National Post (National Edition)
Testing beats tracing: Study
Contact tracing is considered a cornerstone of the fight against COVID-19, a key way to reduce case numbers that can quickly spin out of control.
The idea is straightforward: when someone gets sick with the coronavirus, public-health officials find the people they might have infected and direct them into quarantine.
But a new Canadian modelling analysis suggests that tracing done after someone develops symptoms is of limited value, inevitably requiring economically damaging lockdowns.
In the best-case scenario, contact tracing alone catches a third of new infections and soon becomes overwhelmed, say the scientists from Simon Fraser University and University of British Columbia.
They suggest some form of widespread testing of people without symptoms, calling it the quickest way to identify cases and the people they might have infected — a way to essentially get ahead of transmission.
That might make it possible to more safely reopen parts of the economy during the months-long rollout of COVID-19 vaccines, says a paper posted recently on a “preprint” site, meaning it has yet to be peer-reviewed.
“Right now you and I aren't getting any tests and we're all trying to behave as if we're infectious — no gathering with anyone outside your family,” said Caroline Colijn, a Simon Fraser applied mathematics professor who specializes in epidemiology and led the research. “It's a heavy burden to place on individual social lives. It's leading to depression, anxiety, isolation.”
Outside experts said Tuesday the research seems sound, but warned that any kind of broad testing strategy would require significant additional resources to work.
That would include offering financial support and isolation quarters to the asymptomatic people who test positive, said Dr. Zain Chagla, an infectious-disease specialist at McMaster University.
“There's nothing magical about mass testing,” he said. “You have to tie it all together … You have to be able to add those supports, otherwise you won't be able to get people to come out.”
There are already signs, though, that contact tracing has been largely thwarted by the rapid spread of COVID-19, with Toronto's public health department having abandoned it except for contained, high-risk areas like nursing homes.
The new paper says mathematical modelling shows that contact tracing by its nature lags behind the bug's transmission.
COVID patients can be infectious before they feel sick, then may postpone getting tested when they do have symptoms. The test results often take two or more days to come back. Then, when there is a positive result and the public health unit gets involved, reaching contacts takes more time still.
All the while, the virus is potentially spreading further.
“Even when it's working perfectly … even if it's working amazingly, at best (contact tracing) can get about a third of the onward transmission,” said Colijn.
That's why repeated lockdowns are needed to keep transmission under control, said the mathematician.
One alternative is to actually test all of the contacts of a case, before any of them have developed symptoms, so when some test positive, their own contacts can be traced sooner, the paper says.
Another is mass screening of an asymptomatic population, possibly using rapid tests that don't require a nasal swab and produce results quickly. They're considered less sensitive than the standard polymerase chain reaction (PCR) test. But even if they found only 80 per cent of COVID-positive people — when none of those individuals are being screened now — that would help immensely in choking off transmission, Colijn argued.
Ottawa says it has ordered 38 million rapid tests, with about 4 million reportedly delivered by last month, but few appear to be in use yet.
A third option raised by the research is “pooled testing,” where a batch of samples from a group of people — say a college class — are tested together, using fewer lab resources. If a batch comes back negative, it would mean none have been infected. If it comes back positive, then either the whole group would be isolated, or individual samples tested.
Dr. Jeff Kwong, a public health professor at the University of Toronto, called it an “elegant” paper that seems to confirm his own suspicions about contact tracing.
“This explains why we've had a hard time controlling this in so many different places,” he said. “The contact tracing just can't happen fast enough.”
But he questioned whether Canada has the resources to use PCR tests, at least, to screen large, asymptomatic groups of people.
As it is, for instance, there's insufficient testing in long-term-care homes, where outbreaks are once again causing scores of deaths among residents, Kwong noted.