International travel restrictions were a key intervention in curbing the spread of COVID-19, study shows

A study of the transmission of COVID-19 variants in Canada and across Canada shows that international travel restrictions were a key intervention in reducing or slowing the spread, according to a report published today in eLife.

The results suggest that reducing the number of virus imports that can cause domestic outbreaks within a country through dynamic travel bans gives governments more time to prepare for a new variant, by increasing 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 movement of virus variants internationally. , especially the variants of concern or interest. These methods have been widely used in the UK, US, Brazil, New Zealand, and Europe, and have illustrated the variation in epidemic dynamics between countries that took different public health approaches to containing the virus.

Until now, large-scale genomic epidemiology analyzes of SARS-CoV-2 in Canada have been limited to a study on the early epidemic in Quebec. We wanted to deepen this research with a nationwide analysis for 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 into 2021.”

Angela McLaughlin, Lead Author, Research Assistant at the British Columbia HIV/AIDS Center of Excellence and PhD Candidate in Bioinformatics, University of British Columbia, Canada

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 origins of the viruses. From this, they identified more than 2,260 introductions of new variants into Canada, including 680 sublineages: viruses introduced from other countries that then circulated among the Canadian population. They also identified 1,582 unique viruses: introduced viruses that did not appear to be spreading within the Canadian population.

Just as travel restrictions were introduced in April 2020, the rate of importation peaked (58.5 sub-lines per week), including 31.8 from the US and 31.2 introduced into Quebec alone. Two weeks after the travel restrictions went into effect, the overall sublineage import rate had dropped 3.4 times and in four weeks it had dropped 10.3 times.

However, despite these reductions, 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 sublineages introduced in the summer, when prevalence and immunity were low, contributed the largest proportion of COVID-19 cases in the second wave. In turn, this implies that even a low level of ongoing virus imports of similarly transmissible variants may contribute to viral persistence. In mid-October, travel restrictions were further relaxed and import rates quickly recovered, contributing to the second wave.

By categorizing transmission sources as intra-provincial, inter-provincial, US, and other international sources, the team was able to see where new virus imports were originating. They found that the majority of first-wave virus introductions (January to July 2020) were from the US, followed by Russia, Italy, India, Spain, and the UK, and imported primarily into Quebec and Ontario. In the second wave (August 2020 to the end of February 2021), the origin of new sublineages was still dominated by the US, with larger relative contributions from India, the UK, Asia, Europe, and Africa.

The authors did not anticipate the US to be a large contributor to COVID-19 cases in 2020, given its high prevalence of COVID-19 throughout 2020 and the long shared land border between the two countries. Even as international arrivals to Canada decreased by 77.8% between 2019 and 2020, the number of truck drivers and crew members (air, sea and rail) only decreased by 24.8% and accounted for nearly 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 an area where better public health measures such as tracing of contacts and rapid tests, could 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 show how sublineage-based genomic surveillance can be used to identify gaps in a country’s epidemic response.” lead author Jeffrey Joy, Research Scientist at the British Columbia HIV/AIDS Center of Excellence and Associate 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 an advisable policy in light of the economic shocks. However, our analysis suggests that quick and strict travel bans to locations that are home to a high frequency of a new variant of concern, or an outbreak of an entirely new virus, not yet identified nationally, should be seriously considered to reduce the likelihood of seeding multiple simultaneous outbreaks and overwhelming health care systems.”

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Magazine 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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