Toronto health officials announced that the coronavirus is spreading locally on Monday, pleading for everyone to stay home who can — “social distancing” measures that research shows can massively diminish the spread of COVID-19, if adopted early enough and if followed by nearly everyone.

The city’s medical officer of health, Dr. Eileen de Villa, said Toronto Public Health is investigating three people who tested positive for COVID-19 and who had not recently travelled outside Canada or had close contact with someone who had. The city has 96 active cases overall.

De Villa urged dine-in restaurants, bars, nightclubs, and movie theatres to close, and asked all other businesses to help staff work from home. She told residents to limit gatherings.

“If you can stay home, do,” de Villa said at a City Hall news conference Monday.

“I am calling on our city to rise to this challenge to reduce the spread of this virus. We know that these measures that I’m recommending today work, based on the experiences of other jurisdictions.”

De Villa added: “I cannot overemphasize how important this is, particularly at this critical time.”

Research from other coronavirus-slammed countries strongly suggests that population-wide, early adoption of widespread social distancing, including workplace closures, is necessary to avert dramatic increases in the numbers of COVID-19 patients, hospitalizations, intensive care admissions, and deaths. Keeping some parts of society home and not others is insufficient.

Travel restrictions, like the ones Prime Minister Justin Trudeau implemented Monday, are less effective in blunting the rise of new cases. Trudeau also urged Canadians to stay home.

China, where the outbreak was first detected in December, implemented dramatic measures in late January to slow the spread of the disease. A modelling analysis of those interventions by researchers at the University of Southampton in the U.K. showed that without intense social distancing and aggressive case detection and isolation, China’s COVID-19 case count would have been 67 times higher — more than 5 million sick people, rather than the 81,000 reported so far. China’s travel restrictions were less effective than the combination of social distancing and aggressive detection.

“There’s diminishing returns if you reduce contacts in certain parts of the population but not others,” said Nick Ruktanonchai, one of the lead researchers on a pre-print study that modelled the effects of China’s response.

Their research also showed that the timing of these interventions has a major impact. If China had waited one week later, the number of cases would have tripled to 243,000. Waiting two weeks saw cases balloon by sevenfold, to more than half a million.

If the country had implemented these measures one week earlier, its approximately 81,000 cases could have been reduced by two-thirds, to 26,700.

“What we definitely, decidedly found is that earlier is better,” said Ruktanonchai.

The paper, published on the medical pre-print server MedRxiv, has not been peer-reviewed, like much research on COVID-19, which is being disseminated as quickly as possible given the scale of the pandemic. More than 7,100 people have died worldwide, and some countries, like Italy, have seen hospitals in affected regions functionally collapse by a deluge of critically ill patients.

Other pre-print studies have made similar discoveries. Japan’s decision to voluntarily cancel events was only effective in reducing the infectiousness of COVID-19 by 35 per cent, with cases continuing to spread. On the other hand, researchers in Hong Kong found the outbreak was effectively brought under control through containment, social distancing, and people changing their behaviours.

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The U.K. study supports outbreak modelling by Ontario researchers that suggests half-measures are insufficient to contain the spread of the outbreak. That research, led by scientists at the Universities of Toronto and Guelph, predicted that unless nearly all Canadians dramatically reduce social contacts, critically ill patients will overwhelm the capacity of intensive care units.

U.S. jurisdictions began imposing severe limits on the public as well. Six counties in California ordered residents not to leave their houses for three weeks except to meet essential needs. The governors of New York, New Jersey and Connecticut ordered many non-essential businesses to close. The ability to detect and contain the illness in the U.S. remains severely hampered by a lack of testing.

It remains to be seen whether the urging of Canada’s, Ontario’s and Toronto’s medical officers of health will be enough to prompt widespread-enough social distancing, or whether the measures have come early enough in the outbreak.

De Villa said Monday more widespread closures or stricter enforcement would depend on how the situation unfolds.

“The next thing that needs to happen will depend on how the disease and the outbreak evolves.”