Global analysis of respiratory viral circulation and timing of epidemics in the pre-COVID-19 and COVID-19 pandemic eras, based on data from the Global Influenza Surveillance and Response System (GISRS)

Introduction: The COVID-19 pandemic significantly changed respiratory viruses' epidemiology due to non-pharmaceutical interventions and possible viral interactions. This study investigates whether the circulation patterns of respiratory viruses have returned to pre-pandemic norms by comparing their peak timing and duration during the first three SARS-CoV-2 seasons to pre-pandemic times. Methods: GISRS data from 194 countries (2014-2023) was analyzed for epidemic peak timing and duration, focusing on pre-pandemic and pandemic periods across both hemispheres and the intertropical belt. Analysis was restricted to countries meeting specific data thresholds to ensure robustness. Results: In 2022/23, the Northern hemisphere experienced earlier influenza and RSV peaks by 1.9 months (p<0.001). The duration of influenza epidemics increased by 2.2 weeks (p<0.001), with RSV showing a similar trend. The Southern hemisphere's influenza peak shift was not significant (p=0.437). Intertropical regions presented no substantial change in peak timing but experienced a significant reduction in duration for hMPV and adenovirus (7.2 and 6.5 weeks shorter, p<0.001). Conclusions: The pandemic altered the typical patterns of influenza and RSV, with earlier peaks in 2022 in temperate areas. These findings highlight the importance of robust surveillance data to inform public health strategies on evolving viral dynamics in the years to come.