What we know — and don’t know — about asymptomatic coronavirus transmission

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Wearing a mask and maintaining physical distance from others is important even when none of you are showing symptoms of COVID-19, experts say.

This is partly due to the risk of transmission by asymptomatic people — those who have the novel coronavirus, but aren’t showing any symptoms.

This is playing a role in the spread of COVID-19, the World Health Organization said Tuesday, clarifying some remarks they made just a day before suggesting that it only made a small difference.

“Some estimates of around 40 percent of transmission may be due to asymptomatic (cases), but those are from models. So I didn’t include that in my answer yesterday but wanted to make sure that I made that clear,” said Dr. Maria van Kerkhove, a WHO epidemiologist and technical lead on the pandemic.

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Here’s what we know, and don’t know, about the phenomenon so far.

What is asymptomatic transmission?

Dr. Isaac Bogoch, an infectious diseases specialist at Toronto General Hospital, believes it’s important to draw distinctions between three categories of patients.

These are asymptomatic patients, who have the disease but never show any signs of it, “pre-symptomatic” patients, who are infected but haven’t yet developed a cough, fever or other symptoms, and “Pauci-symptomatic” patients, who show only extremely mild symptoms.

These are sometimes grouped together in the medical literature, he said, which can be a little confusing.

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Coronavirus outbreak: WHO ‘absolutely convinced’ that asymptomatic transmission is occurring

The World Health Organization believes that pre-symptomatic transmission — passing along the virus before you have symptoms — is a big deal.

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“It appears from very limited information we have right now that people have more virus in their body at or around the time that they develop symptoms, so very early on,” van Kerkhove said.

The other types, however, do exist. A recent study from Vietnam documented true asymptomatic cases of COVID-19, Bogoch noted. After following some people who had tested positive for nearly a month, many still didn’t show any symptoms.

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How many infected people don’t show symptoms?

This is one of the big unanswered questions, according to van Kerkhove, though there are estimates.

“We know that probably around 30 per cent of people that get COVID are probably asymptomatic or they have very minimal in the way of symptoms, not the typical cough and fever,” said Dr. Sumon Chakrabarti, an infectious diseases specialist at Trillium Health Partners in Mississauga, Ont.

The research is a bit all over the place though, according to the WHO, which notes that one systematic review found that the proportion of asymptomatic cases ranged from six to 41 per cent, depending on the study.

How much do they spread the virus?

“We know that asymptomatic people can for sure transmit the virus and in fact it might be one of the reasons it was so successful at the beginning of the pandemic,” said Dr. Matthew Pellan Cheng, a clinician scientist at the McGill University Health Centre, who is also on the federal government’s COVID-19 Immunity Task Force.

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How much disease transmission asymptomatic people account for might depend a lot on local conditions, he said.

“If you’re in a situation in a community where there’s tons of symptomatic individuals, then the relative contribution to transmission of asymptomatic people is probably less. But at the onset of a pandemic when very few people are infected, then they probably actually contribute relatively more than symptomatic people.”

Chakrabarti believes that asymptomatic transmission is probably less important than symptomatic transmission, but that it remains a factor in the pandemic.

“It probably does play a role. And it’s one that is very important to be wary of, because if we don’t account for that, then we can actually have ongoing transmission and not know it,” he said.

Dr. Mike Ryan, the WHO’s top emergencies expert, said that people might be at their most contagious before they actually show symptoms.

The novel coronavirus lodges in the upper respiratory tract, he said, making it easier to transmit by droplets than related viruses such as SARS or MERS, which are in the lower tract.

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“Now as we look at COVID-19, we have an infectious pathogen that is present in the upper airway for which the viral loads are peaking at the time you are just beginning to get sick,” he said.

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“That means you could be in the restaurant feeling perfectly well and start to get a fever, you are feeling ok, you didn’t think to stay home, but that’s the moment at which your viral load could be actually quite high,” he said.

Ryan added: “And it’s because the disease can spread at that moment that the disease is so contagious. That’s why it spread around the world in such an uncontained way, is because it’s hard to stop this virus.”

How should this affect our behaviour?

We have to take precautions against asymptomatic or pre-symptomatic transmission, the experts say.

“Because this virus is known to transmit even in the absence of symptoms, you need to be on high alert pretty much all the time,” Cheng said.

That means keeping your distance from others, avoiding crowded places, and wearing a mask when you have to be close, he said.

“You can’t trust yourself not to transmit to other people.”

“Just because you don’t have any symptoms does not mean that the next person will also be asymptomatic.”

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Bogoch believes that even people who only have mild symptoms, like a cough, should still get tested, so that if they do have the disease, they can take appropriate isolation measures.

“With public education and with ensuring access to diagnostic testing, we can probably more readily identify people with mild symptoms in the community,” he said. “And we can further prevent community transmission.”

Questions about COVID-19? Here are some things you need to know:

Symptoms can include fever, cough and difficulty breathing — very similar to a cold or flu. Some people can develop a more severe illness. People most at risk of this include older adults and people with severe chronic medical conditions like heart, lung or kidney disease. If you develop symptoms, contact public health authorities.

To prevent the virus from spreading, experts recommend frequent handwashing and coughing into your sleeve. They also recommend minimizing contact with others, staying home as much as possible and maintaining a distance of two metres from other people if you go out. In situations where you can’t keep a safe distance from others, public health officials recommend the use of a non-medical face mask or covering to prevent spreading the respiratory droplets that can carry the virus.

For full COVID-19 coverage from Global News, click here.

— with files from Reuters

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