Breakthrough in Lassa Fever Vaccine Development
A significant advancement has been made in the development of a vaccine against Lassa fever, a deadly viral hemorrhagic illness prevalent in West Africa. The Phase 1 clinical trial results for IAVI’s Lassa virus (LASV) vaccine candidate, rVSVΔG-LASV-GPC, demonstrate that a single dose elicits robust and long-lasting immune responses while maintaining an acceptable safety profile.
Key Findings from the Trial
The study, published in the New England Journal of Medicine, involved 114 volunteers across multiple sites in the U.S. and Liberia. Participants were randomly assigned to receive either the vaccine candidate at one of four dose levels or a placebo. They were monitored for 12 months post-vaccination to evaluate both safety and immune responses. Scientists from Imperial College London conducted investigations into immune responses among the participants.
Notably, immune responses activating both humoral and cellular branches of the immune system were observed across all dose levels up to 12 months after vaccination. Antibodies induced by the vaccine showed cross-reactivity to other LASV lineages circulating in West Africa, indicating potential broad protection from a single vaccine. No serious adverse events or cases of hearing loss—linked to LASV infection—were reported during the study.
Implications for Public Health
Lassa fever is a major public health concern, causing thousands of deaths annually in West Africa. It is classified as a priority pathogen by the World Health Organization (WHO) and is a target for the Coalition for Epidemic Preparedness Innovations (CEPI). CEPI funded the clinical trial (IAVI C102), which is part of broader efforts to develop vaccines against epidemic threats.
Swati Gupta, Vice President and head of emerging infectious diseases and epidemiology at IAVI, highlighted the importance of these findings: “These encouraging results add to a growing body of evidence that demonstrates the safety and immunogenicity of IAVI’s single-dose Lassa vaccine candidate and further establishes IAVI and our network of partners as a leading force in the development of a vaccine to prevent future Lassa outbreaks.”
Dr. Kent Kester, Executive Director of Vaccine R&D at CEPI, emphasized the significance of the data: “The promising Phase 1 data for IAVI’s vaccine candidate takes us one step closer towards a much-needed Lassa fever vaccine which, if successful, could save thousands of lives and avert millions of dollars of societal costs in the West African countries that bear the burden of this disease.”
Advancing the Vaccine Development Pipeline
Preclinical studies had already shown strong and durable immune responses to VSVΔG-LASV-GPC. Based on the strength of preclinical and Phase 1 clinical data, rVSVΔG-LASV-GPC has advanced to an ongoing Phase 2a clinical trial in West Africa, supported by CEPI. This vaccine candidate uses the same recombinant vesicular stomatitis virus (rVSV) vector platform as ERVEBO®, Merck’s single-dose Ebola vaccine, which is licensed in over a dozen countries.
The rVSV platform underpins IAVI’s broader portfolio of vaccines for emerging infectious diseases (EIDs). Since 2018, IAVI and members of the global Viral Hemorrhagic Fever Consortium have collaborated to accelerate the clinical development of rVSVΔG-LASV-GPC, with support from CEPI and the European & Developing Countries Clinical Trials Partnership (EDCTP).
In September 2025, West African Ministers of Health and the West African Health Organisation endorsed a communiqué reaffirming their commitment to accelerating Lassa fever vaccine readiness as a regional strategic health priority. They pledged to support the development of IAVI’s vaccine candidate through collaborative co-funding and joint action.
Commitment to Global Health
IAVI remains committed to advancing the rVSVΔG-LASV-GPC vaccine to licensure and ensuring affordable access should it prove safe and effective in clinical trials. The vaccine is based on an attenuated strain of vesicular stomatitis virus (VSV) modified to express a Lassa virus protein essential for establishing infection.
The authors of the study include researchers from various institutions, including George Washington University, Imperial College London, and the Partnership for Research on Vaccines and Infectious Diseases in Liberia. Their collective effort underscores the importance of international collaboration in addressing global health challenges.
About IAVI’s rVSV Vaccine Candidates
IAVI licensed the vaccine technology underlying rVSVΔG-LASV-GPC from the Public Health Agency of Canada (PHAC). The vaccine is based on an rVSV vector developed by scientists at PHAC’s National Microbiology Laboratory.
IAVI’s EID portfolio includes vaccine candidates for Sudan virus, Lassa fever, and Marburg virus, supported by various funders such as CEPI, EDCTP, BARDA, and DTRA. Much of the research and development on IAVI’s rVSV platform is conducted at the IAVI Vaccine Design and Development Lab (DDL) in Jersey City, New Jersey.
Funders who have supported the development of IAVI’s rVSV-vectored vaccine candidates include the Gates Foundation, the Government of Canada, the Danish Ministry of Foreign Affairs, and others.
About IAVI
IAVI is a non-profit scientific research organization dedicated to addressing global health challenges, including HIV, tuberculosis, and emerging infectious diseases. Its mission is to translate scientific discoveries into affordable, globally accessible public health tools.
About CEPI
CEPI was launched in 2017 as an innovative partnership between public, private, philanthropic, and civil organizations. Its mission is to accelerate the development of vaccines and other biologic countermeasures against epidemic and pandemic threats.
About PREVAIL
The Partnership for Research on Vaccines and Infectious Diseases in Liberia (PREVAIL) is a joint Liberia-U.S. biomedical research program recognized for conducting research that impacts healthcare delivery. Its mission is to conduct collaborative research and respond to emerging disease threats.