More than 90% of COVID cases from international travellers caught by tests done within one week of arrival: study

March 1, 2021

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An airplane at Toronto's Pearson International airport.
In the largest border study looking at an airport-based COVID-19 surveillance program, researchers from IHPME, including Drs. Vivek Goel and Laura Rosella, and colleagues found testing international travellers on arrival with a follow-up test one week later detects 94% of COVID cases.
By Alisa Kim Researchers from the University of Toronto and McMaster University have released the full results of the largest border study looking at the feasibility of an airport-based COVID-19 surveillance program. The findings show testing international travellers on arrival with a follow-up test one week later detects 94% of COVID cases—providing evidence for a test and reduced quarantine strategy. “The goal was to document how many travellers coming into [Canada] internationally were potentially carrying COVID-19 and how many would be picked up at different stages of quarantine: on arrival, day 7 and day 14,” says Dr. Vivek Goel, a professor at U of T’s Institute of Health Policy Management and Evaluation (IHPME), who is the study’s first author. He and colleagues from U of T including Dr. Laura Rosella, a DLSPH associate professor who is cross-appointed to IHPME, and Dr. Cheryl Regehr of the Factor-Inwentash Faculty of Social Work, worked with researchers from McMaster University to study how many international travellers flying into Toronto’s Pearson airport in September and October 2020 tested positive for the novel coronavirus. The participants, who were asymptomatic, did their own oral-nasal swabs, which were sent to a lab for polymerase chain reaction testing for COVID testing. Of the 16,361 people enrolled in the study, 248 or 1.5% tested positive for COVID-19. Of these, 67% were identified on arrival, 27% on day 7 and 6% on day 14. To be eligible for the study, participants had to be staying in the Greater Toronto Area, speak English or French, and have Internet access. In the face of COVID-19 variants of concern, many countries, including Canada, have closed their borders to foreign travellers, with the exception of essential workers and returning citizens. As of Feb. 22, 2021, a Canadian health order requires air travellers to quarantine for three days at an airport hotel—at their own expense—while awaiting results of their arrival COVID test. Those with a negative result can finish the rest of their mandatory 14-day quarantine at home. Goel notes that mathematical models—developed prior to the emergence of variants of concern—suggested quarantine and testing on day 7 has the same COVID risk reduction as a 14-day quarantine that is perfectly followed. In the absence of data supporting these models, however, the Canadian government has not changed its policy on quarantine—a gap that has now been filled. This data will be valuable when future decisions are made regarding the role of quarantine and testing at the border. “What our results show, which is consistent with the results from some of the mathematical models that have been done, is that with a reduced quarantine strategy and test at day seven, you pick up as many cases from the population as you do with a perfect 14-day quarantine” says Goel. A two-week quarantine is not only a significant barrier to travel, but also has economic, social and psychological consequences, says Goel. “We actually show the decline in general well-being during the time that people were in quarantine in their home,” he says. A closer look at the demographics of the 248 passengers who tested positive reveals 75% of them are between the ages of 18 and 49 years; slightly more than one-half of them were male. Moreover, nearly 75% of the covid-positive travellers were coming from areas designated high risk like the U.S., India and Mexico. The positivity rates were high for travellers from Africa, a region with lower levels of testing. The findings speak to the need for border testing when travel restrictions are lifted, says Goel. “It shows that as we get vaccines implemented in Canada, testing at the border is going to remain very important because we’re going to have disease continue to circulate in other parts of the world where they may not actually be picking it up.” This research was supported by the Government of Canada in partnership with Air Canada and the Greater Toronto Airports Authority.

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