A Retrospective Longitudinal Analysis of Lassa fever outbreaks in Nigeria between 2020 to 2025

Abstract Background Lassa fever (LF) is an acute viral haemorrhagic illness caused by the Lassa mammarenavirus and remains a major public health concern in Nigeria. Between 2020 and 2025, Nigeria experienced repeated LF outbreaks amid broader health system disruptions associated with the COVID-19 pandemic. Evidence suggests that the diversion of molecular diagnostic capacity during the pandemic may have initially affected LF surveillance and reporting. This study analysed national trends in Lassa fever outbreaks in Nigeria between 2020 and 2025, focusing on temporal patterns, geographic distribution, and key epidemiological indicators. Methods A retrospective longitudinal study was conducted using secondary surveillance data from January 2020 to December 2025. Data were obtained from weekly and annual Lassa fever situation reports published by the Nigeria Centre for Disease Control (NCDC). The study covered all 36 states and the Federal Capital Territory. Time-series analysis, seasonal decomposition, descriptive statistics, and spatial analysis were performed using the Python programming language to examine annual trends, seasonal patterns, geographic distribution, and case fatality rates. Results Between January 2020 and December 2025, Nigeria reported 47,814 suspected Lassa fever cases, of which 6,456 were laboratory-confirmed, and 1,008 deaths were recorded, yielding an overall case fatality rate of 15.6%. Confirmed cases declined in 2021 but increased steadily from 2022 to 2024, peaking in 2024 before slightly declining in 2025. Strong seasonality was observed, with 83% of confirmed cases occurring during the dry season and nearly two-thirds of cases occurring in the first quarter of each year. Time-series decomposition confirmed recurrent annual peaks between January and March and troughs between July and September. Geographic analysis showed a marked concentration of cases in five states (Ondo, Edo, Bauchi, Taraba, and Ebonyi). The national case fatality rate increased from 13.36% in 2020 to 18.66% in 2025, with notable regional variability. Conclusions Lassa fever in Nigeria remains persistently endemic, characterized by pronounced seasonal transmission, geographic clustering in high-burden states, and a rising case fatality rate. Strengthening seasonal preparedness, expanding diagnostic and treatment capacity in hotspot regions, and improving clinical management are critical to reducing mortality and controlling future outbreaks