European variant from summer 2020 shows the impact of human behaviour and travel on SARS-CoV-2 spread

Figure legend: The new SARS-CoV-2 variant 20E (EU1) has spread widely across Europe. The diameter of the circles shows the number of sequences (color-coded by country). Spain is shown in red, and Switzerland in purple. Bottom right: The corresponding “family tree” of sequences of the new variant (color-coded by country) indicates that 20E (EU1) traveled between countries multiple times. (Visualization: Nextstrain, Mapbox, OpenStreetMap)
The new SARS-CoV-2 variant 20E (EU1) has spread widely across Europe.

08.06.2021 – As Europe once again begins balancing summer holiday travel with pandemic precautions, a paper out today in Nature looks at how post-lockdown border opening and movement helped to spread a novel SARS-CoV-2 variant across Europe last year. The variant, known as 20E (EU1) and studied by researchers in Switzerland, Spain, and the US, was the most prevalent variant in western Europe by the end of 2020, before the rise of the Alpha variant.

With new variants appearing regularly in headlines, and many countries easing restrictions as vaccination rates rise, lead author Dr. Emma Hodcroft at the University of Bern, says the study highlights the importance of not underestimating the impact of human behavior. “20E (EU1) did not need to be more transmissible to spread rapidly across Europe, but took advantage of loosening restrictions and increasing travel,” she explains, “It’s a timely reminder that not all rises in variant frequencies are due entirely to changes in the virus.”

The study, which was coordinated across the Universities of Basel, Bern, and ETH Zürich in Switzerland, used SARS-CoV-2 sequences to trace how 20E (EU1) expanded initially in Spain, before arriving in multiple introductions with travelers to countries across Europe and spreading onward. Both laboratory work and computational modelling suggest that the mutation associated with the variant, a change in the Spike protein at position 222, does not significantly increase transmission or change antibody binding. “The transient rise in transmission rate we see with 20E (EU1) in our models is more readily explained by travel-associated behavior and demographics than an intrinsic change in the virus,” explains Professor Tanja Stadler of ETH Zürich, one of the study’s principal investigators.

Key to the study’s success was the ability to work closely with collaborators in Spain, where 20E (EU1) first expanded, rising to make up 50% of all Spanish sequences in just a few weeks last June. Professors Iñaki Comas and Fernando González Candelas, both co-authors on the paper, helped investigate the origins and initial spread of 20E (EU1) with the SeqCOVID-SPAIN consortium, responsible for generating the sequences in Spain that allowed the variant’s transmission to be mapped.

As well as making use of SARS-CoV-2 genetic sequences, the study’s authors combined incidence estimates with anonymized mobile phone roaming data, provided by Kido Dynamics, to create a model combining COVID-19 case numbers with when and where visitors from across Europe spent time in Spain and simulate how 20E (EU1) spread.

The study's authors highlight that 20E (EU1) underscores the importance of learning from the past. “Last summer there was essentially no testing on return from holidays, and quarantine was not strongly enforced in most countries,” remarked Professor Richard Neher of the University of Basel, another of the study’s principal investigators, “This year countries are more aware of the potential impact of travel importations, and we’re seeing testing used more broadly.”

This work highlights how viral genomes can shed light on connections between outbreaks across Europe and the importance of collaborative and open data sharing in pandemic response. Professor Volker Thiel, director of the newly formed Multidisciplinary Center for Infectious Diseases (MCIDI) at the University of Bern, praised the work: “These efforts in genomic surveillance of emerging pathogens will play a key role at MCIDI. The center will foster developments in this kind of innovative work to improve our pandemic preparedness, and facilitate the exchange between researchers and public health officials during emergencies.”

Original Publication

Emma B. Hodcroft, Moira Zuber, Sarah Nadeau, Timothy G. Vaughan, Katharine H. D. Crawford, Christian L. Althaus, Martina L. Reichmuth, John E. Bowen, Alexandra C. Walls, Davide Corti, Jesse D. Bloom, David Veesler, David Mateo, Alberto Hernando, Iñaki Comas, Fernando González Candelas, SeqCOVID-SPAIN consortium, Tanja Stadler, and Richard A. Neher: “Spread of a SARS-CoV-2 variant through Europe in the summer of 2020”


Dr. Emma Hodcroft, ISPM, University of Bern, email:

Prof. Dr. Richard Neher, Biozentrum, University of Basel, email:

Prof. Dr. Tanja Stadler, ETH Zürich, Department of Biosystems Science and Engineering, email:

Prof. Dr. Iñaki Comas Espadas, Biomedicine Institute of Valencia (IBV), Spanish Research Council (CSIC), email:

Interview with Emma Hodcroft

«We should not underestimate how important travel can be»

Emma Hodcroft

Emma Hodcroft of the Institute of Social and Preventive Medicine, using the Nextstrain sequencing platform she co-developed, has identified a new coronavirus variant (EU1) that spread rapidly in Europe last summer. Travel during summertime played an important role for the rise of EU1. Summer vacations are coming soon: How can we travel without too much risk? Read more