Molecular surveillance of acute respiratory infections (ARIs) in Bavaria, Germany: The Bavarian Influenza + Corona Sentinel (BIS+C)

Introduction: The Bavarian Influenza + Corona Sentinel (BIS+C) was originally launched in 2009 and has continued to grow and evolve to the present day. The main objective of the BIS+C is to monitor the circulating ARI pathogens Influenza, SARS-CoV-2 and Respiratory Syncytial Viruses (RSV) throughout Bavaria for molecular surveillance.

Materials and methods: The BIS+C is based on a network of general practitioners and pediatricians who send year-round nasopharyngeal swab specimens from patients with acute respiratory infections (ARIs) to the LGL on a weekly basis for virological diagnosis of Influenza, SARS-CoV-2 and RSV. Since its launch, the BIS+C Sentinel has continuously developed and now includes the diagnosis of Influenza A and B cases, including subtypes or lineages, as well as the diagnosis of RSV cases. Due to the pandemic, SARS-CoV-2 diagnostics, including variant screening of positive specimens, have been included in the sentinel. In order to achieve comprehensive coverage of BIS+C sentinel surveillance for ARI pathogens throughout Bavaria, BIS+C sentinel practices were identified and recruited with respect to population density.

Results: Participation in the BIS+C sentinel has more than doubled, from 92 practices in 2021/2022 to 196 in 2023/2024 resulting in over 13.500 nasopharyngeal swabs collected since CW 40/2023 to 39/2024. The results of the diagnostic and epidemiological analyses performed are published in weekly reports throughout the year and describe the trends in the pathogen-specific positivity rates for Influenza, SARS-CoV-2 and RSV infections per CW in Bavaria. According to RKI definitions, the 2023/2024 season revealed an RSV epidemic from CW 45/2023 to 09/2024 caused mainly by the RSV A subtype that affected especially children <4 years of age. From CW 51/2023 to 13/2024, there was an Influenza epidemic that mainly affected children and middle-aged people. It mainly included Influenza A(H1N1)pdm09 cases, followed by Victoria lineage Influenza B cases. As in the years prior to the pandemic, the onset of the epidemic occurred at the expected time. Notably, the onset of the first epidemic of the previous year (CW 44/2022 to CW 2/2023), caused primarily by Influenza A(H3N2), began unexpectedly early. The second, much shorter epidemic caused by Influenza B (Victoria lineage) lasted from CW 12-13/2023. In both BIS+C years, the Influenza B cases (Victoria lineage) occurred towards the change of the fiscal year and lasted until the epidemic’s end¹. In addition, SARS-CoV-2 cases peaked first in winter (CW 50/2023) with a positivity rate of 33.8% and second in summer (CW 35/2024) with a positivity rate of 31.7%, particularly affecting adults and the elderly > 60 years.

Conclusion and Outlook: The BIS+C sentinel network is an excellent tool for monitoring trends in Influenza, SARS-CoV-2, and RSV in the Bavarian primary health care sector. Efforts are currently underway to expand its diagnostic spectrum to a wider range of viral pathogens in order to obtain an even more complete picture of the underlying ARI situation and its causative agents in Bavaria.

Publication History

Article published online:
11 March 2025