Respiratory viruses in Catalan childhood: lessons, status and predictions after the pandemic.

This doctoral thesis has focused on the characterization and study of the dynamics of SARS-CoV-2, the influenza virus (flu) and the respiratory syncytial virus (RSV) in children in Catalonia during the last decade. Through a combination of empirical epidemiological analyses and dynamic modeling, we have explored how the COVID-19 pandemic has altered the seasonality and interaction patterns of respiratory viruses, modifying their epidemiological behavior and impact on public health. We have described the epidemiological changes before, during and after the pandemic, highlighting the distinctive behavior of SARS-CoV-2 in children after the emergence of the Omicron variant and the start of vaccination, ceasing to generate severe disease and predictable epidemic waves like influenza and RSV. These, on the other hand, have shown a progressive return to their pre-pandemic epidemic patterns, although they are not yet in a steady state.

The models have been used to predict their pre- and post-pandemic epidemic peaks (when possible) almost a month in advance, with a maximum margin of error of one week and a magnitude that falls within the 95% confidence interval of the models, although human supervision and intervention improve their performance. To better understand the factors that modulate influenza transmission, we have developed a compartmental SEIR model, using a two-step transmissibility function that has been related to meteorological factors. This model has revealed a significant association between temperature and influenza transmissibility, with a lag of two weeks.

The practical effectiveness of preventive strategies against RSV has also been evaluated, particularly the monoclonal antibody nirsevimab, which has demonstrated high effectiveness in reducing hospitalizations and admissions to the pediatric intensive care unit (PICU), by more than 80%. Furthermore, this work has questioned historical assumptions about the role of children as the main drivers of respiratory virus epidemics. Although their value as a sentinel population for the rapid detection of circulating infections has been confirmed given their high contact rate and active surveillance, the results suggest that they do not necessarily initiate epidemics, especially for viruses such as influenza. This discovery invites us to reconsider public health strategies and control measures traditionally directed at the pediatric population.