KU Leuven: Second corona wave in Europe after summer 2020 mainly due to the introduction of new virus strains

More than half of the COVID-19 strains circulating in Europe in late summer 2020 were not introduced until after June 15. This indicates that easing and international travel played an important role in the emergence of the second wave. Thanks to the vaccination campaign, the cards are different this summer, but vigilance is still required, say researchers from KU Leuven and ULB in Nature .

In the late summer of 2020, Europe was hit by a second wave of COVID-19 that would prove more deadly and more difficult to control than the first. Researchers from KU Leuven and ULB have shown that this wave was largely due to new strains of the virus that only started circulating in the summer.

The team of Professor Philippe Lemey (Rega Institute, KU Leuven) and Simon Dellicour (ULB/KU Leuven) reconstructed the geographical distribution of newly introduced virus strains using a phylogeographic model. They used data on the epidemic, mobility and virus 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 circulating at the end of the summer were found to have been newly introduced since June 15, 2020.

More impact with lower corona figures
The impact of newly introduced strains also appeared to be greater the lower the corona numbers in a particular region. The researchers themselves compare this phenomenon with a forest fire. If there are already a lot of small fires in a forest, a few extra fires make little difference: the fire will spread anyway. But if there are only a few fires here and there, new fires have a much greater impact on the end result.

For example, the introduction of new strains led to relatively fewer infection chains in countries that already had relatively high infection rates in the summer, such as Belgium, Spain, Portugal and France. Variants quickly gained a foothold in the United Kingdom. There, a significant part of the introductions could be traced back to Spain.

Vigilance is still advised
In the summer of 2021, the cards are different, because the vaccination campaign is now running at full speed in the countries studied. For the EU Member States, these campaigns are also part of a broader European strategy in which, among other things, vaccination certificates and regular testing play an important role.

Nevertheless, the researchers emphasize that vigilance is still required as long as not everyone has been able to build up sufficient immunity. That will probably still be the case for the European population this summer. In addition, it is always possible that new variants are more contagious or that the immunity built up after vaccination or can evade a previous infection. The researchers therefore hope that a well-coordinated, uniform implementation of European strategies to combat the spread of SARS-CoV-2 can avoid a new wave of late summer.

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