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Hamburg, 15.05.2026 – Although the infectious disease Lassa fever can be fatal, there is a lack of treatment options with proven efficacy. Researchers at the Bernhard Nocht Institute for Tropical Medicine (BNITM) and the University Medical Center Hamburg-Eppendorf (UKE) have led the first randomised clinical trial with the new drug candidate favipiravir. The promising results have been published in the journal Nature Medicine.
The Lassa virus causes Lassa fever in humans, a severe infectious disease endemic to West Africa. It belongs to the group of viral haemorrhagic fevers, meaning it can cause internal bleeding. Every year, between 100,000 and 300,000 people in West Africa contract Lassa fever. Among hospitalised patients, the mortality rate is around 20 per cent. The disease often begins with non-specific symptoms such as fever and can lead to severe cases involving organ failure and internal bleeding. To date, there is no vaccine against the Lassa virus. For decades, doctors have been using the drug ribavirin as the standard treatment. However, there is a lack of clinical interventional studies on ribavirin – these are studies in which researchers test treatments by administering a specific therapy to patients and examining its effects. It is therefore unclear whether treatment with ribavirin improves survival chances or might even worsen them.
“There is an urgent need for new drugs with proven efficacy for Lassa fever. Our knowledge of ribavirin is limited and based primarily on older reports from the 1980s. Furthermore, the mechanism of action of ribavirin in Lassa fever is unclear,” says Dr Mirjam Groger, laboratory group leader at the BNITM and co-last author of the publication. “We have now conducted the first randomised controlled drug trial to evaluate a new treatment option for Lassa fever. Our aim was to generate reliable evidence on how to treat the disease.”
Favipiravir as a promising candidate
To carry out the study, the BNITM researchers collaborated with long-standing partners from Hamburg, France and West Africa. Together with French and African colleagues, they tested the antiviral candidate favipiravir at the Irrua Specialist Teaching Hospital (ISTH) and the Federal Medical Centre Owo. These world-leading Lassa fever treatment centres are located in Nigeria, a country where Lassa fever is highly prevalent.
“We have been working closely with our partner institution, the ISTH, on site for almost 20 years. Over this period, the necessary infrastructure has been built to conduct this clinical interventional study in line with modern standards,” explains Prof. Dr Stephan Günther, head of the Department of Virology at the BNITM. He initiated the collaboration with the African partner institutions around 20 years ago. Dr Cyril Erameh, first author of the study and principal study physician in Nigeria: “Ongoing investment in diagnostic infrastructure and in therapeutic and clinical research, in building and strengthening the specialist workforce, and in safety measures at the ISTH paved the way for studies of this kind on a highly pathogenic virus such as the Lassa virus.”
Favipiravir is an antiviral agent originally developed to treat influenza. Favipiravir inhibits viral replication in a wide range of RNA viruses, including the Lassa virus. Clinical trials involving healthy volunteers and patients with influenza, COVID-19 and Ebola have shown that the drug is safe and well tolerated. Initial preclinical studies by the Department of Virology at BNITM also indicated that favipiravir could be a promising candidate for treating Lassa fever. Building on this, researchers at the BNITM and their collaborators conducted the clinical Phase II SAFARI trial. In this randomised controlled trial, the researchers evaluated the safety and tolerability of favipiravir in Lassa fever patients compared with the standard therapy ribavirin, and measured the concentration of the antiviral drug in the patients’ blood.
Favipiravir is safe and well tolerated in the treatment of Lassa fever
The international team included 41 patients with mild to moderate Lassa fever in their study. The patients were randomly assigned to receive either ribavirin or favipiravir treatment. To assess the safety and tolerability of the therapy, the researchers recorded so-called treatment-emergent adverse events. Among other things, anaemia, nausea and fever occurred during treatment with both ribavirin and favipiravir. “What makes assessment difficult is that these non-specific events could be either side effects of the drugs or symptoms of Lassa fever,” explains Groger. The study’s results show that favipiravir can be a safe and well-tolerated alternative to the current treatment standard ribavirin.
Ribavirin can contribute to the destruction of red blood cells as a side effect. As Lassa fever can also lead to internal bleeding, it would be advantageous to be able to administer other drugs that do not affect blood cells. Based on the study results, favipiravir could be this alternative candidate in terms of its safety profile.
“The lack of previous clinical trials on the treatment of Lassa fever motivated us to build infrastructure and expertise in this highly endemic region together with our Nigerian partners. Our aim was to establish a sustainable research platform where new drugs and compounds can be evaluated according to high international standards,” explains Prof. Dr Michael Ramharter, head of the Department of Clinical Research at BNITM and head of I. Department of Medicine at UKE, outlining the rationale behind this clinical trial programme. “This world’s first randomised clinical interventional trial marks the first visible success of a long-term study programme designed to significantly improve the treatment of Lassa fever and establish new, better-tolerated treatment standards to reduce Lassa fever mortality,” says Ramharter, describing the future direction for the clinical research projects.
Findings for future studies
The promising results of the SAFARI study provide the basis for further clinical trials investigating favipiravir in patients with severe Lassa fever, either alone or in combination with other medicines. Based on the data obtained on the absorption, distribution, metabolism and excretion of favipiravir, the researchers recommend a different treatment regimen for future clinical trials with favipiravir than the one used in this study. This regimen is now being evaluated as part of the international platform study INTEGRATE, in which the BNITM is involved as a founding member.
The SAFARI study received financial support from the German Center for Infection Research (DZIF), the Federal Ministry of Health, the German Federal Foreign Office and the German Research Foundation (DFG). Further support was provided by the PANDORA-ID (Pan-African Network For Rapid Research, Response, Relief and Preparedness for Infectious Disease Epidemics) network and the CRC (Collaborative Research Centre) 1648 “Emerging Viruses: Pathogenesis, Structure and Immunity”.
About the Bernhard Nocht Institute for Tropical Medicine (BNITM)
The BNITM is Germany’s largest institution for research, healthcare and teaching in the field of tropical and emerging infectious diseases. Current research priorities include malaria, haemorrhagic fever viruses, neglected tropical diseases (NTDs), immunology, epidemiology, implementation and the clinical aspects of tropical infections, as well as the mechanisms of virus transmission by mosquitoes. For handling highly pathogenic viruses and infected insects, the Institute has laboratories of the highest biosafety level (BSL-4) and a biosafety insectarium (BSL-3). In numerous countries of the Global South, the BNITM supports the development of (mobile) laboratory capacities. The Data Science Center has just been set up.
The BNITM is the National Reference Centre for Tropical Infectious Agents, a Consultant Laboratory for Bornaviruses, a WHO Collaborating Centre for Arboviruses and Haemorrhagic Fever Viruses, a WHO Collaborating Centre for Behavioural Research in Global Health (BRIGHT), and an institute within the Leibniz Association.
About the Irrua Specialist Teaching Hospital (ISTH)
The ISTH, located in the Nigerian state of Edo, is a key institution in the fight against Lassa fever. Since 2007, it has operated the Institute for Lassa Fever Research and Control (ILFRC). In 2018, ISTH scientists carried out pioneering work in developing portable genome sequencing technologies. The aim is to reduce morbidity and mortality from Lassa fever through research, training and education.
Dr Cyril Erameh
Bernhard Nocht Institut for Tropical Medicine
cyril.erameh@bnitm.de
Erameh C., Okwaraeke K et al. Favipiravir for Lassa fever: an open-label, randomized controlled phase II trial. Nature Medicine 2026, DOI: 10.1038/s41591-026-04402-w
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