HMN 2026: How New Lassa fever vaccine shows promising results for first-in-human clinical trial

Researchers at the University of Maryland School of Medicine’s Center for Vaccine Development and Global Health, or CVD, report encouraging results from an early clinical trial that tested a new dual vaccine against Lassa fever and rabies. The study, published in the journal Nature Medicine, found that the vaccine was safe and induced immune responses against both viruses. There are currently no vaccines against Lassa fever on the market.

The World Health Organization has identified Lassa virus as a public health threat in West Africa and made Lassa fever a priority disease for research. Like Ebola, it can trigger severe illness and periodic outbreaks in African nations.

Lassa virus infections occur in 300,000 people every year, resulting in 5,000 deaths, according to the Africa Centers for Disease Control and Prevention, but these numbers are likely an underestimate because of limited surveillance. The disease is particularly dangerous in pregnancy, with more than 80% of late-term infections resulting in the death of the mother or fetus.

Additionally, regions where Lassa fever is common, such as parts of West and sub-Saharan Africa, also have a high burden of rabies, with thousands of deaths annually. The disease is almost always fatal once symptoms develop.

“This vaccine is designed to protect against two viruses of global health importance,” said study principal investigator Justin Ortiz, MD, MS, professor of medicine at UMSOM and a vaccine researcher at CVD.

“By combining targets into a single product, it could reduce the need for separate vaccination efforts and streamline delivery in settings where access is limited.”

Trial findings and next steps

In this randomized, controlled trial, 54 healthy adult volunteers from the Baltimore area were assigned to receive different doses of the investigational vaccine, LASSARAB, with an adjuvant or a licensed rabies vaccine control. Participants received two vaccine doses 28 days apart. Immune responses were studied through 61 days post-vaccination for an interim analysis.

The results indicate that LASSARAB was safe, with no serious adverse events reported after vaccination. Additionally, the candidate vaccine induced rapid and robust antibody responses against both Lassa and rabies viruses compared with the control, which only induced an immune response against rabies virus.

This study is ongoing, and vaccine safety and immune responses will be further studied through 394 days post-vaccination. If the results indicate continued elevated immune responses from vaccination, researchers will proceed with more advanced clinical trials.

“This study highlights CVD’s commitment to tackling diseases of global significance,” added Stefan Kappe, Ph.D., director of the Center for Vaccine Development and Global Health and the Myron M. Levine Endowed Professor of Pediatrics.

“LASSARAB not only targets diseases of concern but utilizes a platform that could make distribution attainable in the areas of the world that are most affected by these diseases.”

How the vaccine was designed

This first-in-human trial was designed to evaluate the safety and immunogenicity of an adjuvanted, inactivated rabies virus expressing the Lassa virus glycoprotein complex on the surface of the virus. LASSARAB was developed by a research team at Thomas Jefferson University led by Professor Matthias Schnell, director of the Jefferson Center for Vaccines and Pandemic Preparedness.

Importantly, this investigational vaccine can be freeze-dried for storage, enabling distribution to areas of the world where it may be difficult to maintain cold chains.

“Climate change is causing Lassa fever to extend its reach far beyond its Nigerian and West African origins, putting an estimated 700 million people at risk worldwide,” said UMSOM Dean Mark T. Gladwin, MD.

“By 2070, the number of countries across Africa that will develop ecological conditions suitable for Lassa virus spread could drastically increase, so a vaccine to prevent this deadly infection is desperately needed.”

Last year, before results were available, the trial was highlighted by Nature Medicine in its 2025 feature, “Eleven clinical trials that will shape medicine in 2026,” which identified studies to watch based on their potential to address major unmet health needs.

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