Testing for the novel coronavirus is rapidly expanding in Ontario, with the province’s public health agency actively searching for missed COVID-19 cases at hospitals in six cities and, likely soon, in doctor’s offices too.

So far, all of Ontario’s 24 confirmed cases of COVID-19 have been travel-related: the patients recently returned from countries or cruise ships where the infection is rampant, or are close contacts of those people. There has been no sign yet that the virus is circulating locally within the province.

But if local transmission is already occurring, we will likely find out soon. British Columbia announced its first apparent community transmission case Thursday evening, a woman with no recent travel history who was caught through precisely the same type of program Ontario is now rolling out.

Though it might be anxiety-provoking, any new cases detected by the expanded testing program would be a sign the system is working, said Amy Greer, Canada research chair in population disease modelling at the University of Guelph.

“I don’t think it would be evidence the situation is out of control. I would actually find it comforting to some degree,” says Greer, a sign our health-care system is catching those cases early unlike in the U.S. where late detection has in some cases been lethal.

“Once we know community transmission has started, then we can make better decisions about how we implement control measures,” Greer said.

Last Thursday, Public Health Ontario’s Toronto lab began testing for COVID-19 in samples taken from patients hospitalized in Toronto with acute respiratory illness. Those patients were already being tested for seasonal influenza, which shares some symptoms with COVID-19.

A subset of those flu tests were also tested for COVID-19. Of the 181 that had been processed as of Wednesday morning, all were negative.

“So far, so good,” says Vanessa Allen, chief of medical microbiology at the Public Health Ontario lab.

Now that program is being rolled out across Ontario, testing a subset of samples swabbed from suspected flu patients hospitalized in Ottawa, Kingston, Hamilton, Sudbury, and London, Allen says.

She says that the lab is also “actively exploring” piggybacking on an existing flu-monitoring program to start collecting samples from people who show up at their doctors’ offices with flu-like symptoms — a group likelier to have only mild illness.

Capturing people with mild symptoms who may have COVID-19 is particularly important, but also particularly difficult.

Data from China shows that the majority of people who catch the virus — around 80 per cent — only ever get mild to moderate symptoms. Those people are less likely to show up at hospital, because they aren’t very sick. But they are still capable of shedding the virus and spreading the disease, including to older people and those with underlying health conditions, the group most likely to suffer a severe course of illness and, in some cases, death.

Health officials in Washington state only recently discovered there was community transmission of COVID-19 when a small number of cases were detected, only to be quickly followed by a rash of deaths tied to a nursing home. Genetic analysis of the virus suggested that it had likely been circulating for weeks, potentially infecting hundreds of people who had gone undetected.

Experts have partly blamed limitations on testing for the unrecognized spread of COVID-19 in the U.S. Until the U.S. Centers for Disease Control and Prevention stopped reporting numbers earlier this week, it had processed fewer COVID-19 tests than Ontario, which has a fraction of the population.

Allen says the provincial lab is hoping to use an existing program designed to track how well the flu vaccine works, the Sentinel Practitioner Surveillance Network, to help reach Ontarians with milder respiratory symptoms. Family doctors and nurses voluntarily sign up to be part of the program every year, and send samples to be tested weekly from patients who have flu-like symptoms.

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By testing some of those for COVID-19, the lab can hunt for mild coronavirus cases that have flown under the radar, and would never show up at a hospital. The program is likely to be finalized soon.

“Because this is something we want to know sooner rather than later, we’re building on existing infrastructure in the province,” Allen says.

Expanding such “active surveillance” programs — proactively searching for missed cases — has to be balanced against the increasing demand for testing in patients who have travelled to affected countries and are actively under investigation for COVID-19, experts say. Running too many unnecessary tests can bog down the whole system and make it harder to get accurate results for actual cases.

The lab has also increased its capacity it recent days and is working to increase it further. COVID-19 testing is beginning at Public Health Ontario’s lab sites in Kingston and Hamilton, Allen says, so that tests can be done elsewhere in the province besides Toronto.

Positive tests also no longer need to be sent to Canada’s national lab in Winnipeg for confirmation, saving time, and the lab is hoping to bundle a set of two tests that are carried out on each sample into one run, doubling capacity.