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As Canada braces for another flu season, experts say it's difficult to predict the severity of the virus and the effectiveness of the flu shot this year, but global trends may hold clues as to what's in store for us.

Canada's flu season typically starts in October, when temperatures drop and people are in closer contact with each other indoors. The time to get a flu shot is between now and November, before the virus is circulating widely, the Public Health Agency of Canada (PHAC) recommends.

Just how many Canadians are affected by the flu each year is a bit of a guessing game.

An estimated 3,500 Canadians die of the flu each year and 12,200 are hospitalized, making it one of the 10 leading causes of death in this country according to PHAC, though that's based on a disputed model , which some believe overstates the numbers.

Last year's season saw 3,657 reported hospitalizations and 223 confirmed deaths from the flu, but that doesn't capture the total number of Canadians who went unreported.

What we do know is that only 42 per cent of Canadian adults reported getting the flu shot last season, though it's up from 38 per cent in the 2017-18 season and 36 per cent in 2016-17, according to the national Influenza Vaccine Coverage Survey .

"Despite the fact that the influenza vaccine is much better than nothing ... a lot of people still think that they don't want to get it because it's not good enough," says Dr. Allison McGeer, a physician and infectious diseases specialist at Mount Sinai Hospital in Toronto.

"Influenza vaccines are about 50 or 60 per cent effective in protecting you from hospitalization due to influenza. How good are seat belts at protecting you from dying in a car accident? About 50 per cent. How good are smoke detectors at reducing your risk of death in a house fire? About 35 per cent. So, we have this weird double standard that goes on with vaccines."

Dr. Allison McGee, former director of infection control at Mount Sinai Hospital in Toronto, says vaccinations suffer from a "weird double standard" not applied to other life-saving tools. (Mount Sinai Hospital)

How effective will the flu shot be this year?

There are four different groups of influenza viruses that the flu shot protects against each year – typically H1N1 and H3N2 strains and two types of influenza B strains.

The World Health Organization (WHO) meets with experts from around the world in February and September each year to determine what specific strains of influenza to vaccinate against ahead of each flu season in both the Northern and Southern hemispheres.

In February, WHO delayed its Northern Hemisphere recommendations by a month in a rare move due to "frequent changes" in the H3N2 virus, which also pushed back the manufacturing of the flu shot in Canada for this season.

"That's only the second time in 20 years we've delayed a decision," says Ian Barr, deputy director of the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne, Australia.

"We're making decisions for the whole of the Northern Hemisphere and what we saw in 2018-19 was distinct differences in the types of viruses which were circulating.… So just to have that little extra window of time to see whether things are changing or not can be valuable."

In March, the WHO decided the H3N2 strain the flu shot would target in the Northern Hemisphere this year was one that circulated late in the U.S. flu season. But that strain was different from what hit Canada at the tail end of our season last year – leading some experts to speculate there could be a " mismatch " between the strain and the vaccine here.

"If we have a mismatch this year, say for H3N2, H1N1 still has the possibility of being highly matched," says Dr. Alyson Kelvin, a virologist at the Canadian Centre for Vaccinology in Halifax.

"So even though there might be a mismatch with one, you still have a good chance of being covered in those other three components."

What can other countries tell us about Canada's flu season?

Australia is a country Canada looks to for information on its upcoming flu season because of their advanced surveillance of the virus, but it's not a direct indicator of what will happen here.

Australia saw an earlier start to its flu season this year and a higher number of cases, hospitalizations and deaths — similar to a severe 2017 season that was closely in sync with Canada, Barr said.

"It will be one of the biggest seasons in recorded times, in the last 20 years," he said. "We've had sort of a slow burn if you like, but overall it's still been quite an intense season."

Barr says the variability of H3N2 strains worldwide makes it difficult to predict how effective the Northern Hemisphere flu shot will be this year, especially because the vaccine is formulated six to eight months ahead of the virus actually circulating here.

Canada has not yet seen significant levels of flu activity yet, according to the latest federal government surveillance reports.

"The only predictable thing about the flu is its unpredictability. While we cannot precisely predict what the next flu season will look like, we are prepared and ready for any type of flu season," PHAC spokesperson Anna Maddison said.

"Nevertheless, viruses circulating within a population can sometimes change during the flu season. If this happens, the flu shot may not work as well as expected."

When will the flu shot be available?

Shipments of the flu shot began in mid-September, with about 60 per cent of Canada's total supply order for the upcoming season now available across the country and 90 per cent expected by the end of October, according to PHAC.

The remaining 10 per cent of the necessary supply will be available "as soon as possible," Maddison said.

"Although some vaccination programs may need to adjust their timing to accommodate a somewhat later than usual delivery of vaccine, health professionals at all levels of government are working together to minimize any potential impacts," she said.

"Some provinces and territories may delay the official launch date of their public programs to account for the time required to redistribute the vaccine across their jurisdiction after it is received from the manufacturer."

Maddison added the federal government is working with provinces, territories and vaccine manufacturers to minimize any delays or issues with the availability of the flu shot for this year's season.

Why don't people get the flu shot?

The flu shot is the most effective way to prevent the flu and flu-related complications such as pneumonia, but it also contributes to community immunity and may reduce the severity of symptoms even if you do get the flu, according to PHAC.

Vaccine hesitancy is a concern in Canada, Maddison added, with 20 per cent of those who reported not getting the shot last year saying they didn't think it was necessary, according to survey results.

B.C. provincial health officer Dr. Bonnie Henry says she's frustrated with the skepticism she sees surrounding immunization. (Jonathan Hayward/Canadian Press)

"It's really frustrating when I watch this every year, we see people, and we hear a lot about skepticism about immunization," Dr. Bonnie Henry, the provincial health officer for B.C., told CBC News.

"But we know that it makes a big difference in helping people get through the season without the exacerbation of other illnesses, without ending up at hospital."

About 30 per cent of Canadian seniors didn't get a flu shot this year, according to PHAC, though they are one of the most at-risk groups for hospitalization and death from the flu. Of those who didn't receive a shot, Maddison says about one in five said they were concerned about "vaccine safety."

"We forget that it actually it can be a very severe disease for some people, particularly for elderly people or young people in our lives," Henry said.

"It's way better than doing nothing. If you increased your chances of winning the lottery by 50 per cent, wouldn't you take that? It makes a big difference and it's the best thing we can do."

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