Front-line healthcare workers are bracing for a "surge" in COVID-19 patients, which could hit Ontario hospitals within weeks — though the exact timing, experts warn, is hard to predict.

The urgency comes as the number of lab-confirmed cases across the province keeps ticking upwards, hitting more than 850 by Thursday afternoon and fuelling ongoing questions about hospital readiness and equipment levels.

"It's almost inevitable that we are going to see a surge," said Dr. Andrew Morris, an infectious disease specialist with the Sinai Health System and University Health Network in Toronto.

"When that occurs, it's not precisely known, but I think our best estimates are that we'll start seeing a rise in the next week or so and it will continue to rise for weeks thereafter, if not months."

Close to 60 per cent of Canadians who tested positive for COVID-19 so far likely acquired it in their communities, according to the latest available federal government data. There are also still concerns over transmission tied to travel, given the recent rush of residents returning from abroad amid a flurry of government warnings to come back home.

"It kind of feels like we're all collectively holding our breath, because we have this sense something may be coming — but we don't know when," said epidemiologist Ashleigh Tuite, an assistant professor in the University of Toronto's Dalla Lana School of Public Health.

While Tuite agreed cases could appear to surge upward in the weeks ahead, she stressed it may be partly due to public health agencies slowly ramping up their testing capacity, revealing carriers in the community who could be shedding the virus without knowing it.

Roughly 11,000 Ontario residents who have been tested for COVID-19 are still waiting for their results amid a backlog at the province's public health lab, while untold numbers of potentially-positive cases haven't been tested.

The number of new lab-confirmed daily cases has been largely rising since the beginning of March, however, offering some insight into how the outbreak is progressing.

At the start of the month, public health officials tallied up only a handful of known new cases each day at the most, and sometimes none at all. By the end of the month, new daily case counts were routinely in the double-digits — hitting 85 in a day on March 24, 100 on March 25, and 170 on March 26.

Dozens of people have been hospitalized to date across Ontario, and right now, at least 29 patients remain in intensive care units, including 20 who are on ventilators to help them breathe, provincial officials said on Thursday.

Fifteen people have also died so far, with six deaths reported between Wednesday and Thursday alone.

"In the context of where we are right now in the outbreak, having better numbers in terms of how many cases are requiring hospitalization — and in particular, the number of cases being admitted to the ICU — will be better metrics over the next couple of weeks," Tuite noted.

In an ideal scenario, she said the number of new cases will level off rather than surge upward, preventing the healthcare system from being overwhelmed.

Hospitals across Ontario are bracing for a surge of COVID-19 patients, but it's not exactly clear to epidemiologists when it will occur. (Evan Mitsui/CBC)

'We know it is a marathon ahead'

That's the key reason why public health officials across the country have spent weeks urging residents to stay apart from each other, in hopes voluntary physical distancing measures could reduce the virus' spread.

The federal government also made it mandatory for all travellers returning to Canada to isolate for 14 days and officials have suggested more restrictions may be necessary.

Since the impact of those measures isn't yet clear, clinicians are preparing for the "absolute worst case scenario," said Dr. Chris Mackie, Middlesex-London's medical officer of health.

He stressed patient volumes could prove challenging if case counts quadruple or more each day, bringing local hospitals slightly closer to the overwhelming caseload seen in countries like Italy and Spain.

"We very clearly know we need to be ready at a moment's notice to care for a huge surge in patients that are affected by COVID-19," Dr. Carolyn Snider, chief of emergency medicine at St. Michael's Hospital in Toronto, told CBC News.

The downtown Toronto hospital has been running simulations with staff while developing changes to how people flow through the emergency department — ensuring when patients arrive, they're quickly identified as potentially having COVID-19 and streamed into a separate area.

"We know it is a marathon ahead of us," Snider said. "We'd love to be able to keep delaying that time when the pistol goes off, but if it goes off, we have plans already in place."

As hospitals prepare for a rising number of patients with COVID-19, concerns are growing about potential shortages and rationing of protective equipment. (Paul Borkwood/CBC News)

Concerns over equipment shortages, rationing

Despite hospitals' best efforts, system-wide concerns remain over equipment shortages, following reports of mathematical modelling suggesting Ontario could rapidly run out of intensive-care beds and ventilators and memos obtained by CBC News revealing major Toronto hospitals are urging front-line workers to ration surgical masks.

Provincial officials have strived to allay fears, noting the government has secured millions of masks and gloves to date, and is considering deploying some of the 55-million expired N95 masks the province stockpiled during the SARS outbreak in 2003.

The province also announced major investments on Wednesday aimed at adding 1,000 acute care and 500 critical care beds, boosting surge capacity in long-term care facilities, and purchasing critical medical supplies and protective gear.

Amid all the concerns and precautions, Morris said it's still "very difficult" to predict how well Ontario will navigate what's to come.

"What will really determine our success, and being able to handle the surge, is just the overall number of people (with COVID-19)," Morris added. "And that's really related to social or physical distancing."

Mackie said even if Ontario sees a peak in the number of daily cases, that leaves another half of the wave of cases still to come, which could require residents to stay apart far longer.

And, he added, this new coronavirus could one day prove to re-emerge on a seasonal basis — meaning it may taper off each spring, and return again in winter when temperatures drop and indoor air becomes drier.

"Everything possible to be prepared is being done," Mackie stressed. "That said, this is a disease that's unpredictable."