First-in-Human Trial Reports Promising Dual Lassa–Rabies Vaccine Data

Researchers at the University of Maryland School of Medicine’s Center for Vaccine Development and Global Health (CVD) reported encouraging interim results from an early clinical trial that tested a new dual vaccine against Lassa fever and rabies. The study found the vaccine to be safe and induced immune responses against both the Lassa fever and rabies viruses (RB). There are currently no vaccines against Lassa fever on the market.

“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 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.”

Ortiz is first and corresponding author of the researchers’ published paper in Nature Medicine, titled “Adjuvanted inactivated rabies virus-vectored Lassa virus vaccine in healthy adults: a phase 1 trial,” in which they said, “If efficacy is confirmed, this combination vaccine could help protect populations from two priority pathogens and have a meaningful public health impact in regions where both diseases remain major threats.”

The World Health Organization has identified Lassa virus (LASV) as a public health threat in western Africa and made Lassa fever a priority disease for research. “Transmission occurs primarily through contact with food or household items contaminated by urine or feces from Mastomys rodents, although person-to-person spread can occur via exposure to bodily fluids or contaminated surfaces,” the investigators noted in their paper. Like Ebola, Lassa virus 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 figures cited by the authors, but these numbers are likely an underestimate due to limited surveillance. The disease is particularly dangerous in pregnancy with over 80% of late-term infections resulting in deaths to the mother or fetus. Additionally, regions where Lassa fever is common, such parts of Western and Sub-Saharan Africa, also have a high burden of rabies, with thousands of deaths annually, a disease that is almost always fatal once symptoms develop. The newly reported first-in-human trial was designed to evaluate the safety and immunogenicity of an adjuvanted inactivated rabies virus expressing the Lassa virus glycoprotein complex (GPC) on the surface of the virus. “Scientists at Thomas Jefferson University developed an inactivated rabies-vectored combination vaccine derived from an attenuated rabies strain, LASSARAB, expressing both the rabies glycoprotein and the LASV (Josiah strain) GPC,” the authors explained. “The RABV platform provides a well-established foundation for a dual-target vaccine.”

For the randomized, controlled trial 54 healthy adult volunteers from the Baltimore area were randomly assigned to receive different doses of 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 (AEs) reported after vaccination. Additionally, the candidate vaccine induced rapid and robust antibody responses against both Lassa and rabies viruses when compared with the control, which only induced an immune response against rabies virus.

The study is ongoing, and vaccine safety and immune responses will be further studied through 394 days post-vaccination. “The final study report will be prepared after study completion and will include serious AEs and AEs of special interest through day 394, protocol-defined exploratory LASV and RABV antibody responses assessed at days 121 and 394, and any additional post hoc analyses, as applicable,” the team stated.

If the results indicate continued elevated immune responses from vaccination, researchers will proceed with more advanced clinical trials. 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. 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.

“These data support the feasibility of a bivalent rabies-vectored vaccine integrated into routine immunization for regions where cold-chain capacity is limited,” the team added.

“This study highlights CVD’s commitment to tackling diseases of global significance,” commented Stefan Kappe, PhD, 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.”

Added UMSOM dean Mark T. Gladwin, MD, “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. 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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