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The silent spreaders

As we start to consider what reopening our cities will look like, scientists are still studying the spread of COVID-19 by the asymptomatics, or the “silent spreaders”, and what that means for understanding where this disease is going and where it's been.

Jo Snyder


Studies have found that not everyone who has COVID-19 looks or feels sick, yet they are still able to spread the virus. And there could be more people among us who are not showing symptoms than we originally thought.


What is a silent spreader?


In early April, the British Medical Journal published a study claiming that a large majority of COVID-19 cases are actually carried by those who are showing no symptoms at all.

This claim was repeated earlier in the pandemic by Dr. Robert Redfield, head of the Centers for Disease Control and Prevention, who said that up to 25 per cent of people infected with coronavirus show no symptoms.


But now scientists are starting to think that silent spreaders could count for somewhere between 25-50 per cent of all COVID-19 cases.


How concerned should we be about these silent spreaders?


Shelly Bolotin, an infectious disease epidemiologist with Public Health Ontario says that we don’t actually know how good asymptomatic people are at transmitting the virus.


Dr. Cory Neudorf, chief medical health officer for the Saskatoon Health Region and assistant professor in the Department of Community Health and Epidemiology at the University of Saskatchewan agrees, saying that “most viral illnesses require symptoms to effectively transmit. Coughing and sneezing are better at generating droplets with high amounts of the virus than breathing or talking, for example. But that does not mean that it is not possible to transmit while asymptomatic or pre-symptomatic.”


He adds that asymptomatic spreaders are going to be much more effective at spreading to people who are in very close proximity to them, like those they live with.


In Ontario, half of all COVID-19 infections are among staff and residents of long-term care facilities and health care workers. But how the other half has been infected, exactly, remains unknown.


"This is a sneaky virus," said Patty Hajdu, Federal Minister of Health." Some people don't feel ill at all. And that's why the … physical distancing is so critically important. We have to act as if we are all carrying this virus."


More and more studies are showing that outbreak epicentres, like the Diamond Princess Cruise ship and Vò (ground zero for the first reported deaths in Italy), had large numbers of asymptomatic people testing positive for COVID-19.


In provinces like Ontario, health officials are looking into the 20 per cent of the population that is contracting the virus through community spread and asking exactly how.

Understanding more about community spread will help guide our efforts to reopen the economy safely without risking another outbreak.


How testing will help


One thing everyone can agree on is that testing needs to be made available for everyone, especially as governments implement their reopening plans.


Researchers in Canada are looking into rapid testing to help quickly identify cases at early stages in an effort to reduce the number of people an infected person can spread to. But until that technology is ready, we’re relying on existing tests, about 20,000 a day so far. Canada’s ultimate goal is to administer 60,000 tests a day.


Remember, part of the containment goal is to go from where we were—where each infected person could infect more than two others—to eliminating the virus, where each infected person infects fewer than one other person. That’s how this thing dies out, vaccine or no vaccine.


Truly understanding asymptomatic spread, then, is dependent on our ability to broadly test and count those without symptoms.


Dr. Cory Neudorf says that “it will also be really important to see how serologic testing, where a person’s blood is tested for antibodies to see if they’ve had COVID-19, starts to be used to identify individuals who are probably immune to infection and get them back to work—and use that as sort of an easy win as far as reopening the economy. It’s very important to understand immunity and how well antibodies correlate with immunity.”


In Canada, we are seeing the pandemic slow down. But we’re not out of the woods yet. Keeping up with public health measures until we have more certainty about how this virus is spreading will help to protect the most vulnerable around us.


If you find yourself worried about symptoms, CBC’s Emily Chung provides a detailed staging of symptoms and severity.


Jo Snyder is a seasoned communications professional with expertise on the social determinants of health and health equity. Over her career, she's worked with think tanks, non-profits and big tech to deliver comms of all kinds.


This project has been made possible in part by the Government of Canada.

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