A first-in-human Phase 1 clinical trial of IAVI’s Lassa virus (LASV) vaccine candidate has shown encouraging results, demonstrating robust and long-lasting immune responses and an acceptable safety profile, according to findings published in the New England Journal of Medicine.

The trial, sponsored by IAVI, a nonprofit organisation developing vaccines and antibodies for HIV, tuberculosis, and emerging infectious diseases, addresses a critical gap, as no vaccines or therapeutics are currently licensed against Lassa fever, a viral haemorrhagic illness endemic to West Africa that causes thousands of deaths yearly.

The IAVI C102 trial, funded by the Coalition for Epidemic Preparedness Innovations (CEPI), enrolled 114 volunteers across Liberia and the United States. Participants were randomised to receive one of four doses of the vaccine candidate, rVSV∆G-LASV-GPC, or a placebo, and were monitored for 12 months to assess immune response and safety.

Results showed that both humoral and cellular immune responses were detected across all dose levels for up to a year post-vaccination. Antibodies induced by the vaccine were cross-reactive against multiple LASV lineages circulating in West Africa, suggesting broad protection from a single dose. Importantly, no vaccine-related serious adverse events or cases of hearing loss—a known complication of Lassa infection—were reported.

IAVI Vice President and Head of Emerging Infectious Diseases and Epidemiology, Dr Swati Gupta, described the results as “encouraging,” emphasising that they strengthen IAVI’s position as a leader in the development of a Lassa fever vaccine.

Executive Director of Vaccine R&D at CEPI, Dr Kent Kester, highlighted the urgency of a vaccine due to climate change, migration, and human-animal interactions that may expand Lassa’s footprint across West Africa.

The Phase 1 success has led to the ongoing Phase 2a trial in West Africa, making rVSV∆G-LASV-GPC the most advanced LASV vaccine candidate in clinical development. The vaccine uses the recombinant vesicular stomatitis virus (rVSV) vector platform, the same technology behind Merck’s ERVEBO Ebola vaccine, which is licensed in multiple countries.

Collaboration has been central to the trial, with support from PREVAIL, a Liberia–U.S. clinical research partnership, and funding from CEPI and the European & Developing Countries Clinical Trials Partnership (EDCTP).

In September 2025, West African health ministers and the West African Health Organisation endorsed accelerating Lassa vaccine readiness as a regional strategic health priority, pledging political and financial support for vaccine development.

Principal Investigator at the PREVAIL site in Liberia, Dr Mark Kieh, stressed the vaccine’s potential to provide much-needed relief to communities affected by Lassa fever. IAVI affirmed its commitment to advance the vaccine to licensure and ensure affordable access if proven safe and effective.

The promising Phase 1 findings represent a significant step toward a single-dose Lassa vaccine, which could save thousands of lives and prevent substantial societal and economic costs in West Africa.