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.

 ?? PAUL CHIASSON / THE CANADIAN PRESS FILES ?? Mass screening, possibly using rapid tests that don't
need a nasal swab, could produce results quickly.
PAUL CHIASSON / THE CANADIAN PRESS FILES Mass screening, possibly using rapid tests that don't need a nasal swab, could produce results quickly.

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