The spread of the Delta variant in Catalonia during summer 2021: Modelling and interpretation

Background: The emergence of highly transmissible SARS-CoV-2 variants has posed significant challenges
to public health efforts worldwide. During the summer of 2021, the Delta variant (B.1.617.2) rapidly displaced
the Alpha variant (B.1.1.7) in Catalonia, Spain, leading to a resurgence in infections despite ongoing
vaccination campaigns. Understanding the epidemiological drivers of this outbreak is critical for refining
future mitigation strategies.
Methods: We employed a Bayesian age-stratified epidemiological model, incorporating vaccination status
and variant-specific transmission dynamics, to analyze the outbreak in Catalonia. The model was calibrated
using daily reported cases, hospitalizations, sequencing data, and vaccination coverage across age groups.
We inferred contact patterns dynamically to assess their role in the epidemic resurgence and estimated the
transmission advantage of the Delta variant over Alpha.
Results: Our analysis revealed that increased social interactions among younger, less vaccinated populations
significantly contributed to the surge in infections. The long weekend of Sant Joan (June 23–24) coincided
with a peak in contact rates, driving a rise in the reproduction number, particularly among individuals aged
20–29. We estimated that the Delta variant had a 40–60.
Conclusions: Our findings underscore the critical role of vaccination coverage in mitigating the impact of
emerging variants. The combination of increased social interactions and uneven vaccine distribution exacerbated
the Delta-driven resurgence. NPIs alone proved insufficient in controlling transmission, highlighting
the necessity of targeted vaccination strategies to achieve robust epidemic control. This study
provides a framework for assessing future variant-specific threats and informing tailored public health
interventions.