious-disease experts warn the worst may be yet to come.

So far, flu activity in Canada resembles the 2014-15 season, a brutal year characterized by a hard-hitting H3N2 virus and low vaccine effectiveness rates, said Danuta Skowronski, epidemiology lead for influenza and emerging respiratory pathogens at the B.C. Centre for Disease Control. The elderly are being "disproportionately affected," Dr. Skowronski said, with a number of outbreaks at long-term care facilities being reported. There was a major jump in flu cases over the holidays, as people gathered together, but Dr. Skowronski said she suspects there could be a new surge in the coming weeks.

"We're still spiking," she said. "I can't say that we have peaked."

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According to the Public Health Agency of Canada's FluWatch report, there was an increase in outbreaks and admissions to hospitals from Dec. 11 to 17 (the most recent data available). There were 692 positive flu cases that week, with H3N2 being the most common strain detected, compared with 554 cases the week before. There were 18 laboratory confirmed outbreaks that week, mainly in long-term care homes. People 65 and older accounted for the majority of hospital admissions and deaths related to the flu.

Only a small number of influenza cases are confirmed by a lab, so the actual number of people infected is likely much higher. There are also other respiratory viruses circulating that are often mistaken for the flu, said Michael Gardam, an infectious-diseases expert at Toronto's University Health Network.

While challenges remain, experts are optimistic this year's flu vaccine will offer better protection than the 2014-2015 version. That year, the vaccine effectiveness against the dominant H3N2 strain was less than 10 per cent, the worst rate recorded in the decade Canadian researchers have been using the current formula for estimating how well the flu shot works. Experts realized early on that the vaccine was not a good genetic match for the virus, which helps explain the low effectiveness rates. In addition to H3N2, this year's flu shot also protects against H1N1 and a B strain.

This year, experts believe the flu shot is a much closer genetic match to the H3N2 strain in circulation, which should bring up protection rates. However, Dr. Skowronski cautioned that it still won't offer full immunity against influenza. Flu viruses are constantly mutating and changing, so even though a vaccine protects against the H3N2 strain, that doesn't mean it will be very effective against the virus in circulation, she said.

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Over the coming weeks, experts across the country will be collecting data to determine effectiveness rates and Dr. Skowronski said she'd be happy if it's 40 to 50 per cent.

The flu shot poses a major public-health challenge because protection rates vary due to the constantly shifting nature of the influenza virus. Experts are hopeful they can one day develop a universal vaccine that can protect against all strains of the flu, but until then, the flu shot still offers the best protection. For those vulnerable to the effects of the flu, even a small amount of protection can be valuable, Dr. Skowronski said.

"If you're a person at higher risk of hospitalization or death, cutting your risk in half or even by a third is really important protection," she said.