Epidemiology and Spatial Dynamics of NIE-ZAS-1 and Related Lineages in Nigeria, 2020-2025.

Background: Nigeria continues to face persistent circulation of circulating vaccine-derived poliovirus type 2 (cVDPV2), driven by sustained transmission of dominant viral lineages and intermittent emergence of new strains. Since 2020, the NIE-ZAS-1 lineage has remained the most epidemiologically significant strain, with ongoing spread across multiple states. This study describes the temporal, spatial, and lineage-specific transmission dynamics of NIE-ZAS-1 and emerging related lineages between 2020 and 2025. Methodology: We conducted a descriptive epidemiological and spatial analysis of poliovirus lineage surveillance data across Nigeria from 2020 to 2025. Genetic lineage classifications (NIE-ZAS-1, NIE-YBS-1, NIE-YBS-2, NIE-BOS-1, NIE-KTS-1) were analyzed by state, year, and quarter. Geographic diffusion patterns were reconstructed using state-level detection timelines, frequency counts, and directional spread from presumed origin states. Results: NIE-ZAS-1 originated in Zamfara State in 2020 and expanded to at least 21 states by 2025, accounting for the majority of detections throughout the study period. Transmission showed marked seasonality, with consistent Q3 peaks. In contrast, newer lineages such as NIE-YBS-1, NIE-YBS-2, NIE-BOS-1, and NIE-KTS-1 demonstrated limited spatial spread and short-lived circulation, largely confined to Borno, Yobe, and Kano states. Despite reductions in case counts in some quarters, no lineage showed sustained interruption of transmission by 2025. Conclusion: Persistent cVDPV2 transmission in Nigeria is driven predominantly by the long-standing NIE-ZAS-1 lineage, with emerging strains failing to replace but adding complexity to the transmission landscape. These findings underscore the need for geographically targeted, lineage-informed vaccination strategies, enhanced surveillance in origin and amplifier states, and intensified efforts during high-risk seasonal periods.