PHILADELPHIA, PA., June 9, 2026 – Researchers from Thomas Jefferson University and the University of Maryland School of Medicine have reported encouraging results from the first human clinical trial of LASSARAB, an experimental vaccine designed to protect against Lassa fever and rabies in a single formulation. The findings, published in the journal Nature Medicine , show that the vaccine was well tolerated and induced robust immune responses against both viruses in healthy adult volunteers.
Lassa fever, which is caused by the virus, is a severe hemorrhagic disease that is endemic to parts of West Africa, causing an estimated 300,000 to 500,000 infections and over 5,000 deaths each year. The disease can lead to severe complications, including organ failure, hearing loss and long-term neurological disorders. Currently, there are no licensed vaccines to prevent Lassa fever, but the LASSARAB platform may offer a practical and scalable solution for endemic regions with limited resources.
LASSARAB is based on an inactivated rabies virus vector, which is engineered to express the Lassa virus glycoprotein complex (GPC) and with a synthetic adjuvant, which enhances immune response. LASSARAB leverages the rabies vaccine’s long-established safety record, enduring immunity, existing manufacturing infrastructure and global distribution networks. Among the three Lassa fever vaccines currently under development, LASSARAB is the only candidate based on a killed or attenuated virus.
In this randomized Phase 1 trial, featured as one of Nature Medicine ’s “ Eleven clinical trials that will shape medicine in 2026 ,” 54 male and female adults aged 18 through 50 years old were split into four groups. They either received two intramuscular doses of LASSARAB at increasing (low, medium and high) dose levels or a licensed rabies vaccine as a control. Across all vaccine dose groups, no serious adverse events or severe reactions were observed and reported side effects (such as injection‑site tenderness, pain and mild flu‑like symptoms) were temporary and resolved without additional treatment.
Importantly, the vaccine demonstrated a strong immune response. By 61 days after the first dose, 100% of LASSARAB recipients developed Lassa virus–specific antibodies, while all control participants did not. At the same time, all participants receiving LASSARAB achieved levels of neutralizing antibody against the rabies virus that exceed the World Health Organization’s established threshold for protection, matching responses seen with licensed rabies vaccines.
“These results provide the first clinical evidence that a rabies‑vectored platform can safely generate immunity to Lassa virus in humans,” says senior author Matthias J. Schnell, PhD , director of the Jefferson Center for Vaccines and Pandemic Preparedness and chair of the department of microbiology and immunology at the Sidney Kimmel Medical College at Thomas Jefferson University . Dr. Schnell has played a pivotal role in establishing rabies virus (RABV)–based vectors as a versatile platform for vaccines against high-consequence infectious diseases. “The ability to combine protection against Lassa fever and rabies in a single vaccine could have a substantial public‑health impact in regions where both diseases remain endemic.”
“In this first-in-human study, the LASSARAB vaccine showed promising early results, with the potential to provide dual protection against both diseases in a single product. If confirmed in further studies, this approach could represent a meaningful advance for global health,” added Justin Ortiz, MD, MS, of the Center for Vaccine Development and Global Health and the principal investigator on the clinical trial.
The investigators emphasize that this was an early‑stage trial designed to assess safety and immune responses, not efficacy. Longer‑term follow‑up and larger studies in populations where Lassa fever is endemic will be needed to determine the durability of protection and real‑world effectiveness. Nonetheless, the results support continued clinical development of LASSARAB as a next‑generation, dual‑purpose vaccine targeting two major global health threats.
About the study
The trial was conducted at the University of Maryland School of Medicine’s Center for Vaccine Development and Global Health and was funded by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health under contract number: HHSN272201700082C. The study is registered at ClinicalTrials.gov (NCT06546709).
The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
Media Contact: Deborah Balthazar, 267-254-9851, deborah.balthazar@jefferson.edu
About Jefferson
Nationally ranked, Jefferson, which is principally located in the greater Philadelphia region, Lehigh Valley and eastern Pennsylvania and southern New Jersey, is reimagining health care and higher education to create unparalleled value. Jefferson is more than 65,000 people strong, dedicated to providing the highest-quality, compassionate clinical care for patients; making our communities healthier and stronger; preparing tomorrow's professional leaders for 21st-century careers; and creating new knowledge through basic/programmatic, clinical and applied research. Thomas Jefferson University , home of Sidney Kimmel Medical College, Jefferson College of Nursing, and the Kanbar College of Design, Engineering and Commerce, dates back to 1824 and today comprises 10 colleges and three schools offering 200+ undergraduate and graduate programs to more than 8,300 students. Jefferson Health , nationally ranked among the top 15 not-for-profit health care systems in the country and the largest provider in the Philadelphia and Lehigh Valley areas, serves patients through millions of encounters each year at 33 hospital campuses and more than 700 outpatient and urgent care locations throughout the region. Jefferson Health Plans is a not-for-profit managed health care organization providing a broad range of health coverage options in Pennsylvania and New Jersey for more than 40 years.
Nature Medicine
Randomized controlled/clinical trial
People
Adjuvanted inactivated rabies virus-vectored Lassa virus vaccine in healthy adults: a phase 1 trial
9-Jun-2026
W.H.C. serves as the US FDA IND sponsor representative for the investigational vaccine. J.R.O. is a member of the Moderna Therapeutics New Vaccines Advisory Board, a consultant for ENA Respiratory, and reports research funding from ENA Respiratory and Pfizer provided to his institution. R.R.R. is currently employed by Moderna Therapeutics and may own Moderna shares. M.J.S. has a pending patent application for 3D-6-acyl PHAD-SE. All other authors declare no competing interests.