A study of the transmission of the COVID-19 variant in and through 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 imported viruses that can cause domestic outbreaks in a country through dynamic travel bans allows governments more time to prepare for a new variant – by stepping up testing, contact tracing and vaccination programs .
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 – for understanding 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 differences in epidemic dynamics between countries that have adopted different public health approaches to contain the virus.
Large-scale genomic epidemiological analyzes of SARS-CoV-2 in Canada have so far been limited to a study of the early outbreak in Quebec. We wanted to build on this research with a nationwide analysis of the first and second waves of COVID-19. We also wanted to assess the impact of the March 2020 international travel restrictions on international importation of the virus and understand why the virus persisted in 2021.
Angela McLaughlin, lead author, research fellow at the British Columbia Center for Excellence in HIV/AIDS and PhD student in bioinformatics, University of British Columbia, Canada
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 into Canada, including 680 sublines—viruses introduced from other countries that continued to circulate in the Canadian population. They also identified 1,582 singletons—introduced viruses that do not appear to be spreading in the Canadian population.
Just as travel restrictions were introduced in April 2020, the import rate peaked (58.5 sublines per week), including 31.8 from the US and 31.2 introduced in Quebec alone. Two weeks after the travel restrictions went into effect, the total subline import rate fell 3.4 times, and within four weeks it fell 10.3 times.
However, despite these reductions, new viral variants continued to be introduced at a low level until August 2020, when there was a small spike in cases leading to the second wave. This suggests that wild sublines introduced in the summer, when prevalence and immunity were low, contributed to the highest proportion of COVID-19 cases in the second wave. In turn, this means that even a low level of ongoing virus import of similarly transmissible variants can contribute to virus persistence. By mid-October, travel restrictions were further eased and import rates recovered quickly and contributed to the second wave.
By categorizing transmission sources as intra-provincial, inter-provincial, US and other international sources, the team can see where new virus imports are originating. They found that most virus introductions in the first wave (January to July 2020) came from the United States, followed by Russia, Italy, India, Spain and the United Kingdom, and were imported primarily into Quebec and Ontario. In the second wave (August 2020 to the end of February 2021), the origin of the new sublines was still dominated by the US, with increased relative contributions from India, the UK, Asia, Europe and Africa.
The fact that the US contributed significantly to cases of COVID-19 in 2020 is not unexpected by the authors, given the high prevalence of COVID-19 in 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 rail) fell by only 24.8% and accounted for almost half of all international arrivals after April 2020. Although essential key workers supporting the supply chain, these arrivals may have inadvertently facilitated additional imports from the US – suggesting this is one area where better public health measures, such as contact tracing and rapid testing could help 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 show how sublineage-based genomic surveillance can be used to identify gaps in a country’s epidemic response,” concluded Sr. author Geoffrey Joy, Research Fellow at the British Columbia Center for Excellence in HIV/AIDS and Assistant Professor in the Department of Medicine at the University of British Columbia. “Broad and long-lasting restrictions against non-essential international travel are not necessarily advisable policy in light of the economic impacts. However, our analysis suggests that rapid and strict travel bans to populated areas that harbor a high incidence of a new variant of concern or an outbreak of an entirely new virus that has not yet been identified domestically should be seriously considered. to reduce the likelihood of multiple simultaneous outbreaks spreading and overwhelming health systems.”
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Journal reference:
McLaughlin, A., et al. (2022) Genomic epidemiology of the first two waves of SARS-CoV-2 in Canada. eLife. doi.org/10.7554/eLife.73896.
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