Enhancing respiratory viruses surveillance in Belgium: a validation of the wastewater-based surveillance of influenza, RSV, and SARS-CoV-2 in the 2024/2025 winter season
This study validates a novel wastewater-based surveillance program covering 38% of Belgium's population during the 2024/2025 winter season, demonstrating its high correlation with conventional clinical data and its added value for monitoring SARS-CoV-2 while identifying areas for improvement in tracking influenza and RSV.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
When a virus spreads through a community, it leaves a trace that is often invisible to the naked eye but detectable in the flow of water leaving our homes. This is the foundation of wastewater-based epidemiology, a method that treats sewage not as waste, but as a collective biological sample of an entire population. Instead of waiting for people to feel sick, visit a doctor, and get tested, scientists can analyze the water in treatment plants to see what pathogens are circulating before the full scale of an outbreak becomes apparent. This approach offers a way to monitor health trends across millions of people simultaneously, bypassing the need for individual consent or the biases that come with who chooses to seek medical care. For years, this technique was primarily used to track a single virus, but the question remained: could this same system reliably monitor multiple respiratory viruses at once, and could it serve as a trusted partner to traditional health monitoring?
In the winter of 2024 and 2025, researchers in Belgium put this idea to the test on a massive scale. They launched a program to monitor three distinct respiratory viruses—flu, respiratory syncytial virus (RSV), and SARS-CoV-2—by collecting water samples from thirty different treatment plants across the country. These plants serve approximately 4.5 million people, covering more than a third of the Belgian population. The team worked through a season that proved to be exceptionally challenging for the healthcare system, marked by high numbers of flu and RSV cases while SARS-CoV-2 activity remained relatively low. By comparing the signals found in the sewage with the records of patients visiting doctors and hospitals, the researchers aimed to validate whether this water-based system could accurately reflect the reality of the epidemic on the ground.
The results showed that the wastewater data was remarkably aligned with the clinical data. For SARS-CoV-2, the virus that caused the global pandemic, the signals from the water matched the number of confirmed cases with a high degree of consistency. The researchers found that when the virus was circulating widely among the population, the concentration in the wastewater rose in step, and when cases dropped, the water levels fell accordingly. This correlation was strong enough to confirm that the water samples were providing a reliable picture of the virus's movement, even when the testing strategies for individuals changed or when people stopped seeking medical attention for mild symptoms. The system successfully tracked four distinct waves of the virus over the course of the study, including peaks driven by different genetic variants.
The story was slightly more complex for influenza and RSV, two viruses that have circulated for decades but are harder to detect in water at low levels. Because the concentrations of these viruses in the sewage were often too low to be measured precisely, the researchers could not calculate exact numbers of viral copies. Instead, they focused on whether the virus was present or absent in a given sample. Despite this limitation, the pattern held true: when the number of people sick with flu or RSV increased, the number of treatment plants detecting these viruses also increased. The data from the water showed the same peaks and valleys as the reports from doctors' offices. For instance, during a period in early 2025 when hospitals were overwhelmed by flu cases, the wastewater samples showed a very high level of flu virus activity, confirming the severity of the situation.
However, the study also highlighted where the method still needs refinement. The researchers noted that for RSV, the timing of the data collection missed the very beginning of the epidemic season because the testing for this specific virus started later than planned. Furthermore, the system has a blind spot regarding young children who wear diapers, as their waste does not enter the sewage system. This means that for RSV, which heavily affects infants, the wastewater data might not capture the full extent of the infection in the very youngest part of the population. Additionally, because this was the first full season of monitoring for flu and RSV in this way, the team had to rely on limited historical data to set the thresholds that define what counts as "low" or "high" activity. As more seasons of data are collected, these benchmarks are expected to become more precise.
Ultimately, the study concludes that monitoring wastewater provides a valuable, independent layer of information for public health officials. For SARS-CoV-2, it offers a distinct advantage by providing a constant, unbiased view of the virus that does not depend on people going to the doctor. For flu and RSV, while the system is not yet as precise as clinical testing, it successfully tracks the general rise and fall of outbreaks. The researchers suggest that as the methods improve to detect lower levels of these viruses, wastewater surveillance could become a standard tool for pandemic preparedness, offering a rapid and equitable way to see what is happening in the community before it becomes a crisis in the hospital.
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