Co-occurrence of viral respiratory pathogens: prevalence, seasonal distribution, and symptomatology in outpatients in Bavaria, Germany, from April 2025 - March 2026.

Abstract

Purpose: The Bavarian Influenza Sentinel (BIS + C) monitors the circulation of respiratory viruses in outpatients in Bavaria, Germany. After the Covid-19 pandemic, the molecular surveillance program was expanded for improved insight into the rate of viral infections. Only limited information on viral co-infections is yet available, so this study provides an insight into their co-occurrence, seasonal distribution, and symptomatic outcome.

Methods: From calendar week 15/2025 to 14/2026, general practitioners and pediatricians collected swab samples from outpatients with acute respiratory diseases, that were tested for twelve viral pathogens using qPCR multiplex systems.

Results: At least one virus was found in 58.8% of the outpatients (n = 7,416/12,614) including 4.1% (n = 520) multiple viral detection, primarily in patients younger than 5 years and more frequent in males. The frequency of multiple virus detection ranges from 6.7% in influenza A (n = 118/1,761) to 43.3% in adenoviruses (n = 154/356) positive tested samples. The most common pathogens in multiple detections were entero-/rhinoviruses involved in 11.9% (n = 292/2,459). Differences in the reported symptoms between single- and co-infections have been found, including less muscle/joint pain in influenza A co-infections. The detected Ct values in co-infections compared to mono-infections were lower for SARS-CoV-2, adenoviruses, entero-/rhinoviruses, and coronaviruses.

Conclusion: The frequency of multi-infections follows the seasonality of the corresponding viruses. Especially children under 5 years of age are affected by co-infections. Among the studied viruses, influenza A showed the lowest relative rate of co-detection, in line with its pronounced seasonality - a pattern that warrants further investigation.