Background: Lassa fever, caused by Lassa virus (LASV), poses a significant public health threat in West Africa. Understanding the epidemiological parameters and transmission dynamics of LASV is crucial for informing evidence-based interventions and outbreak response strategies. Methods: We conducted a systematic review (PROSPERO CRD42023393345) to compile and analyse key epidemiological parameters, mathematical models, and past outbreaks of LASV. Data were double extracted from published literature, focusing on past outbreaks, seroprevalence, transmissibility, epidemiological delays, and disease severity. Findings: We found 157 publications meeting our inclusion criteria and extracted 374 relevant parameter estimates. Although LASV is endemic in West Africa, spatiotemporal coverage of recent seroprevalence estimates, ranging from 0.06% to 35%, was poor. Highlighting the uncertainty in LASV risk spatially. Similarly, only two basic reproduction number estimates at 1.13 and 1.19 were available. We estimated a pooled total random effect case fatality ratio of 33.1% (95% CI: 25.7 - 41.5, I^2 = 94%) and found potential variation in severity by geographic regions typically associated with specific LASV lineages. We estimated a pooled total random effect mean symptom-onset-to-hospital-admission delay of 8.3 days (95% CI: 7.4 - 9.3, I^2 = 92%), but other epidemiological delays were poorly characterised. Interpretation: Our findings highlight the relative lack of empirical LASV parameter estimates despite its high severity. Improved surveillance to capture mild cases and approaches that integrate rodent populations are needed to better understand LASV transmission dynamics. Addressing these gaps is essential for developing accurate mathematical models and informing evidence-based interventions to mitigate the impact of Lassa fever on public health in endemic regions.

AC reports payment from Pfizer for teaching mathematical modelling of infectious diseases. PD reports payment from WHO for consulting on integrated modelling. HJTU reports payment from the Moderna Charitable Foundation (paid directly to institution for an unrelated project). All other authors declare no competing interests. The views expressed are those of the authors and not necessarily those of the National Institute for Health and Care Research (NIHR), UK Health Security Agency, or the Department of Health and Social Care. NI-E is currently employed by Wellcome. However, Wellcome had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

All authors acknowledge funding from the Medical Research Council (MRC) Centre for Global Infectious Disease Analysis (MR/X020258/1) funded by the UK MRC and carried out in the frame of the Global Health EDCTP3 Joint Undertaking supported by the EU; the NIHR for support for the Health Research Protection Unit in Modelling and Health Economics, a partnership between the UK Health Security Agency (UKHSA), Imperial College London, and London School of Hygiene & Tropical Medicine (grant code NIHR200908). AC was supported by the Academy of Medical Sciences Springboard scheme, funded by the Academy of Medical Sciences, the Wellcome Trust, the UK Department for Business, Energy, and Industrial Strategy, the British Heart Foundation, and Diabetes UK (reference SBF005\1044). CM acknowledges the Schmidt Foundation for research funding (grant code 6-22-63345). PD, TN acknowledges funding from Community Jameel. GC-D acknowledges funding from the Royal Society. RM acknowledges the NIHR Health Protection Research Unit in Emerging and Zoonotic Infections, a partnership between UKHSA, the University of Oxford, the University of Liverpool, and the Liverpool School of Tropical Medicine (grant code NIHR200907). JW acknowledges research funding from the Wellcome Trust (grant 102169/Z/13/Z). DJ acknowledges funding from the Wellcome Trust and Royal Society (216427/Z/19/Z), RKN acknowledges research funding from the MRC Doctoral Training Partnership (grant MR/N014103/1). KM acknowledges research funding from the Imperial College Presidents PhD Scholarship. The funders of the study had no role in study design, data collection, data analysis, data interpretation, or writing of the report. For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising from this submission.

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