Despite certified measles elimination in 2020, Belgium counted the third-highest measles notification rate among European countries in 2024. To maintain elimination status, it is paramount to confirm interruption of transmission chains and absence of endemic circulation. This study explores the potential of wastewater-based surveillance as a complementary tool to clinical surveillance for monitoring measles. Specifically, it examines whether the detection of measles virus in Belgian wastewater correlates with reported clinical case data.

Wastewater samples were collected from the influents of six wastewater treatment plants selected based on three criteria: high population coverage, proximity to identified risk areas and geographic distribution across the Brussels-Capital, Flemish and Walloon regions. Samples were collected weekly from May to September 2025 and concentrated using ultrafiltration. RNA was extracted and analysed using reverse transcription digital polymerase chain reaction (RT-dPCR). The clinical case data were retrieved from mandatory notifications and lab testing at the National Reference Centre.

Measles virus was detected in 28 percent of the 110 samples analysed, and in five out of six locations sampled between May 5th (week 19) and September 8th  (week 37) 2025. At a national level high case numbers were observed in May and June through clinical surveillance, with a marked decrease from week 24 onwards.

Measles virus was successfully detected in Belgian wastewater, and qualitative trends mirrored those observed in clinical surveillance at a national level. Further data consolidation is required to conduct a correlation study at the provincial level. Results of the correlation analysis will be presented at the ESCAIDE conference. Furthermore, the perspectives of wastewater data to fill knowledge gaps belonging to the clinical-based surveillance will be discussed in this work.