An experimental vaccine designed to protect against both Lassa fever and rabies has delivered encouraging results in its first human clinical trial. Researchers from Thomas Jefferson University and the University of Maryland School of Medicine reported that the vaccine, known as LASSARAB, was safe, well tolerated, and generated strong immune responses against both viruses. The findings were published in the journal Nature Medicine.

Lassa fever is a viral hemorrhagic illness endemic to several West African countries, causing an estimated 300,000–500,000 infections and more than 5,000 deaths annually. The disease can result in serious complications, including organ damage, hearing loss, and long-term neurological problems. Despite its impact, no approved vaccine currently exists for Lassa fever.

The LASSARAB vaccine uses an inactivated rabies virus as a delivery platform engineered to produce the Lassa virus glycoprotein complex. Combined with a synthetic adjuvant that boosts immune responses, the vaccine builds on the proven safety profile, manufacturing capacity, and global distribution network of existing rabies vaccines. Among the leading Lassa fever vaccine candidates, LASSARAB is the only one based on an inactivated or weakened virus platform.

The randomized Phase 1 trial enrolled 54 healthy adults aged 18 to 50. Participants received either two doses of LASSARAB at varying dose levels or a licensed rabies vaccine as a control. No serious adverse events or severe reactions were reported. The most common side effects, including injection-site pain, tenderness, and mild flu-like symptoms, were temporary and resolved without treatment.

The vaccine also demonstrated strong immunogenicity. Sixty-one days after the first dose, every participant who received LASSARAB developed antibodies against the Lassa virus, while no such response was observed in the control group. In addition, all vaccinated participants reached rabies-neutralizing antibody levels above the protective threshold established by the World Health Organization, matching the performance of licensed rabies vaccines.

According to senior author Matthias J. Schnell, director of the Jefferson Center for Vaccines and Pandemic Preparedness, the study provides the first clinical evidence that a rabies-virus-based vaccine platform can safely induce immunity against Lassa virus in humans. He noted that combining protection against both diseases in a single vaccine could have a major public health benefit in regions where Lassa fever and rabies remain widespread.

Lead investigator Justin Ortiz emphasized that the study was primarily designed to evaluate safety and immune responses rather than real-world effectiveness. Larger trials and longer follow-up periods will be needed, particularly in areas where Lassa fever is endemic, to determine how long protection lasts and how well the vaccine prevents disease.

Nevertheless, the results support further development of LASSARAB as a next-generation vaccine capable of addressing two significant global health threats with a single immunization strategy.