Patterns in the dynamics of respiratory viruses after the COVID-19 pandemic: an observational, retrospective study...

BACKGROUNDRespiratory viruses are a major cause of morbidity and mortality worldwide. Their circulation patterns were markedly altered by the COVID-19 pandemic. The Information System for Infection Surveillance in Catalonia (SIVIC), established in 2022, provides continuous, population-representative sentinel surveillance integrating syndromic and microbiological data.AIMOur aim was to describe the timing, regularity and overlap of epidemics across 18 respiratory viruses using microbiologically confirmed data from SIVIC.METHODSWe conducted an observational, retrospective study using weekly data from the SIVIC sentinel network in Catalonia between October 2022 and September 2025. We calculated weekly positivity for each virus and defined epidemic start, peak and end based on week-to-week positivity growth and decline. Mean timing and variability across epidemiological years were estimated after applying a 3-week moving average to compensate for fluctuations.RESULTSAmong 32,782 analysed samples, 21,278 (64.9%) tested positive for at least one virus. Distinct and recurrent epidemic patterns were observed. Respiratory syncytial virus and influenza viruses exhibited consistent sequential peaks in late autumn and early winter, while rhinovirus and enterovirus showed bimodal peaks in autumn and spring. Some viruses (human adenovirus, human parainfluenza 1 and 4, human coronavirus NL63) circulated year-round at low levels. In contrast, SARS-CoV-2 showed a persistent summer epidemic pattern.CONCLUSIONMost respiratory viruses in Catalonia have re-established regular seasonal dynamics, with SARS-CoV-2 displaying a distinct summer peak. Year-round sentinel surveillance for timely monitoring of viral circulation can help anticipate the timing and sequence of epidemics peaks.