What flu strain is circulating now and why are so many kids getting sick?

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TORONTO — The flu has returned with a vengeance after being absent for a couple of years during the COVID-19 pandemic. It's hitting children especially hard. Here's what doctors say is happening, and why.

What flu strains are we seeing this flu season?

Based on predictions from the World Health Organization on what strains would appear, this year's flu vaccine protects against two Influenza A strains -- H3N2 and H1N1 -- and two Influenza B strains which are called B/Victoria lineage and B/Yamagata lineage.

Right now, H3N2 is causing the vast majority of flu cases in Canada. It's possible that the other strains could start circulating more widely and peak later in the flu season, doctors say.

Over the last decade, Influenza B strains have tended to arrive later in the season, said Dr. Allison McGeer, an infectious diseases specialist and microbi ologist at Sinai Health Systems in Toronto.

Does H3N2 make people sicker than the other strains?

H3N2 can cause more severe illness than other strains, especially among seniors who are more vulnerable, partly because it mutates faster so people don't have as much acquired immunity protection, said McGeer.

That's because people appear to accumulate some degree of protection throughout their lives to H1N1 and Influenza B strains, but not for H3N2.

"(For) kids who haven't really been exposed to influenza before to any significant degree, there's not a lot of difference in severity between H1N1, H3N2 and B (strains)," McGeer said.

Why are so many kids getting sick right now?

The flu season started early this year, doctors say, which means kids started getting sick earlier. And COVID-19 safety measures, like masking, also kept other viruses like the flu at bay the previous two cold and flu seasons.

"We're now seeing all our old viral foes coming back to play after being on hiatus for a couple of years," said Dr. Justin Penner, pediatric infectious diseases specialist at the Children's Hospital of Eastern Ontario and member of the Canadian Paediatric Society's infectious diseases and immunization committee.

Kids under five years old are most at risk of getting seriously ill with flu, doctors say. And because of the COVID-19 pandemic precautions, the immune systems of many children under three haven't previously come in contact with the virus.

"Kids between, you know, zero to three have about a 40 per cent chance of acquiring influenza in any given year," McGeer said.

"We've now got like two and a half cohorts of kids who haven't been exposed to influenza at all. So they're all getting sick at the same time."

Why do some people land in hospital with the flu?

In the vast majority of cases, flu can be treated at home, doctors say.

But like many illnesses, some cases end up being more serious, McGeer said.

"There's an entire range (of reasons) from just severity of the influenza illness itself to complications," she said.

Some flu patients might get pneumonia, for example, or a bacterial infection on top of the virus.

Myocarditis and encephalitis are other possible complications of the flu.

People who are immunocompromised can also be at higher risk of severe illness.

"Influenza can also trigger exacerbations of underlying illness," McGeer said, including heart disease and stroke.

Seniors are also more prone to being hospitalized with the flu, but a higher risk in the older age demographic appears to be offset this year by public health measures such as continued masking in long-term care homes, she said.

Why are so many people reluctant to get the flu vaccine?

The Public Health Agency of Canada says it wants to achieve an 80 per cent flu vaccination rate by 2025, but the uptake falls far short of that each year.

The influenza vaccine has a "bad reputation" because it's effective but not perfect, said Dr. James Kellner, a pediatric infectious disease specialist and epidemiologist at the University of Calgary.

Based on data from Australia, which already had its flu season, this year's vaccine appears to be about 50 per cent effective in reducing severe illness from the flu, he said.

That's a difference that people tend to undervalue, Kellner said.

"Looking at 50 per cent prevention of severe outcomes could certainly have saved a lot of misery across the country (this year)."

The fact that flu vaccinations are needed every year instead of getting a one-time shot also puts some people off, he said.

Plus, "there's so much fatigue" for public health measures from COVID-19, Kellner said.

"COVID vaccine initial uptake was so successful for a variety of reasons, but a big part of it was that there was this sense of public spirit around it in the collective willingness and desire to do the right thing," he said.

"It seems that we've lost that quite a bit right now."

Many people also don't take the flu seriously enough or realize how severe it can be, especially for young children, Kellner said.

"There's a sense that flu vaccine is not that important and so it's missed out on a lot."

Some people also mistakenly think they got the flu even though they were vaccinated, said Penner of the Canadian Paediatric Society.

"People's perception of what influenza or the flu is, you know, could be a wide variety of different viruses," he said.

Plus, if people do get the flu even though they've been immunized, the vaccine can make the illness less severe, Penner said.

How can we protect ourselves and our families?

The most effective ways to guard against the flu, experts say, are:

-- Get the flu vaccine. Doctors say it's not too late.

-- Stay home if you feel unwell

-- Masking

-- Frequent handwashing

This report by The Canadian Press was first published Dec. 16, 2022.

Canadian Press health coverage receives support through a partnership with the Canadian Medical Association. CP is solely responsible for this content.