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Canada has confirmed its fourth case of the novel coronavirus in a London, Ont., university student who had so little of the virus in her system that public-health authorities nearly missed her case.

Public Health Ontario’s laboratory failed to detect any traces of the new virus in a twentysomething woman who fell mildly ill after returning from Wuhan, the epicentre of an outbreak that has killed at least 259 people, sickened nearly 12,000 and spread to 22 countries outside China.

Follow-up tests performed by the National Microbiology Laboratory in Winnipeg confirmed the woman had acquired the virus, known as 2019-nCoV.

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The fact that the woman initially tested negative for 2019-nCoV has prompted the province to change its reporting practices – it will now list negative cases as "presumptive negative” until the Winnipeg lab runs more sensitive tests.

Ontario’s Chief Medical Officer of Health David Williams and other public-health leaders said on Friday that the near-miss had nothing to do with the quality of Ontario’s testing and everything to do with how little of the virus was in samples from a patient with mild symptoms.

"The fact that it wasn’t picked up by the test means it’s very likely that the patient wasn’t even infectious at the height of her illness,” said Chris Mackie, medical officer of health for the Middlesex-London Health Unit.

News of the latest Canadian case came as debate intensified about whether people who are not yet sick can spread 2019-nCoV – one of many questions researchers have raced to answer as the Wuhan outbreak evolves.

Dr. Williams and his colleagues continued to say on Friday that most of the evidence showed asymptomatic people could not transmit the virus.

That is why the officials declined to reveal which flight the woman was on, except to say she arrived in Toronto on Jan. 23 and was transported in a private vehicle to London, where she attends Western University.

The woman has not been to campus since returning from China, the university said in a statement.

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Barbara Yaffe, the province’s associate chief medical officer of health, said there was “no need” to contact passengers who sat within a few feet of the woman on the plane, as federal authorities did in the case of a man who last Saturday became the first person in Canada confirmed to have 2019-nCoV.

The patient, a Toronto man in his 50s, had a mild cough on the plane. He was released from Sunnybrook Health Sciences Centre on Friday.

In the new case, the patient did not start feeling sick until Jan. 24, the day after she returned.

She visited a hospital in London and was deemed well enough to go home, where she put herself in isolation with regular follow-up from the local health unit. She has fully recovered, but is keeping herself isolated for now.

“The bottom line,” Dr. Yaffe said, “is she was completely asymptomatic on the flight, plus wearing a mask on top of it. So there’s no risk to the other passengers on the flight from her.”

One of the lucky breaks that public-health authorities caught in 2003 during their fight against severe acute respiratory syndrome (SARS) – a relative of 2019-nCoV – is that the virus could be transmitted only by people who were symptomatic, making it easier to identify cases before they could be passed along.

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On Thursday, the New England Journal of Medicine published a letter from German doctors that said an asymptomatic patient brought 2019-nCoV to Germany.

A 33-year-old German businessman caught 2019-nCoV in Munich after meeting with a healthy female colleague visiting from Shanghai.

The woman did not display symptoms until she was flying home.

Three more employees at the German company later tested positive for the virus, only one of whom had contact with the woman, the paper said.

"There’s no doubt after reading this paper that asymptomatic transmission is occurring,” Anthony Fauci, the director of the U.S. National Institute for Allergy and Infectious Diseases, told CNN on Friday. “This study lays the question to rest.”

If 2019-nCoV can be transmitted easily by people with no symptoms, it will likely be harder for public-health authorities to control the outbreak.

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But the toll the new virus takes could depend more on how deadly it is than on how fast and far it spreads.

A new study released on Friday in The Lancet estimated that as many as 75,800 people may already have been infected in the City of Wuhan as of Jan. 25.

That estimate, based on mathematical modelling, has an important silver lining, said Matthew Miller, a viral immunologist at the Michael G. DeGroote Institute for Infectious Diseases Research at McMaster University in Hamilton.

“It tells us that the death rate is probably much, much lower than we’re reporting based on the number of confirmed cases and confirmed deaths,” he said.

The SARS virus killed about 11 per cent of those it infected. Middle East respiratory syndrome, caused by another member of the coronavirus family, kills about one-third of patients.

“Right now, the current reported death rate, based on confirmed cases versus deaths with this new coronavirus outbreak, is about 2 per cent,” Dr. Miller said, "and it’s been steadily declining for at least the last 10 days as more cases get identified.”

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