Lassa Fever Vaccine Study Enrollment Begins

NIH announced today that enrollment has begun for participants in a Lassa fever vaccine study at the University of Maryland School of Medicine, which the federal agency is sponsoring. The trial will enroll up to 55 healthy adults between the ages of 18 and 50 years to test the safety and immunogenicity of 3 different concentrations of the vaccine candidate. Participants will receive 2 injections, delivered 28 days apart, of either the vaccine candidate, LASSARAB, or a FDA-licensed rabies vaccine (control).¹

LASSARAB is based on an attenuated rabies vaccine that was subsequently inactivated to make the vaccine candidate. The experimental vaccine is then modified so that it expresses all the rabies proteins found in inactivated rabies vaccine along with a Lassa virus surface protein called the glycoprotein precursor complex (GPC). If LASSARAB is shown to be safe and elicits a good immune response to both the rabies proteins and the Lassa GPC, it could be used to prevent both diseases pending further testing in clinical trials and subsequent approval by the FDA. ¹

“The candidate vaccine being tested in this trial was developed by an NIH-supported research team at Thomas Jefferson University in Philadelphia,” said NIAID Director Jeanne Marrazzo, MD, MPH. “The progression of this candidate from the lab to a first-in-humans clinical trial is a promising step towards a vaccine to prevent Lassa fever.” ¹

Matthias Schnell, PhD, and colleagues at Thomas Jefferson University tested the experimental vaccine, in nonhuman primates. They found that 2 doses of the vaccine, delivered 28 days apart, protected all the immunized animals that were exposed to large and lethal amounts of Lassa virus 6 weeks after the second inoculation. ¹

Transmission and Presentation

According to the Centers for Disease Control and Prevention (CDC), Lassa fever can be a severe viral, hemorrhagic illness. It is spread by the multimammate rat. Lassa fever is transmitted by having contact with infected rats or their saliva, urine, or droppings, and there can be human-to-human transmission.²

CDC reports that 8 in 10 people have mild symptoms that include fever, fatigue and weakness. However, in people with severe cases, symptoms can include bleeding, vomiting, difficulty breathing, facial swelling, body pain, and shock. Symptoms usually present 1 to 3 weeks after infection has occurred.²

Lassa fever is endemic to West Africa, including the countries of Sierra Leone, Liberia, Guinea, and Nigeria. Additionally, individuals who live in neighboring countries are also at risk because the rat that spreads Lassa fever lives throughout the region. These animals are the only known carrier of the Lassa virus.²

A US Case

Last fall, a case of Lassa fever was reported in a US traveler that visited West Africa. The patient was hospitalized in isolation at the University of Iowa Health Care Medical Center where they died. The Iowa case was the ninth known occurrence since 1969 of Lassa fever in travelers returning to the United States from areas where Lassa fever is commonly found.³

References

1. NIH-sponsored trial of Lassa vaccine opens. NIH press statement. March 17, 2025. Accessed March 17, 2025.
https://www.nih.gov/news-events/news-releases/nih-sponsored-trial-lassa-vaccine-opens
2. About Lassa Fever. CDC. January 31, 2025. Accessed March 17, 2025.
https://www.cdc.gov/lassa-fever/about/index.html
3. Lassa Fever Suspected in Death of U.S. Traveler Returning from West Africa. CDC.
https://www.cdc.gov/media/releases/2024/s1028-lassa-fever.html