Researchers are finding mounting evidence that asymptomatic COVID-19 carriers are "driving" the rapid spread of the virus around the globe — contradicting what Canadian officials were saying just weeks ago.

At the end of January, Canada's chief public health officer, Dr. Theresa Tam, told the House of Commons health committee that based on what is known about past coronaviruses, asymptomatic transmission is a "rare event." In fact, she said, epidemics are not driven by that kind of transmission.

A few days after that, Health Minister Patty Hajdu echoed Tam in an interview on CBC Radio's The House:

"The best evidence around the virus that we have is that the virus is not contagious when people are not symptomatic."

But a growing body of research indicates they were wrong. In fact, people don't have to appear ill at all to infect others.

Jeffrey Shaman, a professor of environmental health sciences at Columbia University in New York, says he is frustrated when people deny that asymptomatic spread can happen.

"We have so much evidence that that is going on," he said. "It's ridiculous."

It remains unclear if understanding the threat earlier might have affected policy. Had the threat been fully realized, different decisions might have been made regarding travel restrictions, quarantines and physical distancing.

Asymptomatic patients among COVID-19 caseload: studies

Shaman and other researchers argue that even two months ago, officials like Tam and Hajdu should have been more open to the possibility of asymptomatic transmission, considering by that point there was a flurry of research being undertaken by scientists racing to understand how the virus was spreading so fast and far. Many of those researchers suspected asymptomatic transmission.

As the results of much of that research are so new, some of the findings have yet to be peer-reviewed. But, they appear to point in one direction.

"That is what our Science paper suggests, that it is undocumented infections that are driving it," said Shaman. "They're silent spreaders; it's stealth transmission."

Shaman is co-author of one of several studies looking to figure out what proportion of people with COVID-19 are asymptomatic and how infectious they are.

A joint Japanese-U.S.-U.K. study looked at data from the passengers on the Diamond Princess cruise ship who were quarantined for two weeks in Japan after the ship had an outbreak in February. The researchers determined that nearly 18 per cent of those who were infected never showed symptoms.

A smaller Japanese study of 565 Japanese evacuees from Wuhan, China in February found that out of those who tested positive for COVID-19, nearly 31 per cent were asymptomatic.

In the Italian town of Vò, where Italy recorded its first death, every one of its 3,300 inhabitants was tested. The majority of the three per cent who tested positive had no symptoms.

Large-scale testing in Iceland found about half of those who tested positive had no symptoms.

Viral shedding starts early, German study says

But that's only half the story. The asymptomatic can also spread it, and do so to a significant degree.

Shaman's study used mathematical modelling techniques based on real COVID-19 testing data to simulate what was happening among people in 375 cities in China.

His research found that while undocumented cases — those with mild or no symptoms who did not have a confirmed diagnosis at the time — were only half as infectious as symptomatic ones, they were the source for nearly 80 per cent of documented cases. That's because people who feel fine are the ones out and about, travelling and interacting with more people.

Asked whether it's fair to say these so-called silent spreaders are "super-spreaders," Shaman replied: "Yes."

A study of COVID-19 patients in Munich, Germany showed that viral shedding starts early after infection onset, making this virus very different from severe acute respiratory syndrome (SARS). It also found that the capacity to shed virus can last after symptoms clear up. Another study of patients in China found that the viral load in both symptomatic and asymptomatic people was similar.

In other words, people don't have to appear very sick to infect others.

"It's an unfortunate characteristic of the virus because this really facilitates the spread through the community and the world," said Dr. Sumon Chakrabarti, an infectious disease physician at Trillium Health Partners in Mississauga, Ont.

Chief public health officer Dr. Theresa Tam speaks during a news conference in Ottawa last week. On Thursday she acknowledged the growing evidence that COVID-19 can be spread by asymptomatic people. (Adrian Wyld/The Canadian Press)

On Thursday, Tam acknowledged the growing evidence.

"We're always looking at science as it evolves. We've, of course, had cases that are pre-symptomatic or asymptomatic. What we didn't know before was how much does that play in the force of transmission," she said at a news conference in Ottawa.

"And so if we have to change our approach, we will."

The health minister's thinking has clearly changed, too.

"This is a sneaky virus," Hajdu said Wednesday. "Some people don't feel ill at all. And that's why the … physical distancing is so critically important. We have to act as if we are all carrying this virus."

Expect physical distancing measures to continue

Dr. Allison McGeer, an infectious disease specialist with Sinai Health System in Toronto, argues while Tam's messaging about asymptomatic spread back in January "might not have been ideal," there's a good chance people would not have been onside with major restrictions to their lives before they could see proof of how serious the problem was. After all, Canada only reported its first case of COVID-19 in late January.

"There is evidence that if officials appear uncertain about things, people lose trust. They get angry. They don't follow guidance. So we put this demand on public health people to have answers when there are no answers. And that's just an impossible situation," she said.

Doctors and researchers say more testing — even of those without symptoms — is a must. And they say Canadians should prepare for a continuation of distancing measures, including business and school closures. In some countries, it might mean overt directives for the general public to wear face masks.

Just like lessons were learned from the handling of SARS in 2003, so too are lessons being learned this time around.

"If ... we ever have another pandemic of a respiratory virus, we want to consider the possibility of asymptomatic spread right from the beginning," said Chakrabarti.