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After Canada introduced international travel restrictions during the first waves of the COVID-19 pandemic, the number of cases that were being imported into the country was reduced by tenfold.
Even though the acute reduction did not translate into fewer cases, because the decrease was insufficient to prevent new outbreaks, researchers say this information is critical for understanding the evolution of the virus and the impact of travel restrictions.
The findings, published on August 2 in eLife, were based on genomic data analyzed so as to infer the viruses’ geographic origins.
The first COVID-19 case in Canada was detected on January 25, 2020, in a traveler from Wuhan, China, to Toronto. By March 5, the first community transmission case was identified, write Angela McLaughlin, a PhD candidate in bioinformatics at University of British Columbia, and colleagues.
Canadian officials enacted travel restrictions on the entry of foreign nationals fairly swiftly. On March 14, the Canadian government issued a travel advisory to avoid all nonessential travel outside of the country. Two days later, all non-Canadian or nonpermanent residents were banned from entering the country.
How successful were those measures?
To find out, McLaughlin and her team accessed publicly available viral genome sequences to create a detailed timeline of how the virus entered Canada from January 2020 to March 2021, as well as the chains of transmission that ensued.
“The COVID-19 pandemic has highlighted the importance of genomic epidemiology in deciphering the origin and spread of SARS-CoV-2 lineages across local and global scales to aid in directing responses,” the authors write.
The genomic data were used to infer the viruses’ geographic origins. In all, the researchers identified 2263 instances in which COVID-19 was imported into the country.
During the first wave, 49% of cases came from the US. They were primarily introduced into the provinces of Quebec (39%) and Ontario (36%).
In the second wave, the US remained the predominant source; 43% of cases originated from that country, although India contributed 16%, and the UK contributed 7%.
Travel restrictions did curb the entry of new cases into Canada, the researchers found.
Just 4 weeks after implementation of restrictions on the entry of foreign nationals in March 2020, the number of COVID-19 cases entering Canada declined by tenfold, going from 59 cases per week to 10 cases per week.
Despite this dramatic reduction, COVID-19 variants emerged and contributed to the persistence of cases in the second wave.
More cases were imported in November 2020 in conjunction with the easing of entry exceptions for foreign nationals, the shortening of quarantine, and the introduction of newly emergent variants of concern and variants of interest.
“Travel restrictions have a diminishing return if domestic transmission is high, if highly transmissible variants become widespread globally, or if there are many individuals exempt from travel restrictions and quarantine without access to rapid testing,” McLaughlin said in a statement.
By the end of February 2021, an estimated 30 unique genetic sublineages of the Alpha variant (B.1.1.7) had been imported into Canada. That variant increasingly displaced the original SARS-CoV-2 virus by the second half of the second wave and into the third wave.
Travel restrictions and quarantine periods would have had to have been sustained to fully curtail the number of COVID-19 cases in Canada, the authors note.
“This study provides important observations about the transmission of SARS-CoV-2 lineages within Canada and the importation of lineages into Canada over the first year of the COVID-19 pandemic,” writes eLife reviewing editor Sarah E. Cobey, PhD, of the University of Chicago, Chicago, Illinois.
“This information is critical for understanding SARS-CoV-2 evolution and epidemiology, including the potential impacts of travel restrictions,” Cobey concludes.
The Canadian COVID-19 Genomics Network Consortium and the Canadian Public Health Laboratory Network provided access to data used in the study. McLaughlen was supported by a Canadian Institutes for Health Research doctoral grant and a Natural Sciences and Engineering Research Council of Canada CREATE scholarship.
eLife. Published online August 2, 2022. Full text