The interplay of influenza and COVID-19 in Germany, January 2020 - December 2022: a study of competitive disease dynamics with quarantine measures and partial cross-immunity.

We study the dynamics of coexisting influenza and SARS-CoV-2 by adapting a well-established age-specific COVID-19 model to a multi-pathogen framework. Sensitivity analysis and adjustment of the model to real-world data are used to investigate the influence of age-related factors on disease dynamics. Our findings underscore the critical role that transmission rates play in shaping the spread of influenza and COVID-19. Furthermore, our analysis highlights the significant interaction between influenza and SARS-CoV-2 transmission rates, particularly in scenarios with partial cross-immunity. This underscores the importance of comprehensive interventions that simultaneously target both pathogens to effectively control their spread in coexisting environments. Our results demonstrate the importance of age-structured models in capturing the dynamics of influenza and COVID-19, underscoring the importance of accounting for age structure. Notable disparities emerge in estimated transmission rates between single-pathogen and multi-pathogen models, shedding light on the complex interactions between pathogens and their impacts on disease transmission across different age groups.