Second coronavirus wave in Europe after summer 2020 mainly due to introduction of new virus strains

More than half of the COVID-19 strains that were circulating in the late summer of 2020 in Europe were only introduced after 15 June. This indicates that the relaxing of rules and international travel have played an important role in the emergence of the second wave.

More than half of the COVID-19 strains that were circulating in the late summer of 2020 in Europe were only introduced after 15 June. This indicates that the relaxing of rules and international travel have played an important role in the emergence of the second wave. Thanks to the vaccination campaign, things are different this summer, but even now vigilance is still required, researchers from KU Leuven and ULB say in Nature.

In the late summer of 2020, Europe was hit by a second wave of COVID-19 that turned out to be deadlier and more difficult to control than the first one. Researchers from KU Leuven and ULB have shown that this wave was mainly due to new virus strains that only started to circulate in summer.

The team of Professor Philippe Lemey (Rega Institute, KU Leuven) and Simon Dellicour (ULB/KU Leuven) reconstructed the geographical spread of newly introduced virus strains by means of a phylogeographic model. For this purpose, they used data on the epidemic, mobility and viral genomes of ten countries: Belgium, France, Germany, Italy, the Netherlands, Norway, Portugal, Spain, the United Kingdom and Switzerland. In most of those countries, more than half of the COVID-19 strains that were circulating at the end of summer were newly introduced after 15 June 2020.

Furthermore, the impact of newly introduced strains proved to be greater as the coronavirus figures in a certain region were lower. The researchers themselves compare this phenomenon with a forest fire. Simon Dellicour (ULB/KU Leuven): "If there are already a lot of small fires in a forest, a few seats of fire more will make little difference: the fire will spread no matter what. But if there are only a few seats of fire here and there, then new fires will have a much greater impact on the end result."  

This way, the introduction of new variants led to comparatively fewer chains of infection in countries that already had relatively higher infection rates in the summer, such as Belgium, Spain, Portugal and France. In the United Kingdom, on the other hand, variants quickly gained a foothold. A substantial part of the introductions there could be traced back to Spain.

Vigilance is still required

The situation is different in the summer of 2021 because in the countries under study, the vaccination campaigns have reached cruising speed. For EU member states, those campaigns are also part of a broader European strategy, in which vaccination certificates and regular testing play an important role. 

Yet, the researchers emphasise that vigilance is still required as long as not everyone has been able to build up sufficient immunity. This will probably be the case this summer for the European population. Besides, it is always possible that new variants are more contagious or have the ability to bypass the immunity built up after vaccination or a previous infection. Therefore, the researchers hope that a well-coordinated and uniform implementation of European strategies to stop the spread of SARS-CoV-2 will prevent a new late-summer wave. 


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