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The COVID Alert app is seen on an iPhone in Ottawa, on July 31, 2020.Justin Tang/The Canadian Press

In mid-February, British Columbia had counted just five confirmed infections from the new coronavirus. Of those, four were linked to Wuhan, the Chinese city where the virus was first detected, and one to Shanghai. Then the contract tracing team at the Fraser Health Authority made a troubling discovery.

Case 6, a woman in her 30s, had recently returned from Iran – a country that had just started acknowledging its first COVID-19 cases. It hit Aamir Bharmal, the medical health officer overseeing a small contact tracing team, that the disease was much more widespread than anyone had realized.

“This was a sign that there were countries where there was probably a lot more COVID activity that people were just not aware of,” Dr. Bharmal recalled. “It was pretty momentous. It was also an indication that we needed to step up, in terms of how we were going to approach it.”

Contact tracing, an old pillar of communicable disease control in public health, has been a vital tool in the fight against COVID-19. When a person is confirmed with an infection, disease detectives work to identify, manage and assess those who have been exposed, breaking chains of onward transmission. The information uncovered also sheds light on how a new disease, such as COVID-19, is spread.

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Fraser Health’s Iran case sparked an international investigation, with the World Health Organization’s director-general, Tedros Adhanom Ghebreyesus, telling a news conference days later that such cases were “very worrisome” and that the window to control the outbreak could be closing.

But for contact tracing to be effective, public-health units must have robust and timely testing systems in place, paired with a comprehensive public-health response and public buy-in. In Toronto, Ottawa and Montreal, soaring infection rates have outstripped case and contact management resources, necessitating stricter population-level restrictions until new infection rates become more manageable. Quebec Health Minister Christian Dubé has said up to 30 per cent of people are not responding to calls or e-mails from the province’s health department, while contact tracers in Alberta are reporting more people refusing to provide tracing information.

These are serious setbacks in a strategy that is centred on speed. In a study published in The Lancet in July, Dutch researchers found contact tracing has the potential to control virus transmission – and to lessen the need for more restrictive measures, such as lockdowns – but only if all delays are reduced as much as possible.

In a best-case scenario where a person is tested and receives results as soon as symptoms develop, the mathematical modelling study predicted the number of people that person infects – called the R0, or reproduction number – is reduced to 0.8, from 1.2. For infection to slow or stop, the R0 must be below one.

In that scenario, at least 80 per cent of infectious people who develop symptoms would have to be tested, 80 per cent of their contacts reached the same day, and all adhere to isolation protocols.

With a one-day testing delay, the model predicted a reproduction number of one. At two days, tracing had to be completed within one day and at least 80 per cent of contacts reached to keep the reproduction number below one. Once testing delays reached three days, the model found that even perfect contact tracing could not keep the reproduction number below one.

“We conclude that reducing the testing delay – i.e. shortening the time between symptom onset and a positive test result, assuming immediate isolation – is the most important factor for improving contact tracing effectiveness,” the researchers wrote.

In Ontario, which is seeing a seven-day average of about 800 new cases a day and has a backlog of more than 26,000 tests, public-health units have had to prioritize their contact tracing.

Contact tracers in Toronto are calling only confirmed cases and asking them to notify their high-risk close contacts to self-isolate. Ottawa is doing similarly, following up only with contacts of confirmed cases who are linked to higher-risk settings such as schools, hospitals or long-term care facilities. Lower-risk contacts are directed to an online form to enter their own information.

Premier Doug Ford on Wednesday said the province has 2,750 contact tracers, along with 600 Statistics Canada employees onboarded since July, and will hire 500 more contact tracers and managers by mid-November.

Montreal is contacting only those who have tested positive and those at high risk of exposure; regional health authorities have posted information online explaining what constitutes a close contact, with directions for positive cases to notify those contacts themselves.

In Alberta, Chief Medical Officer of Health Deena Hinshaw pleaded with Albertans to co-operate with contact tracers.

“It is understandable that people are tired of COVID and angry at the ways that their lives have been disrupted," she said Tuesday. “Unfortunately, choosing not to work with contact tracers does not make that better; it makes it worse. If we are not able to trace contacts and prevent the virus from spreading, the impacts will continue to grow."

In B.C., which implemented broad public-health measures early in the epidemic and hired hundreds of new contact tracers in anticipation of a second wave, health units are still calling every person who may have been exposed to someone who tested positive for COVID-19.

Dr. Bharmal’s team, which grew to about 200 members from 14, has tracked more than half of the province’s 10,892 cases.

Ashleigh Tuite, an infectious disease epidemiologist at the Dalla Lana School of Public Health at the University of Toronto, said it makes sense to suspend contact tracing when resources are limited and case counts high, as the tool loses its effectiveness.

“But pulling back from contact tracing means that you should compensate in some other way, which would be having more restrictive measures in place,” she said.

Susy Hota, the medical director of infection prevention and control at the University Health Network in Toronto, said she is worried about putting the onus to notify family, friends and co-workers on people who have just learned they’re infected, some of whom will be seriously ill.

“The overall risk is that it’s now up to the individuals who are diagnosed with COVID-19 to be telling their close contacts about their diagnosis,” Dr. Hota said. “Some people will not feel comfortable doing so because of fears of repercussions, especially if you’re talking about close contacts in a workplace. You might feel alienated or like you’d be stigmatized.”

The federal government’s COVID Alert smartphone application is playing a role in facilitating anonymous exposure notifications.

A study by a team at Oxford University’s Nuffield Department of Medicine found app-based contact tracing can stop transmission if approximately 60 per cent of the population uses the app and can still slow transmission with lower uptake rates.

As of Wednesday, more than 4.3 million people have downloaded the Canadian app, and seven provinces and territories are online.

With a report from Kelly Grant

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