The largest-ever Lassa fever research programme in West Africa

Overview

Through the Enable programme, CEPI underscores the significance of collaborative, integrative, and sustainable efforts in future Lassa virus research, including the execution of vaccine trials. Building on previous experience and knowledge gathered in the Enable 1.0 study, Enable 1.5 is now underway to continue to shed light on the epidemiology of Lassa fever in West Africa. Country ownership and lasting community engagement are paramount for the global endeavor to eliminate the threat of Lassa fever.

Background

Lassa fever, a zoonotic acute viral haemorrhagic disease caused by the Lassa virus, has persisted as an endemic threat in Western Africa since its discovery in 1969. While most cases are asymptomatic or mild, severe cases require hospitalisation and often result in death due to organ failure. Survivors may suffer from permanent hearing issues, known as sensorineural hearing loss (SNHL). Current primary treatments for patients involve early supportive care (e.g. fluids & pain relief) and the administration of the anti-viral drug ribavirin; however, the effectiveness of the latter remains debated. Therefore, the need persists for a tailored preventive intervention against Lassa fever infection – leading to the pursuit of an effective vaccine.

Leveraging its unique position as a global partnership between public, private, philanthropic, and civil society organisations, CEPI is a driving force behind these collaborative efforts to combat Lassa fever, setting the stage for ongoing vaccine development and epidemiological research. To address knowledge gaps identified by the World Health Organization for informing the design of future Lassa virus vaccine trials and delivery strategies, CEPI launched a prospective multi-site cohort study and research capacity strengthening effort named the “Enable Lassa Research Programme” in 2019.

Enable 1.0

In Enable 1.0 over 23,000 participants were followed up in 2019–2023 to estimate the incidence of Lassa fever infection and disease across five West African countries (Benin, Guinea, Liberia, Nigeria, and Sierra Leone). Preliminary results indicate that Lassa fever baseline seroprevalence — a measure of the proportion of people who have antibodies against Lassa virus at a given time, indicating previous exposure — was around 30%, ranging from 2% in Benin to 48% in Liberia. During the study, 39 cases of symptomatic Lassa fever were observed. The results of this study are soon to be published. Notable successes of the programme include the high retention rates of participants due to active local engagement, the collection of extensive data to inform future work, going beyond Lassa infection and disease incidence, the establishment of valuable regional partnerships, and the building of community trust for vaccine research and development.

Despite the wealth of collected data, persistent gaps in our understanding of the epidemiology of the disease remain. For example, the disease burden in infants remains ill-defined — as under 2s were excluded in Enable 1.0 — and the frequently observed co-infections with malaria and Lassa fever need further examination. Furthermore, mounting evidence of mild or subclinical Lassa fever cases with different disease expressions suggests that some non-febrile Lassa fever disease cases may have been missed by the study, which hinged its case definition on fever lasting for at least 48h. Lastly, we observed in some study sites a lack of appreciation of vaccines and health system hesitancy, posing challenges for future Lassa fever vaccine trials.

Enable 1.5

Aiming to fill knowledge gaps identified in Enable 1.0 (see above), CEPI is coordinating the continuation of the study under Enable 1.5, focusing on Lassa fever at five sites in Liberia (one site), Nigeria (three sites), and Sierra Leone (one site). Over 5,000 participants were recruited from Lassa fever hotspots and followed for 12 months. Enable field staff teams assessed participants’ symptoms every 2 weeks and drew blood every 3 months for serological (antibody) testing. Suspected Lassa fever cases underwent thorough evaluations, including clinical assessments and laboratory testing including RT-PCR and malaria blood smear microscopy. Data aggregation, cleaning, and analysis are underway, while serological testing is also beginning.

