Phase 1 Trial Shows Safety and Immunogenicity of LASSARAB Lassa Fever and Rabies Vaccine

An interim report published June 9, 2026, in Nature Medicine shows that a new combination vaccine, LASSARAB + 3D-6-acyl PHAD-SE, is safe and triggers an immune response against both Lassa fever and rabies viruses. This first-in-human phase 1 trial tested the adjuvanted, rabies-vectored vaccine in healthy adult participants.

The study focused on the immunogenicity of a specific vaccine candidate that uses an inactivated rabies virus as a vector to deliver Lassa virus antigens. According to the report, the combination of the LASSARAB vaccine and the 3D-6-acyl PHAD-SE adjuvant successfully induced the desired immune responses without causing significant safety concerns in the initial cohort.

How does the LASSARAB vaccine work?

The LASSARAB candidate employs a rabies-vectored strategy. In this approach, an inactivated rabies virus serves as the delivery vehicle, or vector, to carry antigens from the Lassa virus into the body. This allows the immune system to recognize and develop defenses against both the rabies virus and the Lassa virus simultaneously.

To strengthen this response, researchers added 3D-6-acyl PHAD-SE, a specific adjuvant. Adjuvants are ingredients added to vaccines to create a stronger immune response, which often allows for lower doses of the active antigen or fewer booster shots.

By combining these elements, the vaccine aims to provide dual protection. This is a distinct departure from traditional single-target vaccines, as it leverages the known stability of inactivated rabies virus technology to address Lassa fever, a disease for which there is currently no widely available approved vaccine.

Why is a Lassa fever vaccine necessary?

Lassa fever is caused by the Lassa virus, a member of the arenavirus family. It is primarily endemic to West African nations, where it spreads through contact with food or household items contaminated with rodent urine or feces.

While many infections are mild, the disease can progress to severe hemorrhagic fever. According to public health data, severe cases often involve high fever, facial swelling, and bleeding from the mouth, nose, or gastrointestinal tract. A significant complication of Lassa fever is sensorineural deafness, which occurs in a substantial percentage of survivors.

Because treatment options are limited and the mortality rate for hospitalized patients can be high, the development of a safe, effective vaccine is a priority for global health security and the reduction of regional disease burdens.

What were the results of the phase 1 trial?

The primary goal of a phase 1 trial is to assess safety and determine if the vaccine triggers an immune response, known as immunogenicity. The interim report from the June 9, 2026, publication confirms that the LASSARAB + 3D-6-acyl PHAD-SE combination was well-tolerated by healthy participants.

The researchers observed that the vaccine induced immunogenicity to both the Lassa and rabies viruses. This means the participants’ immune systems produced the necessary antibodies or T-cell responses to potentially fight off these viruses if exposed in the future.

The use of the 3D-6-acyl PHAD-SE adjuvant was central to achieving these results, as it helped the body mount a more robust response to the inactivated vector. The safety profile observed in this first-in-human study provides the necessary baseline to move the candidate into larger trials.

What happens next for this research?

While the phase 1 results are positive, they do not yet prove that the vaccine prevents Lassa fever or rabies infections in real-world settings. Phase 1 trials are designed for safety and dosage, not for efficacy.

The next steps in the development process typically involve phase 2 and phase 3 trials. These subsequent stages will test the vaccine in larger, more diverse groups of people, including those in regions where Lassa fever is endemic. These trials will determine if the immune response observed in healthy adults translates into actual clinical protection against the disease.

Researchers will also continue to monitor the long-term safety of the rabies-vectored approach and the specific effects of the 3D-6-acyl PHAD-SE adjuvant over longer periods of time.