A study of the transmission of COVID-19 variants to and through Canada shows that international travel restrictions have been a key measure to reduce or slow the spread, according to a report published today in eLife.
The findings suggest that reducing the number of virus imports, which can trigger domestic outbreaks within a country through dynamic travel bans, gives 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 – to understand the origin and international movement of virus variants, particularly those of concern or interest Variants . 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 epidemic in Quebec,” says lead author Angela McLaughlin, research assistant at the British Columbia Center for Excellence in HIV/AIDS, and a graduate student in bioinformatics. University of British Columbia, Canada. “We wanted to deepen this research with a national-level analysis for the first and second waves of COVID-19. We also wanted to assess the impact of the March 2020 international travel restrictions on international imports of the virus and understand why the virus persisted into 2021.”
The team used available sequence data from Canadian COVID-19 cases 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 sublineages – viruses that were introduced from other countries and then circulated within the Canadian population. They also identified 1,582 singletons – introduced viruses that did not appear to be spreading within the Canadian population.
Just as travel restrictions were introduced in April 2020, the import rate reached its maximum (58.5 sub-lines per week), including 31.8 from the US and 31.2 imported exclusively in Quebec. Two weeks after the travel restrictions came into force, the total import rate of the sub-lines had fallen 3.4 times and 10.3 times within four weeks.
Despite these reductions, however, new virus variants continued to be introduced at low levels until August 2020, when there was a small surge in cases leading to the second wave. This suggests that wild-type sublines, introduced in the summer when prevalence and immunity were low, accounted for the highest proportion of second-wave COVID-19 cases. This in turn implies that even low levels of ongoing virus imports of similarly transmissible variants can contribute to viral persistence. Travel restrictions were eased further by mid-October and import rates recovered quickly, contributing to the second wave.
By categorizing sources of transmission as intra-province, inter-provincial, US and other international sources, the team was able to see where the new virus imports came from. They found that most of the first wave virus imports (January to July 2020) came from the United States, followed by Russia, Italy, India, Spain and the United Kingdom, and were mainly imported into Quebec and Ontario. In the second wave (August 2020 to late February 2021), the origin of new sublineages was still dominated by the US, with increased relative contributions from India, UK, Asia, Europe and Africa.
That the US had a large proportion of COVID-19 cases in 2020 was not unexpected by the authors given the high COVID-19 prevalence in 2020 and the long land border between the two countries. Even as international arrivals in Canada fell 77.8% from 2019 to 2020, the number of truck drivers and crew members (air, ship and train) only fell 24.8%, accounting for almost half of all after April 2020 international arrivals. Although these arrivals, which were key supply chain support workers, 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 help. to 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 genome surveillance can be used to identify gaps in a country’s epidemic response.” , concludes senior author Jeffrey Joy. Research Scientist at the British Columbia Center for Excellence in HIV/AIDS and Assistant Professor at the Department of Medicine, University of British Columbia. “Broad and long-standing restrictions on non-essential international travel are not necessarily a prudent policy given the economic impact. However, our analysis suggests that rapid and strict travel bans are needed towards locations with a high frequency 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 assess the likelihood reduce multiple simultaneous outbreaks and health systems overload.