Getting tested too soon after contact with a confirmed case can result in a false negative, experts say
Since the start of the pandemic, public health messaging around COVID-19 has emphasized the importance of testing and self-isolation for anyone who has been in close contact with a positive case.
But the definition of “close contact” has become more complicated in recent months as social circles have grown larger.
In early October, Toronto Public Health announced they had to stop contact tracing after the amount of close contacts for each confirmed case became too much for the city’s 700-plus team of contact tracers to manage.
For close contacts and close contacts of close contacts, policies around testing and self-isolation might be confusing to follow. But infectious disease specialists Anna Banerji and Isaac Bogoch say the protocol depends on the incubation period and when contact occurred.
Banerji, a pediatric infectious disease specialist at the University of Toronto, says the longest an incubation period can last is 14 days. The incubation period refers to the days between a person is infected with COVID-19 and when symptoms appear.
“Unfortunately, some people never show symptoms and they could be infectious, which makes it really tricky,” she says. “But the average incubation period is usually five days.”
On top of that, it can take a minimum of three days for an infected person to start spreading the virus.
So, if Person A has tested positive for COVID-19, Person B is a close contact, and Person C is a close contact of Person B, the possibility of infection and the need for testing and self-isolation depends on time of contact.
In the previous example, Bogoch says Person B should self-isolate for 14 days, regardless of the situation or the timing.
“Just because you test negative, if you’re a close contact, that doesn’t grant you a free pass,” he emphasizes.
Bogoch, who is an infectious diseases physician and scientist with the University Health Network, says that period of self-isolation is critical for a close contact because they could show symptoms at any point during the 14-day incubation period.
He notes a close contact doesn’t always need to get tested if they aren’t showing symptoms. But if they do, they should wait a few days from the day of contact with Person A before getting tested.
“Let’s say you were exposed on Monday morning, and then you found out on Monday afternoon that that person who is a close contact was positive,” he says. “I wouldn’t run out Monday night to get a test, because it’s going to be negative.”
Banerji says getting tested too early or too late can be a major problem, because it’ll result in a false negative. “We have about a false negative rate about 30 per cent, which means you’re going to think you don’t have it when you might,” she says.
That’s why it’s essential for close contacts to self-isolate for 14 days, regardless of what your test results show and whether you have symptoms.
Bogoch says Person B generally doesn’t have to inform Person C that they’ve been in contact with positive Person A until after they’ve either shown symptoms or tested positive.
“Do the close contacts of the close contact need to isolate? Typically, they don’t,” he says.
Person C also usually doesn’t need to self-isolate until Person B tests positive or shows symptoms.
It gets complicated if Person B lives in the same household as Person A, and had continued contact with Person A throughout their incubation period.
A person who tests positive for COVID-19 will be infectious for 10 days after the onset of symptoms. “At any point in time, if you’re living in the same household, Person B could be exposed to the coronavirus,” Banerji says.
Person B may be infected on day 10, and so Person B’s incubation period of 14 days should start after day 10.
Ultimately, Bogoch says it’s safest to call your local public health unit when you’re experiencing any confusion around symptoms or self-isolation.
“There are extra personnel around who can walk you through the various scenarios, because there’s a lot of nuance to this sometimes,” he says.
Here are some examples of possible COVID-19 infection scenarios, dependent on timing: