On the eve of the summer vacation, a study assesses how newly introduced viral lineages have contributed to the resurgence of COVID-19 in Europe. It shows that in the majority of European countries, more than half of the lines circulating at the end of summer 2020 were the result of new introductions since June 15. Despite the progress of vaccination campaigns, the authors conclude that conditions similar to those demonstrated in the study could provide fertile ground for the spread and resurgence of the virus.

Following the first epidemic wave of SARS-CoV-2 in the spring of 2020, Europe was the scene of a resurgence of the virus from the end of the summer. Although it is established that international travel during the summer of 2020 had an impact on the circulation of the virus, it remains difficult to assess how these trips have been able to restructure the epidemic in different European countries.

In a new study published in the journal Nature on June 30, 2021, Philippe Lemey – Rega Institute, KU Leuven,  Simon Dellicour – SpELL, Spatial Epidemiology Lab, École Interfacultaire de Bioengénieurs, Université libre de Bruxelles  and visiting professor at KU Leuven, and their collaborators implemented a phylogeographic model to assess how newly introduced viral lines, as opposed to persistent viral lines, contributed to the resurgence of COVID-19 in Europe. Their model is based on epidemiological data, mobility data and viral genomic data from ten European countries (Germany, Belgium, Spain, France, Italy, Norway, Netherlands, Portugal, United Kingdom, Switzerland).

Their analyzes show that in the majority of the countries studied, more than half of the lines circulating at the end of the summer resulted from new introductions since June 15. The Belgian team also shows that the success of transmission of the newly introduced lineages was predicted by the local incidence of COVID-19: in countries that experienced a relatively high summer incidence (e.g. Spain, Portugal, Belgium and France), the introduction events led to proportionately fewer active transmission channels after August 15.

Their results indicate, among other things, that introduction events in the UK have been particularly successful in establishing local transmission chains, with a considerable fraction of introductions originating from Spain.

“Imagine a fire: if there are already quite a few outbreaks in a forest, lighting a few more will not change the fate of the forest; the fire will spread anyway. On the other hand, if there are only a few sporadic outbreaks, then lighting new ones can accelerate and increase the violence of the fire to come ”explains Simon Dellicour – author of the article, FNRS Qualified Researcher at ULB .

These results illustrate the threat of viral spread via international travel, a threat that must be carefully considered by strategies to control the current spread of more transmissible and / or immune variants of the virus.

The pandemic exit strategy offered by vaccination programs is a source of optimism which has prompted member states of the European Union to propose the implementation of vaccine passports in order to boost travel and the economy. In addition to implementation challenges and equity issues, there are risks associated with such strategies when vaccination is incomplete, as will likely be the case for the European population this summer.

The study authors conclude that conditions similar to those demonstrated in the study could provide fertile ground for the spread and resurgence of the virus, with such a resurgence now also potentially involving the spread of variants that elude immune responses triggered by the virus. vaccine protection or previous infection. The authors hope that a well-coordinated and unified implementation of European strategies to mitigate the spread of SARS-CoV-2 will reduce the risks of future waves of infection.