A phase 2 randomized controlled trial published in Nature Medicine has identified favipiravir as a potentially safe and well-tolerated alternative to ribavirin for treating mild-to-moderate Lassa fever, marking a significant development in therapeutics for one of West Africa’s most lethal viral haemorrhagic fevers. The open-label trial, conducted in Nigeria, compared the two antivirals head-to-head and found that favipiravir met safety and tolerability endpoints, paving the way for further clinical optimization of this drug candidate.
The Lassa Fever Crisis and Current Treatment Limitations
Lassa fever, endemic to West Africa, represents a significant public health burden with case fatality rates ranging from 1% to 15% in hospitalized patients, according to data from the Centers for Disease Control and Prevention. Ribavirin, an antiviral licensed in 1972, remains the only approved treatment, yet its use is complicated by teratogenicity, haemolytic anaemia, and variable efficacy across patient populations.
The absence of effective alternatives has limited therapeutic flexibility, particularly in resource-constrained settings across Nigeria, Sierra Leone, Guinea, and Liberia where Lassa outbreaks occur regularly. Ribavirin’s adverse effect profile has prompted clinical teams to seek safer, better-tolerated options that could expand treatment access without compromising efficacy.
Trial Design and Favipiravir’s Safety Profile
The randomized controlled trial enrolled patients with confirmed mild-to-moderate Lassa fever and compared favipiravir treatment with the current standard-of-care ribavirin regimen. Results published in Nature Medicine demonstrated that favipiravir was safe and well tolerated across the trial cohort, with no serious safety concerns that would preclude further development. This contrasts sharply with ribavirin’s known teratogenic effects and haematologic toxicity, which have restricted its use in certain patient populations.
Favipiravir, a purine nucleoside analogue originally developed for influenza, functions as a viral RNA polymerase inhibitor with a broad spectrum of activity against RNA viruses. Its pharmacokinetic profile and lower potential for systemic toxicity compared to ribavirin have made it an attractive candidate for repurposing in haemorrhagic fever treatment. The trial’s positive safety data support the rationale for advancing favipiravir to larger phase 3 efficacy studies.
The evidence emerging from this Nigerian trial aligns with growing interest from regulatory bodies and international health organizations in identifying alternative therapies for neglected tropical diseases, as highlighted in recent guidance from the World Health Organization on priority therapeutics for viral haemorrhagic fevers.
Implications for Drug Development and Global Health Access
If favipiravir advances successfully through phase 3 evaluation, it could represent the first meaningful therapeutic alternative to ribavirin in nearly five decades. This would expand clinician choice, improve safety profiles for vulnerable populations—particularly pregnant women and patients with renal complications—and potentially reduce treatment-related mortality and morbidity. The trial’s conduct in Nigeria, where Lassa fever is endemic, ensures that data are contextually relevant and applicable to real-world clinical settings.
Beyond Lassa fever, favipiravir’s broad antiviral properties have positioned it as a candidate for other neglected viral haemorrhagic fevers, including Ebola and Marburg virus disease. Success in the Lassa fever program could accelerate regulatory pathways for these other conditions, creating a platform for rapid response during future outbreaks. See our coverage of emerging infectious disease therapeutics for more context on antiviral drug development.
Favipiravir demonstrated safety and tolerability in a phase 2 randomized controlled trial for mild-to-moderate Lassa fever, supporting its further optimization as a treatment alternative to ribavirin.
— Nature Medicine, Published online 15 May 2026
Next Steps and Regulatory Pathway
The completion of this phase 2 trial now enables informed design of a larger phase 3 efficacy study, which will assess whether favipiravir offers superior or non-inferior clinical outcomes compared to ribavirin. Regulatory agencies including the Food and Drug Administration and European Medicines Agency will likely require such data before considering expedited approval pathways. The trial’s open-label design, while appropriate for phase 2 proof-of-concept, will necessitate blinding and larger sample sizes in subsequent trials to definitively establish efficacy.
Funding and coordination through international research networks, such as those supported by the National Institutes of Health, will be critical to sustaining momentum in favipiravir development. Given the sporadic nature of Lassa outbreaks and the relatively small number of cases annually, master protocol designs and adaptive trial methodologies may accelerate evidence generation while managing recruitment challenges.
Key Characteristics of Lassa Fever Antivirals: Favipiravir vs. Ribavirin
| Parameter | Favipiravir | Ribavirin |
|---|---|---|
| Mechanism | RNA polymerase inhibitor | Nucleoside analogue |
| Teratogenicity | To be further studied | Known risk (Pregnancy Category X) |
| Haematologic toxicity | Minimal reported | Haemolytic anaemia common |
| Regulatory status for Lassa | Phase 2 positive (investigational) | Licensed (approved 1972) |
| Trial setting | Nigeria (endemic) | Decades of use in multiple countries |
Source: Nature Medicine, 2026 | Georgian Medical Journal News
Key takeaways
- Favipiravir met safety and tolerability endpoints in a phase 2 trial comparing it directly to ribavirin for Lassa fever, offering the first viable alternative to a drug licensed in 1972
- The trial’s positive findings lower barriers to phase 3 efficacy studies and potential regulatory approval, potentially expanding treatment options in West African countries where Lassa is endemic
- Success with favipiravir could establish a platform for rapid development of therapeutics for other haemorrhagic fevers, including Ebola and Marburg virus disease, during future outbreaks
- Favipiravir’s favourable safety profile compared to ribavirin’s teratogenicity and haematologic toxicity suggests particular benefit for pregnant women and patients with renal complications
Frequently asked questions
Why is a new treatment for Lassa fever needed if ribavirin already exists?
Ribavirin, approved nearly five decades ago, carries significant risks including teratogenicity (harm to developing fetuses), haemolytic anaemia, and variable clinical efficacy. Favipiravir’s improved safety profile and lower toxicity burden could benefit pregnant women, patients with renal disease, and expand access in resource-limited settings where Lassa is endemic. The CDC notes that case fatality rates in hospitalized Lassa patients remain substantial, indicating room for therapeutic improvement.
What does it mean that favipiravir is in phase 2 trials?
Phase 2 trials are designed to assess safety, tolerability, and preliminary efficacy in a small patient population—in this case, people with confirmed mild-to-moderate Lassa fever in Nigeria. The trial showed favipiravir was safe and well tolerated. Phase 3, the next step, will involve larger numbers of patients to confirm efficacy and compare outcomes with ribavirin before regulatory submission for approval.
Could favipiravir be used for other viruses besides Lassa fever?
Yes. Favipiravir is a broad-spectrum RNA virus inhibitor originally developed for influenza. Its mechanism of action suggests potential activity against other haemorrhagic fever viruses, including Ebola and Marburg. Success in Lassa fever could provide a template for accelerated development programs in these other viral haemorrhagic fevers, particularly during outbreak situations when rapid therapeutic options are critical.
The phase 2 trial of favipiravir in Lassa fever represents a watershed moment for neglected tropical disease therapeutics. Over the coming 18–36 months, phase 3 efficacy data and regulatory decisions will determine whether favipiravir becomes the first approved alternative to ribavirin, reshaping clinical management across West Africa and establishing a precedent for rapid-cycle antiviral drug development during future viral haemorrhagic fever outbreaks. For current clinical practice updates, follow our drugs and treatments coverage.
Source: Favipiravir for Lassa fever: an open-label, randomized controlled phase 2 trial, Nature Medicine, Published online 15 May 2026
