`HOT-SPOT' STRATEGY EYED FOR SHOTS
Would focus on super-spreaders for more impact
Forest firefighters call it hot-spotting: targeting their efforts to where a wide-ranging blaze seems to be expanding fastest or poses the gravest threat.
Canadian researchers suggest the same strategy could be used to snuff out a different kind of inferno, the COVID-19 pandemic.
As authorities carefully dole out a limited supply of coronavirus vaccine, the scientists are recommending an infectious-disease version of the concept. It would use smartphone contact-tracing apps to identify potential superspreaders of the virus, then prioritize those people to receive the precious resource first.
The strategy could achieve herd immunity with less than half as much vaccine as if it was distributed more evenly across the Canadian population, scientists from the Universities of Waterloo and Guelph, the Perimeter Institute for Theoretical Physics and drugmaker Sanofi Pasteur conclude in a new paper.
“Hot-spotting is dramatically more efficient than uniform allocation,” they say in the article, which was posted recently on a “pre-print” site without independent scrutiny. “We conclude that hot-spotting could enable public health authorities to greatly reduce the social and economic costs of COVID-19 until vaccine supply catches up with demand.”
The authors declined to comment until their research is published in a peer-reviewed journal.
One outside expert called it an excellent paper that should be seriously considered by Canadian authorities, but cautioned that it would only work if the vaccines actually prevent infection with COVID-19, not just symptoms of the virus.
Another expert, Queen's University infectious-disease specialist Dr. Gerald Evans, said it's also unclear if the apps would truly identify likely transmitters or just “highly mobile persons.”
The worldwide scientific effort to produce a COVID-19 preventive has had unprecedented success, with two vaccines approved by Health Canada less than a year after the virus that causes the disease was isolated.
Canada is among the first to take delivery of a Pfizer product, with supplies of the Moderna shot to arrive soon. Health-care workers and the most vulnerable Canadians are being treated now. But the federal government says it could be next fall before most people who want it are immunized.
The paper by mathematicians and physicists addresses that reality — that rollout of the vaccine will not occur overnight. They cite previous theoretical research that suggests targeting potential super-spreaders — people who infect several others — increases the effectiveness of a vaccine.
Identifying those individuals could be relatively easy with tracing apps that harness the Bluetooth function on phones to warn if someone has been close to a person who tested positive for COVID-19. Except the app — and the Bluetooth handshakes it makes with others' phones — would be used simply to pinpoint those who have an exceptional number and duration of contacts.
The authors use mathematical modelling to test the impact of prioritizing those people, versus more uniformly administering vaccine. Their conclusion: less than half as many would need to get shots to reach herd immunity, the point at which enough people are vaccinated that spread is significantly curbed.
It's a well-reasoned paper and should be considered, said Mahesh Nagarajan, an applied mathematician and business professor at the University of British Columbia. But there is at least one caveat, he said.
The Pfizer and Moderna shots were found 95 per cent effective in phase-three trials — but that effectiveness was based on preventing symptomatic COVID disease, what's called protective immunity.
The studies did not collect data on sterilizing immunity — whether the vaccines actually prevent infection from the virus.
If they do not stave off infection, singling out super spreaders wouldn't help much, as they could still pass on the pathogen to others, noted Nagarajan.
“Let's assume the vaccines do nothing to stop the spread,” he said. “Removing high-risk individuals doesn't really change anything.”
Then there is the potential unintended consequence of such a strategy — giving people an incentive to violate social-distancing rules so they can jump the vaccine queue, said Nagarajan.