Mid-Term Site Visits - Progress and Insights

In June 2025, CEPI's Enable 1.5 team, in collaboration with MMARCRO, IVI, and MRC Gambia, conducted mid-term site visits across the three participating countries. The valuable field trip began in Kenema, Sierra Leone, continuing through to Phebe, Liberia, and concluding in Abuja, Nigeria. In October 2025, CEPI and partners (IVI, NCDC, Epicentre and MMARCRO) visited Enable 1.5 sites in Edo, Ondo, and Abakaliki States in Nigeria. The team reviewed scientific progress and deliverables, while also evaluating laboratory and clinical infrastructure, and troubleshooting as needed. All sites were progressing with the study as expected. Discussions highlighted the crucial role of sustained and enhanced community engagement to mitigate participant attrition, as well as continuous communication regarding the impact of Enable’s capacity-strengthening efforts. 

Study expands to Bauchi State

Bauchi State, northeastern Nigeria, reports one of the highest Lassa fever burdens in the country, along with Edo and Ondo States. In April 2026, the Enable 1.5 study expanded to include Bauchi State, commencing enrollment of 1000 patients for follow up over 12 months. The case definition used allows for the identification of infections with and without fever to ensure all disease presentations are captured, and an accurate incidence rate is measured; the current case definition also prioritizes epidemiological linkage to confirmed cases. As in other sites already in Enable 1.5 (Edo, Ondo, and Ebonyi States), participants will give blood samples for RT-PCR and ELISA analyses. Follow-up is currently ongoing, with data entry expected to finish in mid-2027, followed by data tidying and analysis, and the publication of results. CEPI is grateful to be collaborating with ATBUTH and NCDC in this important study expansion.

Sociobehavioural study (SBS) commenced

The Enable 1.5 Socio-Behavioural Study has commenced, assessing community attitudes and acceptance of vaccination with a future licensed Lassa fever vaccine, and participation in Lassa fever vaccine and therapeutics clinical trials in West Africa. The SBS is being conducted across six sites in three countries – Nigeria (four sites), Liberia (one site), and Sierra Leone (one site). A Training and Harmonisation Workshop was held on 1–2 April 2026 to align the four Nigerian sites and key partners ahead of the implementation of the Socio-Behavioural Study. It provided a practical forum to clarify the study approach, confirm site-level expectations, and support readiness for field activities.

Sensorineural hearing loss assessment

Infection with the Lassa virus may result in sensorineural hearing loss (SNHL), which can affect survivors for a lifetime. Understanding the baseline level of SNHL in communities where CEPI is studying Lassa fever is key, as well as ascertaining the incidence of new SNHL cases due to new Lassa virus infections. As part of Enable 1.5, CEPI has provided KUDUwave audiometers with laptops, and newly trained staff are testing participants' hearing.

Resources and Publications

Incidence of symptomatic Lassa virus infection in West African countries (Enable 1.0 Lassa Research Study): a prospective, multisite, cohort study | By Issifou A, Deha A, Yadouleton A, et al. |  The Lancet Global Health (June) 2026

Post-acute sequelae after Lassa fever: A systematic review and meta-analysis | By Wei Q, Zhang T, Schmit N. |  Journal of Infection Vol 92, Issue 5 (May) 2026

Serologic Evidence of Hemorrhagic Fever Virus Spillover in Rural Liberia | By Schafer AM, Kerkula E, Lee C, et al. |  Open Forum Infectious Diseases Vol 13, Issue 3 (March) 2026

Prospective cohort study to evaluate Lassa fever incidence, symptoms and coinfection with malaria in West Africa: the Enable Lassa Research Programme (‘ENABLE 1.5’) – study protocol | By Mandi H, Asogun D, Ayodeji O, et al. |  BMJ Public Health 3:e001960 (July) 2025

From the pain of Ebola to the passion to fight Lassa - an interview with Dr Donald Grant, Sierra Leone Ministry of Health 

A deadly viral illness is exploding in West Africa. Researchers are scrambling to figure out why  Science | Vol 383, Issue 6685 

Events

Incidence of Lassa fever disease and Lassa Virus infection in five West African countries: a prospective, multi-site, cohort study presentation at the American Society of Tropical Medicine and Hygiene Conference by Professor Danny Asogun | New Orleans on 16 November 2024

The ENABLE Lassa research programme: addressing key epidemiology gaps for Lassa fever vaccine development - a symposium at the World Congress of Epidemiology (25 September 2024, Cape Town) Programme | World Congress of Epidemiology 2024 (wce2024.org)