Molecular Epidemiology of Circulating Vaccine-Derived Poliovirus Type 2 in Zamfara and Sokoto States

Circulating vaccine-derived poliovirus type 2 (cVDPV2) transmission in North West Nigeria represents the most active ongoing poliovirus outbreak globally, driven by low oral polio vaccine coverage and dense population movement across state borders. A detailed molecular epidemiological understanding of cVDPV2 transmission chains linking Zamfara and Sokoto states is essential for outbreak interruption strategy. This study performs molecular epidemiological analysis of cVDPV2 isolates from acute flaccid paralysis cases and environmental samples in Zamfara and Sokoto states between 2022 and 2025. Complete VP1 gene sequences from WHO-confirmed cVDPV2 isolates will be retrieved from the GPEI poliovirus sequence database, supplemented by environmental surveillance poliovirus sequences from sewage samples collected at sentinel sites. Bayesian phylogeographic analysis using BEAST v2 will reconstruct spatiotemporal transmission trees, estimate time-scaled phylogenies, and identify cross-state transmission linkages. Population immunity modelling will integrate demographic vaccination coverage data from national immunisation plus days. This integrative molecular and epidemiological approach addresses the key question of whether Zamfara-Sokoto transmission represents a single evolving cluster or independent reintroductions. Findings will directly inform the GPEI's North West Nigeria Outbreak Response operations, guiding the geographic scope and timing of outbreak response immunisation activities. Keywords: cVDPV2, molecular epidemiology, poliovirus, Zamfara, Sokoto