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.

RSV roars back after pandemic drop

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.

Virus transmission ebbs and flows each year

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.

On its own, RSV is largely known for causing mild symptoms in most children and healthy adults, but infants, older adults, and people with compromised immune systems can experience serious illness.

Research suggests that in some cases, infections in infancy may be linked to the later development of severe RSV or other long-term respiratory system impacts and it's a virus that's capable of infecting people over and over again throughout their lives.

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"RSV was the leading cause of infant hospitalizations long before anybody had ever discovered COVID-19," said infectious diseases physician Dr. Amesh Adalja, who works with the Johns Hopkins University Center for Health Security. "And RSV had a considerable toll on adults, similar to influenza."

And it was always bound to come back, along with flu and other temporarily-suppressed pathogens, he added.

"It wasn't like we would always not have respiratory viruses because of COVID," Adalja said. "And you have children that were born around the pandemic who just have not gotten RSV."

One team of Canadian researchers analyzed more than a decade of respiratory virus seasons, alongside mathematical modeling for possible future seasons, and found "striking regularity" in how multiple viruses alternated between mild and severe winter peaks, year by year.

Their 2021 study, published in the scientific journal JAMA, suggests a certain level of predictability, said University of Alberta infectious disease specialist Dr. Lynora Saxinger.

Lower transmission years are typically followed by a stretch of more aggressive transmission, due to the loss of immunity across an entire community, she said. That could partially explain this year's return of RSV, and the aggressive start to the flu season not long after.

"Next year we would expect a smaller wave if most kids get exposed this year," Bowdish said.

What role did COVIDplay?

In the meantime, as scientists are scrambling to understand the current viral landscape, other theories attempting to explain the ongoing pediatric health-care crisis involve potential immune system impacts from prior bouts of COVID-19, perhaps in line with already-documented post-COVID health issues.

After getting infected with SARS-CoV-2, some children face a serious condition known as multisystem inflammatory syndrome, or MIS-C. The cause remains unclear, though it's thought to be an overactive immune response.

In fairly rare circumstances, other kids and more often, adults can face lingering, sometimes debilitating, long COVID symptoms long after an infection, which might also be tied to the virus' impacts on the immune system.

In both cases, experts note the mechanisms at play are still not fully understood.

But does this mean childrens' immune systems are going haywire after they catch COVID, putting them at risk of falling seriously ill from other bugs? That's not likely, several medical experts told CBC News.

Bowdish called the notion "unproven but not impossible," while Bhattacharya stressed there's scant evidence suggesting COVID leads to lasting immune suppression against other infections.

During the acute phase of COVID-19, the T-cells in your blood which are a part of the human immune system that target specific foreign invaders do drop in number. "But then they come right back to normal after you recover," Bhattacharya said.

"I do think it's possible that additive inflammation, if you get COVID and RSV in close succession, could make you really sick," Bhattacharya said. "But that is very different than COVID causing immunodeficiency lasting long into recovery."

Dr. David Naylor, co-chair of Canada's COVID-19 Immunity Task Force, said the "plausible" role for COVID in the current pediatric crisis is as a co-factor, disrupting immune responses and exacerbating the impact of all those first exposures to other viruses.

"The evidence for that concept is weak so far," he said in an email to CBC News, "but this nasty virus is full of unpleasant surprises."

Calls for masks, vaccination campaigns

Parents and health-care workers alike are struggling to care for children at a time when hospital beds are full and medication shelves are empty.

In recent weeks, multiple pediatric health-care facilities across the country have reported being well over capacity and short on staff, while federal officials say they've finally secured a foreign supply of children's fever and pain medication to ease a months-long shortage, hopefully giving parents a greater ability to treat sick children at home.

Conversations around the use of masks to mitigate virus transmission are also ramping up, with leading public health officials in several regions calling on the public to mask up this fall and winter.

Widespread vaccination campaigns for flu and COVID are important as well, Saxinger said, alongside other protective measures.

Adalja, meanwhile, called for better viral surveillance. RSV has remained a "severe pediatric burden for decade after decade after decade," he said, adding testing for the virus needs to become more routine.

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CBC News is granted rare access inside Toronto's Hospital for Sick Children as staff try to keep up with a spike in seriously ill children sick with respiratory illnesses.

In Ontario, recent epidemiological surveillance data does show some early, hopeful signals that respiratory virus pressure on hospitals may begin easing, and at some point in the years ahead, there could be vaccines against not just flu and COVID, but also RSV, should clinical trials go well.

Still, kids will always be more vulnerable to these kinds of pathogens, including some too young to be vaccinated, warned virologist Alyson Kelvin, a researcher with the University of Saskatchewan's Vaccine and Infectious Disease Organization.

"We know that they make up one of the highest groups of hospitalizations for respiratory viruses," she said. "And we always want to make sure that we're protecting them."