Young and old more likely to face severe flu. Here's why doctors think it happens

Canadians have been getting sick enough with seasonal flu to land in hospital, say doctors with suggestions on who is most at risk and what it could mean for festive gatherings.

"We're starting to now see the effect of flu on certain populations, particularly very young children and very older people, in making them sick enough that they need to come into hospital," said Dr. Gerald Evans, chair of the division of infectious diseases at Queen's University and Kingston Health Sciences Centre.

During the depths of the COVID-19 pandemic, air travel declined. It's one of the suspected reasons that influenza all but disappeared, Evans said.

Flu viruses need human hosts travelling between the southern and northern hemispheres to gain a foothold during winter on both ends of the planet, according to influenza experts.

For about 100 years, doctors have known that the youngest and oldest are most at risk for serious flu. Why hasn't been nailed down, but there are a few possible reasons — including what strains were circulating when you were first exposed.

Canadian and international research on humans as well as in animal models suggest that the first strain of flu virus you're infected with tends to prime or shape the immune system. The result is that our immune system responds best to the original type of flu infection it faced.

"That's why we believe that older people who are mostly primed with H1N1 don't do very well during an H3N2 year like we're having this year," Evans said.

The 2009 H1N1 pandemic also continues to affect how younger ones do with flu.

Those aged 13 and under were probably primed to H1N1 after 2009, just as their grandparents were in their childhoods, Evans said.

If so, today's kids could be more vulnerable to severe disease from flu now than their parents' generation who first encountered an H3N2 strain.

Evans added it's also thought that older people may have more severe outcomes from flu because of underlying problems such as heart disease, lung disease or treatments for cancer.

Another reason why young children are being hit hard by flu and RSV this year: recent pandemic public health measures meant those under two haven't seen flu at all and preschoolers haven't experienced it or another respiratory virus known as respiratory syncytial virus, or RSV, for a couple seasons.

"The boost of immunity they get from having had some prior exposures in the year before are missing and so they're tending to get infected more," Evans said. 

Dr. Upton Allen, chief of infectious diseases at the Hospital for Sick Children in Toronto, pointed to a few other possibilities.