A study of the transmission of the COVID-19 variant in and across Canada shows that international travel restrictions were a key intervention to reduce or slow the spread, according to a report published today in eLife.
The results suggest that reducing the number of virus imports that can fuel domestic outbreaks within a country through dynamic travel bans allows governments more time to prepare for a new variant—by increasing testing programs, tracing contacts and vaccination.
The COVID-19 pandemic has highlighted the importance of genomic epidemiology — that is, the genetic sequencing of SARS-CoV-2 samples from different regions and times — to understand the origin and movement of virus variants internationally, especially variants of concern or interest. . These methods have been widely used in the UK, US, Brazil, New Zealand and Europe and have illustrated the difference in epidemic dynamics between countries that took different public health approaches to control the virus.
“Large-scale genomic epidemiology analyzes of SARS-CoV-2 in Canada have so far been limited to a study of the early epidemic within Quebec,” says lead author Angela McLaughlin, Research Assistant at the British Columbia Center for Excellence in HIV/ AIDS. and PhD candidate in Bioinformatics, University of British Columbia, Canada. “We wanted to elaborate on this research with a national-scale analysis of the first and second waves of COVID-19. We also wanted to assess the impact of international travel restrictions in March 2020 on international imports of the virus and understand why the virus persisted until in 2021”.
The team used available sequence data from Canadian cases of COVID-19 and data on the prevalence of circulating variants in other countries to estimate the geographic origin of the viruses. From this, they identified more than 2,260 introductions of new variants in Canada, including 680 sublines — viruses introduced from other countries that continued to circulate within the Canadian population. They also identified 1,582 singletons – introduced viruses that did not appear to have spread within the Canadian population.
Just as travel restrictions were introduced in April 2020, the import rate reached its maximum (58.5 sublines per week), including 31.8 from the US and 31.2 entered in Quebec alone. Two weeks after the travel restrictions came into effect, the overall import rate of the bottom line had fallen 3.4 times, and within four weeks it had fallen 10.3 times.
Despite these reductions, however, new variants of the virus continued to be introduced at a low level until August 2020, when there was a small increase in cases leading to the second wave. This suggests that wild-type sublines introduced in the summer, when prevalence and immunity were low, contributed to the higher proportion of COVID-19 cases in the second wave. In turn, this implies that even a low level of persistent virus imports of similar transmissible variants may contribute to viral persistence. By mid-October, travel restrictions were further eased, and import rates rose quickly and contributed to the second wave.
By categorizing sources of transmission as within the province, between the province, the US, and other international sources, the team could see where new virus imports originated. They found that most virus introductions in the first wave (January to July 2020) came from the US, followed by Russia, Italy, India, Spain and the UK, and were mainly imported into Quebec and Ontario. In the second wave (August 2020 to the end of February 2021), the origin of new sublines was still dominated by the US, with increasing relative contributions from India, the UK, Asia, Europe and Africa.
That the US was a large contributor of COVID-19 cases in 2020 was not unexpected by the authors, given the high prevalence of COVID-19 throughout 2020 and the long land border shared between the two countries. Even as international arrivals to Canada fell by 77.8% from 2019 to 2020, the number of truck drivers and crew members (air, ship and train) only fell by 24.8%, and accounted for almost half of all international arrivals after April 2020. Although key workers supporting the supply chain, these arrivals may have inadvertently facilitated additional imports from the US – suggesting this is an area where better public health measures, such as contact tracing and rapid testing, could to have helped prevent the movement of new variants.
“These analyzes shed light on the natural epidemiological history of SARS-CoV-2 in the context of public health interventions and demonstrate how sublineage-based genomic surveillance can be used to identify gaps in a country’s epidemic response,” concluded the author of old Jeffrey Joy. Research Scientist at the British Columbia Center for Excellence in HIV/AIDS and Assistant Professor in the Department of Medicine, University of British Columbia. “Broad and prolonged restrictions against non-essential international travel are not necessarily an advisable policy in light of the economic impacts. However, our analysis suggests that rapid and strict travel bans to localities harboring a high frequency of a variant new concern or an outbreak of an entirely new, as-yet-unidentified virus domestically should be seriously considered to reduce the possibility of multiple, simultaneous outbreaks and overwhelming health care systems.”
Materials provided by eLife. Note: Content may be edited for style and length.