Lassa fever in West Africa: a systematic review and meta-analysis of attack rates, case fatality rates and risk factors.

Background: Lassa fever is an acute viral haemorrhagic disease endemic to West Africa, with Nigeria, Sierra Leone, and Liberia bearing the greatest burden. Despite repeated outbreaks and rising incidence, a regional synthesis of epidemiologic indicators remains lacking. This systematic review and meta-analysis aimed to estimate pooled attack rates (proportion of a population affected during an outbreak), case fatality rates (CFRs), and identify consistent risk factors associated with Lassa fever in West Africa.

Methods: Following PRISMA 2020 guidelines, we systematically searched PubMed, Scopus, AJOL, and Web of Science for observational studies published between 1969 and 2025. Eligible studies reported attack rates, CFRs, or risk factors for Lassa fever among human populations. Only English-language, peer-reviewed studies were included and included study quality was assessed using the Newcastle-Ottawa Scale. Data were analyzed using random-effects meta-analysis. Heterogeneity was assessed using the I² statistic, and subgroup analyses were conducted by country, diagnostic method, study design, and setting.

Results: Out of 333 identified studies, thirty-five studies were included. The pooled attack rate was 21.0% (95% CI: 18.0%-24.0%, I² = 98.6%) and CFR was 27.0% (95% CI: 20.0%-35.0%, I² = 99.4%), with significant heterogeneity. Subgroup analysis showed higher CFRs in Sierra Leone (48%) and Liberia (40%) compared to Nigeria (16%). Cohort studies and hospital-based settings reported markedly higher attack rates and CFRs. RT-PCR-based diagnostics were associated with lower CFRs than serological or mixed-method studies. The pooled odds ratio for risk factors was 1.29 (95%CI: 1.10-1.49), with environmental exposures (rodent infestation, poor housing) having the strongest association (OR = 2.18). Evidence also pointed to a shift in disease transmission from rural to peri-urban settings, driven by urbanization, habitat loss, and climate change.

Conclusion: Lassa fever continues to pose a severe public health threat in West Africa, characterized by high transmission, elevated mortality, and evolving ecological dynamics. To reduce morbidity and mortality, integrated strategies are needed such as early diagnosis via decentralized RT-PCR, environmental hygiene, risk communication, and vaccine preparedness within a One Health framework.