Younger Canadians represent one in three of all reported COVID-19 cases, and experts say they could be unknowingly accelerating the spread of the virus in Canada and around the world.

Of the 4,186 COVID-19 cases for which the Public Health Agency of Canada has provided epidemiological data , 29 per cent are aged 20 to 39 and four per cent are under 19 — meaning one-third of cases in Canada involve people who are younger than 40.

Canada's chief public health officer, Dr. Theresa Tam, said Sunday that those under 40 made up about 12 per cent of hospitalized cases .

"This statistic is important because it shows that younger age groups are also experiencing illness severe enough to require hospitalization," Tam tweeted .

Steven Hoffman, director of the Global Strategy Lab and a global health law professor at York University, said that while the rates of hospitalization and death for younger Canadians are lower than older age groups, they're not insignificant.

"It's not even close to zero," he said. "Twelve per cent is still a significant number that should make any younger person stop and pause to recognize that this represents a threat not only to elder members of our society, but to everyone."

Hoffman said communicating the risks of dangerous activities to young people who think they're "invincible" is always a challenge, but the damage in ignoring containment measures like physical distancing and self-isolation can have devastating effects.

"It's just amplified during a pandemic because usually it's young people who, in ignoring understanding of risk, are endangering themselves and themselves alone," he said.

"In this context, young people's actions also can put other people at risk, especially the more older people that they come in contact with."

All age groups at risk of COVID-19

Dr. Raywat Deonandan, a global health epidemiologist and an associate professor at the University of Ottawa, said it's important for younger people to remember we've never encountered this new coronavirus, so we've built up no immunity to it.

"Everybody is susceptible," he said. "Everybody."

A man wearing a mask to protect against COVID-19 is seen in downtown Toronto on March 26, 2020. Canadians under 40 make up about 12 per cent of COVID-19 cases in hospital. (Evan Mitsui/CBC)

Deonandan said a misconception that older age groups are solely susceptible to serious complications from COVID-19 came from early data on China's cases, which showed elderly people — particularly those who smoked — were more likely to have bad outcomes.

"But by the time it made it to Europe, it was shifting dramatically," Deonandan said. "In Europe, we're seeing people being hospitalized a lot in their 30s, 40s and 50s and dying in that age group as well."

Deonandan said one of the key underlying vulnerabilities that puts every age group at risk is respiratory impairment, especially in people with conditions such as obesity, diabetes and asthma.

"That's almost everyone," he said. "I know people in their 20s and 30s who are a little overweight, who smoke, who vape, who enjoy inhaling marijuana — all of these things compromise your lung function sufficiently to make you vulnerable."

What role do children play in the spread of COVID-19?

Hoffman said that while youth under 19 only make up about four per cent of COVID-19 cases in Canada, they may be catalysts for spreading infection more widely because they interact with more people on a daily basis than the general population.

"There is a reason why school closures are one of the first things that happen in a pandemic," he said. "Schools are a place where social networks collide."

Hoffman said students from different social circles come into close contact with peers and teachers daily, meaning that if a virus were to spread in a school it would quickly jump to many different groups.

"Young people actually play a disproportionate role in infectious disease spread," he said. "And the expectation is that that trend would continue for COVID-19."

Hoffman said teenagers and young adults may also be driving the spread of the virus to older age groups in places like Italy, which has been hit particularly hard in the pandemic, with more than 100,000 cases and 12,000 deaths.

"Of course, in Italy, the big story has been about the elderly because it's the second-oldest country in the world after Japan and we've seen that toll in Italian hospitals," he said.

"But another part of that story is how has it spread so quickly? And there's been a lot of discussion and hypotheses about the role that young people have played in actually transmitting it even though they themselves haven't been as severely affected."

Dr. Alyson Kelvin says research shows children may pose a significant risk in the spread of COVID-19 to older age groups. (Liam Richards/The Canadian Press)

One peer-reviewed study released early in the journal Pediatrics this month looked at 731 confirmed cases and 1,412 suspected cases of COVID-19 among children aged two to 13 in China.

Researchers concluded that 50.9 per cent of the children had mild symptoms, 38.8 per cent had moderate symptoms and 4.4 per cent were completely asymptomatic.

Another smaller study published in the Lancet Infectious Diseases retroactively tested 36 children in the same households of confirmed adult COVID-19 cases in the eastern Chinese province of Zhejiang.

Researchers found that more than half of the cases were mild — seven had respiratory symptoms while 10 showed no symptoms at all.

"The data suggests that because children aren't developing as severe disease, they could be the facilitators of transmission," said Dr. Alyson Kelvin, an assistant professor at Dalhousie University in Halifax. "So bringing it into their homes and infecting their parents where their parents are more susceptible to getting sick."

Older Canadians are at significant risk of serious cases of COVID-19, but younger age groups may be putting them at more risk unknowingly. (Evan Mitsui/CBC)

Kelvin, who is also a member of the Canadian Centre for Vaccinology and part of a research team for a clinical trial of a COVID-19 vaccine , wrote about the significant risk children play in spreading the virus in a commentary on the study .

"They actually have a higher percentage of being asymptomatic. We don't know if they have it, so parents should stop sharing food with the kids," she said, "and make sure there are no quarantine playdates."

'Need to scale up testing'

Even though some children in the study were asymptomatic, Kelvin said one important finding was that they still presented evidence of pneumonia in their lungs — something she thinks is particularly significant for parents.

"If you had a child that had maybe some health issues, they could be more susceptible to hospitalization because their friends might not show that they're infected," she said. "But interaction might lead to something more severe in certain children."

Hoffman said that because young people are more likely to show milder symptoms of COVID-19, they're likely not going to hospital, being tested or being added to the daily confirmed cases in Canada and around the world.

"We need to scale up testing of whether someone has this virus," he said, "but also whether someone had the virus and whether they've developed immunity."

Hoffman said serological testing — blood testing that can detect if someone has been infected and detect antibodies — is the next key factor in determining the true spread of COVID-19 and developing an effective treatment.

"We're not there yet. We'll be there soon," he said. "And then youth will become a very important part of that."