Open this photo in gallery Kirt Browne, left, and Calvin Langille, clean the subway touch points with disinfectant during a visual demo of the enhanced cleaning measures being used by the TTC at the Wilson Yard in North York on March 3, 2020. Passengers who don’t show symptoms and are not contacted by health authorities have no need to seek medical attention, officials said. Tijana Martin/The Globe and Mail

A man diagnosed with COVID-19 used Toronto’s local transit system on three days this week after returning from a trip to Las Vegas, sparking an internal effort to identify the specific vehicles he rode.

The case was announced Friday and is the first known instance of someone carrying the novel coronavirus being on the Toronto Transit Commission, which transports about 1.7 million riders daily. His travels Monday through Wednesday also included rides on GO Transit buses and coincides with transit workers agitating for the right to wear masks on the job.

Public health officials said the risk to other transit riders and to staff remains low and that no extra precautions have been ordered. The city’s chief medical officer of health said her family would continue to use transit.

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“This information is being provided out of an abundance of caution. We’re talking about low-risk situations,” said Eileen de Villa, Toronto’s top public health doctor, in a briefing.

The man boarded the subway at Bathurst station each morning around 8:50. He travelled westbound to Islington station before transferring to a 108N Mississauga Way express bus, which he took to his workplace. The man got back on transit around 6:10 each evening, taking the 27 Milton GO bus to Yorkdale station. He continued from Yorkdale to St. George station followed by Bathurst station. On March 4, the man also rode the 511 Bathurst streetcar. Dr. de Villa said more details would be provided once they are available.

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Passengers who don’t show symptoms and are not contacted by health authorities have no need to seek medical attention, officials said.

“The TTC takes the health and safety of its employees and customers very seriously and is in daily communication with Toronto Public Health,” the transit agency said in a statement. “At this time, the direction is that no additional measures are required. The TTC continues to be a safe method of travel.”

On Friday, Ontario’s chief medical officer of health confirmed six new positive cases of COVID-19, bringing the total number of confirmed cases in the province to 28. The virus is not circulating locally, officials said.

Also Friday, Royal Bank of Canada was investigating a “possible presumptive case” of COVID-19 at its Meadowvale office building in Mississauga. The investigation is focused on an RBC employee, who the bank has not identified. There is no evidence at this time that the cases are linked.

“We have taken a number of steps to protect the health and safety of our employees, and are working with Ontario Public Health to understand the case and the next steps,” said RBC spokeswoman Gillian McArdle.

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The bank declined to detail the steps it has taken, but contingency plans to continue serving clients “are already in progress,” Ms. McArdle said. The Meadowvale building has not been evacuated, and work continues at the offices.

The RBC employee is under self-quarantine, and the bank is in direct contact with employees who may have been exposed to that person “to ensure they self-quarantine until advised otherwise as a precautionary measure,” Ms. McArdle said.

“We are committed to providing a healthy and safe workplace, and to following the guidance provided by local, federal and international public health and government authorities,” she said.

Toronto Public Health began investigating the case of the transit user Thursday night, and that probe is still ongoing. The city’s public health team is in “constant communication” with health authorities in Peel Region, Joe Cressy, chair of the Toronto Board of Health and a city councillor, said in an interview.

Toronto Public Health is also “ in touch with [RBC],” he said, but “it is too early to say or to speculate as to whether there may be a link” between the two cases.

Cities around the world are rushing to protect public confidence in the safety of their transit systems, implementing cleaning regimes as frequent as every four hours.

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One study suggests, though, that transit, in spite of its inherent crowding, may have a modest role in the spread of infectious disease. A 2011 paper in the Journal of Urban Health that modelled an influenza epidemic in New York City found that just 4.4 per cent of infections would occur on the subway. Close to one-third of infections would occur in other community spaces, such as bars and restaurants, 30 per cent would occur in the household and about one-quarter would occur at work.

Metrolinx, the regional transit agency that oversees GO Transit, reported its first COVID-19 case nearly a week ago, saying it was a passenger who took the bus from the airport. Both agencies have stepped up their cleaning efforts, but both prohibit their employees wearing masks, arguing they are ineffective and might alarm the public.

Union leaders representing employees at both Metrolinx and the TTC are pushing for the option of masks, saying that they should be allowed if they provide even a small amount of protection.

The TTC and Metrolinx say they are being much more aggressive at cleaning their vehicles and facilities. The TTC has moved from a once-weekly cleaning of stair rails, grab poles and other touch points to doing it every day. Metrolinx has been distributing hand sanitizer throughout GO stations and buses and is applying a long-acting, anti-microbial agent to surfaces across its fleet.

With files from Carly Weeks