More young people are being infected with COVID-19, creating the potential for a severe outbreak, scientists warn.

They say the change in demographics could be attributed to lockdown fatigue, increased testing or feelings of invincibility.

Brett Snider, a water resources PhD student at the University of Guelph, first noticed a higher number of infections in the under 20 and 20-29 age groups in the Toronto and Peel regions in May.

Snider's supervisor, Ed McBean, said he was puzzled by rising infections in the under 20 age group because daycares and schools were closed.

"And we thought 'whoa'. The little ones don't tend to get violently ill, but they will bring it home. And if you have multiple generations or even visitors, you get that uptick as a result," said McBean, an engineering professor at the University of Guelph.

While the numbers have stabilized in Peel and Toronto, McBean said there's been an increase in infections in younger people in other parts of the country.

"That's critical because once you start to get an uptick, it can explode very quickly," he said.

McBean said he's especially worried about the increase in infections in the under-20 age group.

Children who may be sick don't usually show symptoms and when they share a space or toys, they might spread the infection to others and then carry it home, he said.

"If we start to see an increase in the younger age category there's no doubt that it'll eventually spread from the parent to the grandparent, and it can have severe impacts on older populations."

McBean said his message for public health officials is to keep daycares and schools closed.

Data from British Columbia, Ontario, Alberta, Saskatchewan, Quebec, Manitoba, Nova Scotia, New Brunswick and Prince Edward Island show an increase in COVID-19 infections in the 20-29 age groups. The figures also show more women are testing positive for COVID-19.

Numbers from the Public Health Agency of Canada show that during the second week of July, the largest proportion of new cases reported -- 22 per cent of female cases and 28 per cent of male cases -- was among the 20-29 age group.

Since the start of the pandemic, 14 per cent of female cases and 15 per cent of male cases have been up in that age group.

The younger age groups are now making up a higher percentage of positive tests, while older age groups are falling overall, the data show.

A clinical professor at the University of B.C.'s school of population and public health said the increase could be because it's summer and fewer employment opportunities during the pandemic mean more time for young people to spend with friends.

Stephen Hoption Cann said when they get together in larger groups, young people can potentially spread the virus because they may not show symptoms or have a mild infection.

"They have to be cautious in social gatherings, pubs and bars," he said.

"You never really know when you're close to the person who might be infected so you have to be more cautious."

Dr. Barbara Yaffe, Ontario's associate chief medical officer of health, said health officials have noticed the same trend, noting healthy people either don't show symptoms, have very mild symptoms or they don't know they have COVID-19.

"I think that contributes to potentially, some complacency," she said at a recent news briefing.

While young people may not see the infection as a "big deal," they may pass it on to a person they're close with who has an underlying medical condition, Yaffe said.

B.C. public health officials said early on many long-term care and assisted living facilities were hard hit and now most of the province's new cases are in the broader community.

"We are concerned about the increase in new cases in recent days as COVID-19 continues to silently circulate in our communities," B.C.'s health minister and provincial health officer said recently in a statement.

Another factor for consideration is a larger number of infections among women.

Julia Smith, a research associate at Simon Fraser University's faculty of health sciences, said Canada had a higher proportion of women infected than other countries near the start of the pandemic.

Age and employment may be factors, she said.

Most residents of long-term care homes tend to be women, she noted. Such homes were the first to be hit with COVID-19 in Canada, killing thousands of seniors.

Women also tend to make up a larger share of health-care workers who would have more interactions with patients, putting them at greater risk of infection, Smith said.

McBean said the messaging to maintain safe social distancing and frequent handwashing, wear a mask and get together in small groups outdoors remains the best way to avoid infection.

Keeping the Canada-U.S. border closed for as long as possible would also help prevent external infections from coming into the community, he said.

McBean said provinces need to continue to follow strict protocols to keep the virus contained, and keep the R value below one. The R value or reproduction rate represents the average number of new people who will catch the disease from one infected person.

If R value is less than one, then the epidemic is being brought under control. If R is greater than one, then the epidemic is growing.

"We're on a very fine line as it is right now in Ontario. So, we just have to be very careful in that number doesn't start to creep up again and be above one," McBean said.

McBean said provinces need to continue to follow strict protocols to keep the virus contained.

"Basically, we're doing pretty well," he said. "I mean this is a terribly infective virus and so easily transmitted. It's much worse than the others. This one's pretty bad."

This report by The Canadian Press was first publish July 19, 2020.