Kids are getting hit hard by respiratory viruses. Here's what scientists know — and what they don't

Pediatric hospitals remain under intense pressure in Canada amid a resurgence of childhood respiratory viruses, ongoing shortages of children's pain medication, and the return of the annual flu season. 

Across the country, respiratory syncytial virus (RSV) infections are above expected levels for this time of year and keep increasing, federal data shows. Influenza is also spreading widely, and earlier than usual. 

It's clear health-care workers are scrambling to care for "unprecedented" levels of seriously ill young patients, with some Canadian facilities now resorting to surgery cancellations and patient transfers in order to make space.

What's less clear is — why are there so many sick kids, all at once?

While scientists say it's difficult to know the full answer, they do have a few theories.

When it comes to the pressure from RSV, an infection so widespread that most people catch the virus by the time they're toddlers, there may be a ripple effect from the early days of the COVID-19 pandemic.

Immunologist Deepta Bhattacharya, a professor at the University of Arizona, said RSV cases fell to very low levels in 2020, "presumably because of COVID mitigations" — a range of precautions that included social distancing, mask wearing, and widespread lockdowns during which a significant portion of the workforce began working from home.

There's a population-wide impact from skipping a year of infections, he said, since the immune system's antibody production after an RSV infection drops off fairly quickly.

That means more people are susceptible to infection in the first place, and those that do get infected don't have as many antibodies to slow the virus down, and mothers who weren't exposed recently aren't transferring as many antibodies to their infants through breast milk, Bhattacharya explained.

It's a situation several Canadian scientists warned about in a commentary on RSV published back in July 2021 in the Canadian Medical Association Journal.

"For many months, Canada has seen virtually no cases of RSV infection, which may mean that pregnant women and infants have had lower exposure and therefore pediatric immunity levels may be low," the group wrote, adding a resurgence in cases could stretch resources in pediatric intensive care units across the country.

Canada eventually experienced a mid-pandemic spike in RSV cases in early 2021 — but it didn't lead to the current level of hospital pressure. Notably, by that point, influenza still hadn't fully resurfaced, with just a slight rise in cases quite late into the typical flu season.

With flu now firmly back in the mix, alongside a slate of respiratory infections including COVID, co-infections might also be a factor in rates of severe disease, said Dawn Bowdish, an immunologist with McMaster University in Hamilton, Ont.

Each virus operates — and impacts the body — a little differently. Research on adults suggests people who are infected with multiple viruses at once, such as those behind COVID, along with flu or RSV, can face poorer outcomes, Bowdish said.