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After Canada imposed international travel restrictions during the first waves of the COVID-19 pandemic, the number of cases imported into the country was reduced 10-fold.
Even if the acute drop didn’t result in fewer cases because the drop wasn’t enough to prevent new outbreaks, the researchers say this information is crucial for understanding the evolution of the virus and the impact of travel restrictions.
The results, released on 2.8 eLifewere based on genomic data analyzed to infer the geographic origin of the viruses.
The first case of COVID-19 in Canada was detected in a traveler from Wuhan, China to Toronto on January 25, 2020. On March 5, the first case of community transmission was identified, write Angela McLaughlin, a PhD student in bioinformatics at the University of British Columbia, and colleagues.
Canadian officials were quick to place travel restrictions on foreigners entering the country. On March 14, the Canadian government issued a travel advisory to avoid all non-essential travel outside of the country. Two days later, all non-Canadian or non-permanent residents were banned from entering the country.
How successful were these measures?
To find out, McLaughlin and her team accessed publicly available viral genome sequences to create a detailed timeline of how the virus got to Canada from January 2020 to March 2021, and the ensuing chains of transmission.
“The COVID-19 pandemic has highlighted the importance of genomic epidemiology in deciphering the origin and spread of SARS-CoV-2 lineages at local and global scales to help guide responses,” the authors write.
The genomic data were used to infer the geographic origin of the viruses. In total, the researchers identified 2263 cases where COVID-19 was imported into the country.
During the first wave, 49% of cases came from the US. They were introduced primarily to the provinces of Quebec (39%) and Ontario (36%).
In the second wave, the US remained the dominant source; 43% of the cases came from that country, although India contributed 16% and the UK 7%.
Travel restrictions slowed the entry of new cases into Canada, the researchers noted.
Just 4 weeks after imposing foreign entry restrictions in March 2020, the number of COVID-19 cases entering Canada dropped 10-fold, increasing from 59 cases per week to 10 cases per week.
Despite this dramatic decline, COVID-19 variants have emerged and contributed to the persistence of cases in the second wave.
More cases were imported in November 2020 related to the easing of entry exemptions for foreigners, the shortening of quarantine and the introduction of emerging variants of concern and interest.
“Travel restrictions have a diminishing effect when domestic transmission is high, when highly transmissible variants are spreading globally, or when many individuals are exempt from travel restrictions and quarantine without access to rapid testing,” McLaughlin said in a statement.
As of late February 2021, an estimated 30 unique Alpha variant (B.1.1.7) genetic sublineages have been imported into Canada. This variant increasingly displaced the original SARS-CoV-2 virus in the second half of the second wave and in the third wave.
Travel restrictions and quarantine periods should have been maintained to fully contain the number of COVID-19 cases in Canada, the authors note.
“This study provides important observations on the transmission of SARS-CoV-2 lineages within Canada and the importation of lineages into Canada during the first year of the COVID-19 pandemic,” she writes eLife Review Editor Sarah E. Cobey, PhD, from the University of Chicago, Chicago, Illinois.
“This information is critical to understanding the evolution and epidemiology of SARS-CoV-2, including the potential impact of travel restrictions,” concludes Cobey.
The Canadian COVID-19 Genomics Network Consortium and the Canadian Public Health Laboratory Network granted access to the data used in the study. McLaughlen was supported by a doctoral fellowship from the Canadian Institutes for Health Research and a CREATE grant from the Natural Sciences and Engineering Research Council of Canada.
eLife. Published online August 2, 2022. full text