Seasonality of respiratory viruses and viral co-infection across two winter seasons in the UK.

<bold>Background:</bold> The SARS-CoV-2 pandemic significantly impacted circulation and seasonality of other respiratory viral infections (RVIs). With the re-emergence of other RVIs, co-infection can occur and may worsen clinical outcomes. <bold>Aims:</bold> · Understand seasonality patterns of RVIs in post-pandemic period. · To describe viral co-infection within seasonal patterns of RVIs. <bold>Methods:</bold> Hospitalised adults with acute respiratory illness (ARI) underwent viral PCR testing between August 2022 and January 2024 as part of the UNIVERSAL study. This multi-centre UK prospective cohort study aims to characterise RVI in hospitalised adults. <bold>Results:</bold> In 2022-2023, peak incidences of RSV and influenza A occurred in December and overlapped. In winter 2023-2024, peak incidence of influenza occurred later, in line with common pre-pandemic patterns. 26/502 (5%) cases of viral co-infection were identified. SARS-CoV-2 was the commonest virus in combination with others (17/26 – 65%). The highest numbers of co-infection were detected in January 2023 (6/26 - 23%) and March 2023 (6/26 - 23%), coinciding with peak detection of SARS-CoV-2. SARS-CoV-2 co-infection occurred most frequently with other human coronaviruses, but also with influenza, adenovirus, human metapneumovirus, entero/rhinovirus and parainfluenza viruses. Age, co-morbidities, length of stay, admission oxygen requirement were not significantly different in co-infection. <bold>Conclusion:</bold> Seasonality of RVIs shifted back towards pre-pandemic patterns in winter 2023-2024. SARS-CoV-2 co-infection occurs with a range of other RVIs. Further studies are required to understand the impact of respiratory viral co-infections.