Canada's top doctor says there isn't enough evidence to back herd immunity as a way to reopen society, as Quebec's premier is considering the approach to restart his province's economy.

"The idea of ... generating natural immunity is actually not something that should be undertaken," Canada's Chief Public Health Officer Theresa Tam said Saturday, urging people to be "extremely cautious" about the concept.

Herd immunity is conferred when enough people in a given population have been infected with a virus, marking them immune to reinfection and slowing down the rate at which the virus spreads on its own.

The World Health Organization (WHO) published a brief Friday stating that there is "currently no evidence" that people who have recovered from COVID-19 and have antibodies would be protected from a second infection, but clarified Saturday that most people infected would end up with "some level of protection."

Earlier today we tweeted about a new WHO scientific brief on "immunity passports". The thread caused some concern & we would like to clarify:<br><br>We expect that most people who are infected with <a href="https://twitter.com/hashtag/COVID19?src=hash&ref_src=twsrc%5Etfw">#COVID19</a> will develop an antibody response that will provide some level of protection. <a href="https://t.co/AmxvQQLTjM">pic.twitter.com/AmxvQQLTjM</a> —@WHO

Tam's comments come as Quebec Premier François Legault expressed interest in herd immunity this week as a means to reopen businesses and allow children to return to school.

"The idea is to gradually — and that's the important word — to gradually let people go out, let children go out," he said Thursday, adding that those under 60 years of age might be candidates for developing immunity.

Tam rejected the suggestion that in the absence of a vaccine, some members of Canada's population could offer protection to society's most vulnerable.

"Even a young person might get severely sick or get into the ICU, so it's not a concept that should be supported," she said.

In response to Tam's remarks, Premier Legault's office said the province plans to forge ahead with easing lockdown restrictions, but only with the approval of Quebec's public health department.

'More caution'

Legault is not alone in searching for solutions to reopen his province's economy, as nationwide shutdowns pass the one-month mark.

On Friday afternoon, Prime Minister Justin Trudeau and Canada's premiers agreed to work on a joint set of national guidelines that would lay out how to carry out the process.

The prime minister said Saturday that those plans do not rely on using immunity as an interim form of protection.

"In the approach that we're taking very carefully around the provinces and across the country on looking at reopening, I don't believe that there are any plans that hinge on certain people or individuals being immune or having immunity to COVID-19," Trudeau said.

The federal government has committed millions of dollars toward a new COVID-19 immunity task force focused on researching immunity testing and developing a vaccine — something Tam said is still in its early stages.

"Until we have those answers, we need to... err on the side of more caution," Trudeau said.

WATCH | Trudeau discusses joint guidelines for reopening the economy:

Prime Minister Justin Trudeau discussed how Canada's provinces are working on a jointly-drafted set of guidelines to establish principles for reopening the country's economy. 0:42

'Immunology' of the virus not known

Allowing people with protected immunity status back into society has captured international attention, with countries like Chile moving to issue "immunity passports" to those who have recovered from the virus.

The passes have been touted as a way to gradually exempt people from restrictions put in place to limit the spread of COVID-19, permitting them to return to work, attend mass gatherings or even travel across borders.

But Tam warned that research on whether immunity is possible — and how long it might last — remains murky.

"It is a very novel virus," she said. "We don't understand the immunology of this virus very much."

WHO cautioned in its brief that due to a lack of evidence, doling out passes based on recovery from the virus could lead to people ignoring public health advice when they could still get re-infected and continue the chain of transmission.

WATCH | Dr. Tam says it's too early to think about immunity passports:

Canada's Chief Public Health Officer Theresa Tam said that without a clear understanding of immunity tests for COVID-19, it's too early to think about issuing passes for those who might be protected. 0:53

Passports may become necessary, doctor says

For Dr. Kumanan Wilson, a physician at the Ottawa Hospital and innovation adviser at the Bruyère Research Institute, immunity passports will be key to returning to society if the science is there to support it.

Wilson is hoping to partner with the federal government to launch a digital identification system that shows who is protected from the virus should immunity tests become available.

"It is possible that the science doesn't demonstrate that you do develop long-lasting immunity, in which case [passports] would not be an appropriate way to proceed," Wilson said. "But if the science does demonstrate that this is a good solution, it will be important to have a technological solution in place."

Wilson has proposed a system that would arm Canadians, or people around the world, with a digital record of their level of protection, which would be scanned to enter workplaces, sports stadiums and airports.

LISTEN | Dr. Kumanan Wilson speaks to CBC Radio's The House about immunity passports:

Ottawa physician Dr. Kumanan Wilson discusses the concept of immunity passports — and their repercussions — as a stepping stone to reopening the economy in the absence of a COVID-19 vaccine. 12:49

He acknowledged that classifying people based on protection status could lead to other concerns, such as some citizens having better access to immunity tests than others, or those compelled to falsify their status to re-enter society.

But he said he's also conscious of the need for life to return to normal as quickly as possible.

"We know that for every week, every day that we wait to get people out of social isolation, it's causing quite a bit of harm."