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. 2023 Nov;28(45):2300018.
doi: 10.2807/1560-7917.ES.2023.28.45.2300018.

Investigation of an international water polo tournament in Czechia as a potential source for early introduction of the SARS-CoV-2 Omicron variant into Belgium, Switzerland and Germany, November 2021

Christoph Rudin  1 , Nena Bollen  2 , Samuel L Hong  2 , Fanny Wegner  3   4 , Lida Politi  5   6 , Kassiani Mellou  7 , Caspar Geenen  8 , Sarah Gorissen  8 , Bruno Verhasselt  9 , Keith Durkin  10 , Coralie Henin  11 , Anne-Sophie Logist  2 , Simon Dellicour  12   2 , Tobias Resa  13 , Tanja Stadler  14   15 , Piet Maes  2 , Lize Cuypers  16   8 , Emmanuel André  16   8 , Adrian Egli  3   17   4   18 , Guy Baele  2
Affiliations

Investigation of an international water polo tournament in Czechia as a potential source for early introduction of the SARS-CoV-2 Omicron variant into Belgium, Switzerland and Germany, November 2021

Christoph Rudin et al. Euro Surveill. 2023 Nov.

Abstract

BackgroundThe earliest recognised infections by the SARS-CoV-2 Omicron variant (Pango lineage B.1.1.529) in Belgium and Switzerland suggested a connection to an international water polo tournament, held 12-14 November 2021 in Brno, Czechia.AimTo study the arrival and subsequent spread of the Omicron variant in Belgium and Switzerland, and understand the overall importance of this international sporting event on the number of infections in the two countries.MethodsWe performed intensive forward and backward contact tracing in both countries, supplemented by phylogenetic investigations using virus sequences of the suspected infection chain archived in public databases.ResultsThrough contact tracing, we identified two and one infected athletes of the Belgian and Swiss water polo teams, respectively, and subsequently also three athletes from Germany. In Belgium and Switzerland, four and three secondary infections, and three and one confirmed tertiary infections were identified. Phylogenetic investigation demonstrated that this sporting event played a role as the source of infection, but without a direct link with infections from South Africa and not as a superspreading event; the virus was found to already be circulating at that time in the countries involved.ConclusionThe SARS-CoV-2 Omicron variant started to circulate in Europe several weeks before its identification in South Africa on 24 November 2021. Accordingly, it can be assumed that travel restrictions are usually implemented too late to prevent the spread of newly detected SARS-CoV-2 variants to other regions. Phylogenetic analysis may modify the perception of an apparently clear result of intensive contact tracing.

Keywords: B.1.1.529; COVID-19; Omicron; SARS-CoV-2; contact tracing; epidemiology; phylogenetics; viral spread.

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Conflict of interest statement

Conflict of interest: None declared.

Figures

Figure 1
Figure 1
Lineage frequencies of SARS-CoV-2 over time since the start of the COVID-19 pandemic, focusing on the main variants of concern, worldwide, January 2020 up to October 2022
Figure 2
Figure 2
Contact tracing results among the SARS-CoV-2 Omicron cases related to the international water polo event, Belgium, Switzerland and Germany, 16 November–1 December 2021 (n = 20)
Figure 3
Figure 3
Timeline of detected primary (n = 6a), secondary (n = 7) and tertiary SARS-CoV-2 Omicron infections (confirmed n = 4; suspected n = 3) in Belgium, Switzerland and Germany, 16 November–1 December 2021
Figure 4
Figure 4
Minimum spanning tree between our samples of interest, 16 November–1 December 2021 (n = 11) and the 10 closest neighbouring sequences in Dataset C for each SARS-CoV-2 Omicron outbreak case, excluding the samples of interest, Belgium, Switzerland and Germany, 19 November–17 December 2021
Figure 5
Figure 5
Phylogenetic analysis of the Belgian, Swiss and German SARS-CoV-2 Omicron cases related to the water polo tournament, 16 November–1 December 2021 (n = 11)
Figure 6
Figure 6
Phylogenetic analysis of the Belgian, Swiss and German SARS-CoV-2 Omicron cases related to the water polo tournament, 16 November–1 December 2021 (n = 11) against a background of genomes from Czechia and South Africa, 2 November–3 December 2021 (n = 2,259)
Figure 7
Figure 7
Phylogenetic analysis of the Belgian, Swiss and German SARS-CoV-2 Omicron cases related to the water polo tournament, 16 November–1 December 2021 (n = 11) with additional genomes from these countries, 17 November–17 December 2021 (n = 3,822)
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