Exploring bilateral collaboration for enhanced pandemic preparedness: A pilot study on joint wastewater surveillance in Denmark and Norway.

The use of wastewater-based surveillance (WBS) in infectious disease surveillance and early warning of new disease outbreaks have gained increased interest and relevance following the COVID-19 pandemic. During the pandemic, countries around the world strengthened their disease surveillance systems by exploring innovative approaches such as WBS. However, maintaining these systems in the "inter-pandemic period" is challenged by limited resources and funding. This pilot study explores the feasibility and value of a joint bilateral approach for WBS to ensure an "ever warm" system using shared resources. Wastewater was sampled biweekly from the capital cities of Denmark and Norway at the onset of the 2024-2025 influenza season. Sampling was performed over a limited time period of four weeks. All samples were shipped to SSI's laboratory in Denmark and analyzed for influenza and SARS-CoV-2 allowing cross-border comparisons. The results, in terms joint and coordinated logistics, laboratory analysis and interpretation, demonstrate the potential of bilateral collaborations to strengthen the early detection capacity and use of WBS in disease surveillance beyond the COVID-19 pandemic. While acknowledging the pilot nature of this study, the findings suggest that establishing bilateral collaborations during "normal time" could facilitate a more effective use of WBS in response to future public health emergencies. Further research with expanded sample size and duration is encouraged to validate these preliminary findings.