Unveiling the circulation of poliovirus vaccinal strains in the Barcelona Metropolitan Area through wastewater surveillance

Poliomyelitis remains a globally significant paralytic disease; widespread use of oral poliovirus vaccine (OPV) and, to a lesser extent, inactivated poliovirus vaccine (IPV) has driven progress toward eradication, with wild poliovirus serotypes 2 and 3 declared eradicated in 2015 and 2019, respectively, while serotype 1 persists in two endemic countries. Environmental (wastewater) surveillance, long established as a complementary tool to syndromic acute flaccid paralysis (AFP) surveillance, is critical for detecting silent poliovirus circulation and for certifying interruption of transmission. Using ongoing wastewater monitoring in the Barcelona metropolitan area, we detected a circulating vaccine-derived poliovirus type 2 (cVDPV2) in 2024 and Sabin-like type 3 (SL3) strains in 2025; the SL3 genomes carried reversion-to-virulence substitutions T6I and A54V/T. Targeted upstream sewer-catchment analysis enabled localization of the source and, in coordination with clinical surveillance, identification of an index case. Although high vaccination coverage in the affected areas was associated with no observed AFP cases, these detections underscore the necessity of sustained environmental surveillance, rapid genomic characterization, and coordinated public-health responses to limit spread and to protect vulnerable populations, particularly immunocompromised individuals, while supporting global eradication efforts.