Following the extensive non-pharmaceutical interventions (NPIs) and behavioral changes in the wake of the SARS-CoV-2 pandemic, an interseasonal rise in respiratory syncytial virus (RSV) cases was observed in Germany in 2021. The aim of this study was to characterize the local molecular epidemiology of RSV infections in comparison to the three pre-pandemic seasons. Additionally, clinical data were retrieved from patient charts to determine the clinical significance of RSV infections. RSV detections peaked in calendar week 40 of 2021, 18 weeks earlier than the usual peak observed in the three pre-pandemic seasons. Sequence analysis revealed a close phylogenetic relatedness regardless of the season of origin. A significantly higher amount of pediatric cases (88.9% of all cases, p < 0.001) was observed for season 2021/2022. For the pediatric cases, significant differences were observed for an increased number of siblings in the household (p = 0.004), a lower rate of fever (p = 0.007), and a reduced amount of co-infections (p = 0.001). Although the mean age of the adult patients was significantly younger (47.1 vs. 64.7, p < 0.001), high rates of comorbidities, lower respiratory tract infections and intensive care unit admissions prevailed. The NPIs in the wake of the SARS-CoV-2 pandemic had a tremendous impact on the epidemiologic characteristics and seasonality of RSV and warrant further epidemiologic studies of this important pathogen.

Introduction: The four endemic human coronaviruses (HCoV-OC43, -NL63, -HKU1, and − 229E) are common respiratory pathogens. Studies on HCoV epidemiology in Germany are limited. Therefore, this study aimed to characterize the epidemiology and clinical significance of HCoV-infections in adult patients over 11 seasons at a tertiary care hospital in Germany. Methods: Over 29,000 respiratory specimens were analyzed using nucleic acid amplification assays for respiratory pathogens between 2012 and 2022. Additionally, clinical data were retrospectively retrieved from patient records for cases with confirmed HCoV infections. Results: During the study period, 469 unique adult cases were identified. HCoV-OC43 (38.6%) and HCoV-NL63 (25.6%) were most frequently detected, followed by HCoV-229E (22.4%) and HCoV-HKU1 (13.4%). HCoVs exhibited strong seasonality, with most detections occurring during the winter months, peaking between January and March. Sporadic detections of HCoV-OC43 and HCoV-NL63 were observed in summer. While HCoV-NL63 was detected year-round, HCoV-OC43 and HCoV-HKU1 displayed a biennial circulation pattern. No consistent pattern was observed for HCoV-229E. Across all HCoV cases HCoV-OC43 were more frequently associated with dyspnea (48.1%, p = 0.004), lower respiratory tract infection (45.3%; p = 0.018), exacerbation of obstructive lung disease (17.6%, p = 0.002), and systemic prednisolone administration (16%, p = 0.007). HCoV-HKU1 infection was more frequently detected in patients with lymphoma (35.6%, p = 0.002) and associated with higher in-hospital mortality (18.6%, p = 0.042). Overall, 119 co-infections (26.0%) were identified, including viral (10.9%) bacterial (10.5%), and fungal (1.7%) pathogens. Co-infections were significantly associated with LRTI (p < 0.001), need for mechanical ventilation (p = 0.001), and ICU admission (p < 0.001). Conclusion: Endemic coronaviruses exhibit complex circulation patterns characterized by distinct seasonality. The cyclic behavior of HCoV-OC43 and HCoV-HKU1 suggests possible immunological interactions within this virus family. The clinical presentation and severity of HCoV infections are highly dependent on the age group, comorbidities of the affected patients, and co-infections.