Tracking of Death Cases of Lassa Fever Outbreak in Nigeria Using a Modified EWMA Control Chart

Lassa fever remains a recurrent and endemic public health threat in Nigeria, with mortality that is often seasonal, over-dispersed, and difficult to monitor using classical, threshold-based surveillance methods. This study applied a Modified Exponentially Weighted Moving Average (MEWMA) control chart, developed by Khan et al. (2017), to monitor weekly Lassa fever deaths in Nigeria using surveillance data obtained from the Nigeria Centre for Disease Control (NCDC) covering January 2021 to September 2025 (244 weeks). In-control process parameters were estimated from a baseline period, and the MEWMA statistic was constructed using the smoothing parameter λ = 0.2, control chart coefficient L = 2.793, k = λ/2, and a nominal in-control Average Run Length (ARL₀) of 370. Descriptive analysis showed that weekly deaths were approximately normally distributed, with no marked skewness or extreme outliers, while a time plot revealed natural fluctuations without an immediately obvious pattern. The Modified EWMA chart, however, detected several statistically significant out-of-control signals, most prominently in 2024, indicating periods when Lassa fever deaths rose above the expected baseline. Similar, less pronounced signals were observed in 2021, 2022, 2023, and early 2025, suggesting a recurring seasonal or periodic outbreak pattern. These findings demonstrate that the Modified EWMA chart is a sensitive and practical early-warning tool for Lassa fever mortality surveillance in Nigeria, and its adoption by public health authorities could support more timely outbreak response.

Lassa fever; Modified EWMA control chart; Statistical process control; Average Run Length; Disease surveillance; Nigeria

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