Why provinces are starting to test people without symptoms for COVID-19
Practice could provide earlier warning of infections, but false negatives are a problem
Not long ago, people with full-blown symptoms couldn't get tested for COVID-19 in some provinces unless they were extremely ill. Now, even those provinces are starting to test people with no symptoms at all.
Why the change? What do they hope to find out? And is there a downside? Here's a closer look.
Why would you want to test asymptomatic people?
There's now lots of evidence that people can spread COVID-19 before showing symptoms, and some people never show symptoms at all. So testing asymptomatic people can:
- Help find cases in the community that would otherwise go unnoticed and help pinpoint hot spots or at-risk areas.
- Make it possible to isolate more people who may not know they have the disease and could spread it to others.
- Provide better information about the prevalence and spread of the disease in the community.
- Provide earlier indications of how the epidemic is progressing (whether it's growing, where outbreaks are) than hospitalizations and deaths, which are documented days or weeks later than a positive test.
Who could spread COVID-19 without showing symptoms?
People who test positive in nasal swab tests are shedding genetic material from the virus from their upper respiratory tract. That could potentially transmit the disease.
With SARS, which is caused by a coronavirus similar to the one that causes COVID-19, people didn't tend to shed the virus until they had symptoms.
However, with COVID-19, research conducted with subjects in Singapore found that there are three categories of asymptomatic people who can test positive:
- Pre-symptomatic people who are incubating the disease, but don't yet show symptoms. Studies suggest people can test positive up to three days before developing symptoms.
- Post-symptomatic people who have recovered and no longer have symptoms, but still test positive (although there is evidence these people may not be contagious).
- Completely asymptomatic people, who have been exposed to the virus and test positive, but never show symptoms.
Is there evidence that asymptomatic people can spread COVID-19?
A positive test doesn't necessarily mean a person is contagious. They may be shedding genetic material from dead viruses, especially if they have recovered.
But lots of other evidence suggests that pre-symptomatic people and perhaps asymptomatic people may be able to spread the virus.
By testing the contacts of people who have tested positive for COVID-19, researchers have found cases of presymptomatic transmission around the world. One study found seven clusters of pre-symptomatic transmission in Singapore that accounted for six per cent of locally acquired cases between mid-January and mid-March. The researchers estimated that 10 per cent of new infections may be sparked by people who carry the virus, but have not yet shown symptoms.
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A study published in Science in April used mathematical modelling to estimate that people with mild symptoms or no symptoms were only half as infectious as people with symptoms, but were responsible for about 80 per cent of the transmission to people who went on to have more severe infections.
What percentage of people with COVID-19 don't develop symptoms?
That's not known. Widespread testing in Vò, Italy, found that well over half of approximately 3,400 people who tested positive never showed symptoms. In other studies, the number of asymptomatic people has ranged from five per cent to 80 per cent. One difficulty is that some people who are asymptomatic when they test positive later develop symptoms.
Children, young adults, and women are more likely to be asymptomatic than older people and men.
Researchers at the Centre for Evidence-Based Medicine at the University of Oxford who did a survey of studies about asymptomatic people with COVID-19 found "there is not a single reliable study" giving an actual proportion relative to total infections.
"It is likely we will only learn the true extent once population based antibody testing is undertaken," they wrote in a blog post. Unlike nasal swab tests, which only measure ongoing infections, antibody testing can tell if someone who is no longer infected has previously been exposed to the virus.
Why weren't most provinces testing asymptomatic people before?
Previously, there was a shortage of tests and testing resources across the country, which required provinces to do more targeted testing to detect more cases with fewer tests.
Most provinces have now ramped up their testing capacity and also have fewer cases than at the peak of the pandemic, allowing them to widen their testing.
What kinds of asymptomatic people are being tested?
Strategies for testing asymptomatic people vary from province to province.
Some provinces are also targeting high-risk groups in an effort to catch infections in places they are most likely to arise.
In Brooks, Alta., provincial officials set up a testing centre for all residents of the town that have no symptoms following an outbreak at a local slaughterhouse. Both B.C. and Alberta are testing people in outbreak situations, such as meat plants and prisons.
Ontario has started to test asymptomatic health care workers, and plans to add first responders, taxi drivers, workers in meat-packing and auto plants and school-aged children.
In northern Quebec, the Cree Board of Health is targeting people who have travelled from an at-risk area or job, such as students returning from classes in cities like Ottawa and Montreal, or mine workers returning to the community, to catch infections being brought in before symptoms begin.
Does anyone do random testing instead of targeting certain groups?
Yes, but it's far from widespread at the moment.
Manitoba is doing "asymptomatic surveillance," where anyone going to a randomly selected health-care facility, such as a doctor's office, will be offered a test. It's similar to what the province already does for influenza, and is intended to get a random sample of community transmission. The idea is to give the province a sense of whether there are people with COVID-19 in the community that they aren't aware of.
Ontario says it will roll out random testing across the province, but hasn't said when.
In the meantime, Ontario Premier Doug Ford has been pushing for more residents to get tested, whether they have symptoms or not.
"If you feel you need a test, you'll be able to get a test," he said Sunday. "So please don't wait."
Quebec officials said earlier this month that random screening of asymptomatic people will not be done in the short term, but is being considered to identify local outbreaks more quickly.
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What are the challenges with asymptomatic testing?
False negatives, which can represent up to 10 per cent of tests, are a problem, especially since they're often caused by testing too early. People tend not to test positive until one to three days before the onset of symptoms.
Because of that, Manitoba's chief provincial health officer Brent Roussin has said a negative test result in an asymptomatic person doesn't hold much weight, and they would need to be retested if they start to show symptoms.
False positives can also be a problem when a large number of people are tested, as the false positive rate is about one per cent.
With random testing, another challenge is coming up with a way to randomly find people to test, said Dr. David Buckeridge, an epidemiologist at McGill University's School of Population and Global Health in an interview earlier in May.
Is asymptomatic testing considered a good idea everywhere?
Most experts agree on the need to test asymptomatic people at the site of outbreaks and asymptomatic contacts of people who have been diagnosed with COVID-19 to better isolate people who might spread the disease. But less targeted testing of people without symptoms is more controversial.
Advocates, such as Buckeridge, say testing asymptomatic people in the general population could help governments and the health system act more proactively by:
Detecting an uptick in infections earlier than is possible now.
Identifying neighbourhoods at risk.
Identifying socio-economic factors that could make people more susceptible to the virus.
However, Dr. Alex Wong, an infectious disease researcher at Regina General Hospital, said he doesn't think widespread testing of asymptomatic people is useful for monitoring the epidemic in places where very few people have the disease, such as Saskatchewan.
In an interview earlier this week, he warned that in places with low disease prevalence, widespread testing of asymptomatic people could turn up more false positives than real positives.