The Spread of the Delta Variant in Catalonia During Summer 2021: Modelling and Interpretation

Background: After the wild-strain of SARS-CoV-2 spread around the entire globe, we experienced the emergence of a series of new, more transmissible variants. Similarly to other regions, in Catalonia, during the beginning of summer 2021, the Delta variant (B.1.617.2) rapidly replaced the Alpha variant (B.1.1.7). Here we analyze how the spread of the Delta variant led to substantial epidemic pressure despite the disposal of vaccines. Methods: We employ an age-stratified epidemiological model that accounts for the vaccination status and incorporates the presence of both the Alpha and Delta variant. We adjust the model in a Bayesian framework to the reported case numbers and the sequencing data. The approach allows to infer the transmission advantage of the Delta variant, as well as the temporal evolution of the age-dependent contact rates. Findings: We show that the combination of an increasing interaction rate among the younger population, a still low vaccination coverage and the spread of the Delta variant resulted in the infection of a substantial part of the younger population. We find only little effect of non-pharmaceutical interventions that were put in place. Our analysis suggests a transmission advantage of the Delta variant between 40\% and 60\% with respect to the Alpha variant. Our inference approach based on this retrospective analysis can be used in the future to forecast the potential impact of new variants of the virus. Interpretation: The easing of restrictions during a vaccine roll out requires close monitoring of the epidemic evolution and the changing contact rates. If the younger, more frequently interacting part of the population did not yet have access to the vaccine, increasing contact rates can lead to a rapid rise in infections in relatively short time. The epidemiological situation is particularly difficult to asses, if the vaccine roll out coincides with the emergence of a new, more transmissible variant.Funding Information: B.S. acknowledges financial support from the European Unions Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 713679 and from the Universitat Rovira i Virgili (URV). G.B. acknowledges financial support from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 945413 and from the Universitat Rovira i Virgili (URV). RG acknowledge funding from project PID2019-106811GB-C31 from MCIN/AEI/10.13039/ 501100011033 and by the Government of Catalonia (2017SGR-896). A.A. acknowledges support by Ministerio de Econom´ıa y Competitividad (Grants No. PGC2018-094754-B-C21 and No. FIS2015-71582-C2-1), Generalitat de Catalunya (Grants No. 2017SGR-896 and No.2020PANDE00098), Universitat Rovira i Virgili (Grant No.2019PFR-URV-B2-41), ICREA Academia, and the James S. McDonnell Foundation (Grant No. 220020325). Declaration of Interests: The authors declare that they have no competing interests.