Explainer

9 Apr 2021

SARS-CoV-2 importations to England: do travel restrictions work?

Dr Dinesh Aggarwal

COG-UK have previously examined the role of travel on the establishment of COVID-19 during the first wave. Understanding how to limit importations of SARS-CoV-2 is vital to managing the pandemic in the future. COG-UK sought to assess the effectiveness of travel restrictions by examining over four thousand SARS-CoV-2 importations over the summer of 2020, in collaboration with a report to SAGE and preprint.

During the summer of 2020, travel restrictions between England and other countries were assigned on a country-by-country basis. Travel corridors were established such that travellers returning from closed travel corridor countries were required to self-quarantine for 14 days, whereas travellers returning from open travel corridor countries were not required to quarantine.

So, what did we learn from this new study?

We found that the implementation of travel-corridors were effective, but not water-tight, in reducing both the number of contacts reported by positive cases and the number of subsequent cases due to onward transmission.

The majority of importations of SARS-CoV-2 in England over the summer of 2020 were from coastal European countries. The highest number of cases and onward contacts were from Greece, which was largely exempt from self-isolation requirements (bar some islands in September at the end of the study period). This highlights the importance of on-going efforts to systematically monitor imported SARS-CoV-2 cases to help refine travel restrictions.

Younger age groups had more contacts than average after returning to the UK. But for those returning from travel restricted countries, the numbers were similar to other age groups — demonstrating compliance with guidelines.

It was also possible to confirm a suspected travel-related cluster by linking genomic information with contact tracing data.

How did COG-UK researchers establish these findings?

COG-UK researchers from a wide variety of academic institutions worked closely with Public Health England to combine national contact tracing data with the large-scale sequencing enabled by COG-UK. We first looked to see if the number of contacts reported from individuals returning from travel restricted countries was more than those from countries with open travel corridors. We found that those from travel restricted countries were 40 percent lower.

However, not all contacts become cases. Using over 800 imported SARS-CoV-2 genomes, we identified index cases in England that were sufficiently unique to be used to search for similar genomes in the UK. We fitted models to the number of these similar genomes, finding more than five times fewer cases linked to countries with a travel restriction than those without.

COG-UK researchers have developed the Polecat Clustering Tool to examine emerging clusters of infection in more detail. We used the tool to examine a particularly distinct cluster of SARS-CoV-2 genomes, suspecting they may have been brought into the UK. We found, by linking these cases to contact tracing data, that these were largely (at least initially) from imported cases from mainland Europe, identifying a novel method to support public health agencies in their monitoring of travel related cases.

How will this information be useful?

This study comes at a vital point when countries aim to monitor and limit the importation and spread of variants of concern. COG-UK are assisting efforts to monitor the importation of variants in real time. We highlight the impact of travel restriction policy on reducing contacts and the subsequent spread of SARS-CoV-2. In addition, this study demonstrates that useful insights can be made through the integration of sequencing and epidemiology on a national scale.

 

COVID-19 Genomics UK (COG-UK)
The current COVID-19 pandemic, caused by SARS-CoV-2, represents a major threat to health. The COVID-19 Genomics UK (COG-UK) consortium has been created to deliver large-scale and rapid whole-genome virus sequencing to local NHS centres and the UK government.

Led by Professor Sharon Peacock of the University of Cambridge, COG-UK is made up of an innovative partnership of NHS organisations, the four Public Health Agencies of the UK, the Wellcome Sanger Institute and academic partners providing sequencing and analysis capacity. A full list of collaborators can be found here. Professor Peacock is also on a part-time secondment to PHE as Director of Science, where she focuses on the development of pathogen sequencing through COG-UK.

COG-UK was established in April 2020 supported by £20 million funding from the COVID-19 rapid-research-response “fighting fund” from Her Majesty’s Treasury (established by Professor Chris Whitty and Sir Patrick Vallance), and administered by the National Institute for Health Research (NIHR), UK Research and Innovation (UKRI), and the Wellcome Sanger Institute. The consortium was also backed by the Department of Health and Social Care’s Testing Innovation Fund on 16 November 2020 to facilitate the genome sequencing capacity needed to meet the increasing number of COVID-19 cases in the UK over the winter period.