In Belgium, a national wastewater-based surveillance program was established in 2020 to monitor SARS-CoV-2. In September 2024, this program was extended to other respiratory viruses: RSV and influenza. Challenges encountered to adapt the existing surveillance program will be discussed in this work.

In Belgium, over 5 million inhabitants representing 40% of the population are monitored throughout 30 wastewater treatment plants since September 2020. Analysis of RSVA, RSVB, influenza A, influenza B, SARS-CoV-2 viral concentrations are performed by RT-ddPCR. On a weekly basis, epidemiological assessments are conducted using wastewater-based indicators, and weekly assessments are published by the Belgian Public Health Institute, Sciensano.

Retrospective analysis of RSV and Influenza were conducted on samples stored since January 2024. Lower viral concentration were measured for RSV and influenza than for SARS-CoV-2. Therefore, two distinct ways for analysing the data were developed. The proportion of data above the limit of quantification being high for SARS-CoV-2, wastewater-based indicators were developed based on quantified measurements. Regarding RSV and influenza, the low proportion of measurements above the limit of detection motivated the development of another reporting system and the number of treatment plants, with a detected target, were summed by date.

Since 2020, the wastewater-based surveillance was able to detect resurgences and peaks of SARS-CoV-2. The surveillance effectively provided information on the virus circulation in non-seeking healthcare population. The retrospective analysis of influenza concluded that the peaks observed in January 2024 by wastewater-based and case-based surveillances were concomitant. However, the RSV analysis were performed after its seasonal peak. Therefore, no conclusions could be made on the retrospective study on RSV.

Since January 2024, two of the three respiratory viruses studied, SARS-CoV-2 and Influenza, were effectively monitored through wastewater-based surveillance at a national level. And, an entire winter of observation is necessary to conclude on the effectiveness of wastewater-based epidemiology to monitor RSV. Such data will be collected during the winter 2024/2025 and presented at the Watermicro conference in June 2025