Phase 1 data from a University of Maryland team show the LASSARAB candidate triggered no serious adverse events and produces antibodies against both diseases, but field efficacy and durability remain untested.
The rabies virus has been used as a vaccine backbone for nearly four decades. Now researchers at the University of Maryland School of Medicine have wrapped a Lassa fever antigen inside that same inactivated rabies shell and tested it in humans for the first time. The construct, called LASSARAB, induced antibody responses against both rabies and Lassa virus in a small phase 1 trial, according to results published in Nature Medicine this week.
The dual readout is a proof of concept, not an efficacy result. It shows the rabies backbone can carry a second disease target and still train the immune system on both, the engineering move that public-health planners in West Africa would need to cover two endemic diseases in a single clinic visit.
"There is no licensed vaccine for Lassa fever, and the disease overlaps geographically with endemic rabies," said Justin Ortiz, MD, MS, the trial's first and corresponding author and a researcher at UMSOM's Center for Vaccine Development and Global Health, as quoted by Genetic Engineering & Biotechnology News. A single product that covers both, he added, "would simplify delivery and reduce the number of injections needed in low-resource clinics."
The trial enrolled 54 healthy adults in the Baltimore area and randomized them to receive either LASSARAB or a licensed rabies vaccine as a control, in two doses 28 days apart. Investigators analyzed immune responses at day 61 and will continue safety follow-up through day 394. No serious adverse events were reported, according to the Nature Medicine paper. Participants who received LASSARAB developed antibody responses against both Lassa and rabies, while those in the control arm showed rabies-specific responses only.
The platform design is what makes the result worth attention. LASSARAB uses an inactivated rabies virus engineered to display the Lassa virus glycoprotein complex (GPC) on its surface, paired with an adjuvant to amplify the immune signal. The rabies backbone has been used in vaccines given billions of times worldwide, and preclinical LASSARAB protected animals against lethal Lassa challenge. The phase 1 readout shows the same backbone can drive antibody production against a payload it was never designed to carry.
That matters because the access gap is severe. Lassa fever, a hemorrhagic illness endemic to West and Sub-Saharan Africa, kills an estimated 5,000 people a year out of roughly 300,000 infections, according to figures cited in the GEN report. Those numbers are widely considered an undercount because surveillance is limited in the affected regions. Mortality exceeds 80 percent in late-term pregnancy cases, and roughly a quarter of survivors develop sensorineural hearing loss. No Lassa vaccine is licensed anywhere, despite the pathogen's WHO priority status.
A second feature of the construct addresses the other barrier to deployment. LASSARAB is formulated to be freeze-dried, removing the cold-chain requirement that complicates vaccine distribution in the rural clinics where both diseases circulate. Freeze-dried rabies vaccines are already in use; pairing that delivery format with a Lassa payload extends the same logistics to a new disease. The clinical reality is that rabies kills tens of thousands of people a year, mostly in low-resource settings, and post-exposure prophylaxis is often unavailable when someone is bitten.
The climate story is less certain. UMSOM dean Mark T. Gladwin has cited estimates that roughly 700 million people could fall within Lassa's expanding ecological range as the disease moves beyond its historical Nigeria and West Africa footprint, with modeling suggesting many more African countries could develop suitable ecological conditions for the virus by 2070, according to the GEN report. Those projections are attributed claims from UMSOM leadership, not peer-reviewed modeling in the Nature Medicine paper, but they frame a longer-horizon argument for why a deployable Lassa vaccine matters even before the next outbreak cycle.
Independent selection by Nature Medicine flagged the trial before the readout. The journal's 2025 feature "Eleven clinical trials that will shape medicine in 2026" pre-identified the LASSARAB program as one to watch, an unusual third-party editorial endorsement that gives the early human data more weight than a routine press release.
What the data do not yet show is whether the Lassa antibody response actually protects people. There is no established immune correlate of protection for Lassa, and the Nature Medicine paper notes that LASV-neutralizing antibodies are not universally detected in survivors, according to the paper. Antibody induction is a signal, not a guarantee. T-cell response depth, durability of the antibody response past day 61, and efficacy against actual Lassa exposure all remain to be tested in larger phase 2 and 3 trials, which would also need to enroll pediatric and pregnant populations and run in endemic settings.
The trial is also a single-site study in healthy U.S. adults. Geographic diversity, real-world transmission settings, and the populations most at risk of severe Lassa disease are not represented. Any deployment claim, from stockpiling to country-by-country rollout, has to wait for those data.
What to watch next: phase 2 design in Lassa-endemic West African countries, including an active comparator against the licensed rabies backbone on its own, and T-cell and durability data past the day 61 interim. The construct cleared the first human checkpoint. The harder questions are next.