A new vaccine candidate against Lassa fever, one of West Africa’s deadliest diseases, has shown strong and long-lasting immune protection in humans, raising hopes for the first-ever approved vaccine against the virus.

Findings from the first-in-human Phase 1 clinical trial of the vaccine, developed by IAVI, were published in the New England Journal of Medicine. The results show that a single dose of the vaccine, known as rVSV∆G-LASV-GPC, is safe, well-tolerated, and produces robust immune responses lasting up to a year.

The vaccine is being developed by IAVI, a nonprofit scientific research organisation that creates vaccines and antibodies for HIV, tuberculosis, and other emerging infectious diseases. The trial was funded by the Coalition for Epidemic Preparedness Innovations (CEPI) — a global partnership that supports the development of vaccines against epidemic threats.

Lassa fever, caused by the Lassa virus (LASV), is a severe viral illness that kills thousands of people each year in West Africa, where it is endemic. The World Health Organisation (WHO) has listed the Lassa virus as a priority pathogen needing urgent vaccine research because of its epidemic potential.

Despite the threat, no licensed vaccine or specific treatment currently exists for Lassa fever.

The IAVI trial, known as IAVI C102, involved 114 volunteers in Liberia and the United States. Participants were randomly given one of four vaccine doses or a placebo and were monitored for 12 months to assess safety and immune response.

Researchers found that the vaccine triggered strong antibody and cellular immune responses against the virus across all dose levels — responses that lasted through the one-year study period. Importantly, the antibodies were cross-reactive, meaning they could protect against multiple strains of the Lassa virus circulating in West Africa.

No serious vaccine-related side effects were reported. Crucially, no hearing loss — a common long-term complication of Lassa fever — occurred among participants.

“These encouraging results add to a growing body of evidence that demonstrates the safety and immunogenicity of IAVI’s single-dose Lassa vaccine candidate,” said Dr. Swati Gupta, Vice President and Head of Emerging Infectious Diseases at IAVI.

“A vaccine is urgently needed as shifting patterns in climate, migration, and human-animal interaction may expand Lassa’s footprint across West Africa and even further afield,” she said.

A Step Toward Protection

Executive Director of Vaccine R&D at CEPI, Dr. Kent Kester, said the new data bring the world one step closer to protecting millions of people.

“Lassa fever is a cruel disease which has plagued West Africa for decades, including a deadly outbreak in Nigeria this year,” he said.

“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.”

Principal Investigator at the PREVAIL Redemption Hospital site in Liberia, Dr. Mark Kieh, added that communities have long awaited such progress.

“The development of a vaccine to prevent Lassa fever will provide much-needed relief to our communities impacted by the virus.”

Built on Proven Vaccine Technology

The Lassa vaccine uses the same rVSV vector platform as ERVEBO®, the single-dose Ebola vaccine licensed by Merck and approved in more than a dozen countries. The Ebola vaccine has already proven successful during outbreaks in Africa, providing a strong foundation for the Lassa candidate.

Encouraged by the positive Phase 1 data, IAVI and CEPI have already advanced the vaccine to an ongoing Phase 2a clinical trial in West Africa. It is currently the most advanced Lassa vaccine candidate in the world.

In September 2025, West African Ministers of Health and the West African Health Organisation (WAHO) met in Abidjan, Côte d’Ivoire, where they reaffirmed political support for speeding up Lassa vaccine readiness. They endorsed a regional plan to co-fund vaccine development and mobilise resources through advocacy and collaboration.

The move positions the Lassa vaccine as a strategic health priority and a key part of pandemic preparedness for West Africa.

“As a nonprofit product development partnership, IAVI is committed to addressing this unmet medical need together with local governments, communities, funders, and other experts with aligned global health priorities,” Dr Gupta said.

Both IAVI and CEPI say they are committed to ensuring that, if proven safe and effective, the Lassa vaccine will be affordable and accessible to those most at risk.

For the people of West Africa, where Lassa fever remains a constant threat, this breakthrough promises a future where a single shot could save countless lives.