Favipiravir Just Beat Ribavirin in Lassa Fever—Now Comes the Hard Part

Picture a clinical trial coordinator in Kenema, Sierra Leone, pulling overnight mortality data from an open-label randomized controlled trial that nobody in the global infectious disease community was certain could even be completed. Lassa fever kills fast—case fatality rates among hospitalized patients reached 16.3% in Nigeria in 2024, according to the WHO—and the standard of care, ribavirin, has been in use since a 1986 study whose evidentiary foundation has been questioned ever since. Now, for the first time, a Phase 2 randomized controlled trial published in Nature Medicine has compared favipiravir directly against that legacy drug. The hazard ratio for mortality in the favipiravir arm: 0.50. Half the risk of death.

That number deserves to sit alone for a moment.

An HR of 0.50 in a hemorrhagic fever trial, conducted under open-label conditions in an endemic West African setting, against a comparator that has dominated treatment protocols for four decades—that is not a modest signal. That is a result that should force a serious conversation about what comes next, who is responsible for convening it, and whether the regulatory infrastructure that governs rare tropical disease therapeutics is remotely equipped to handle it.

The Evidence Ribavirin Never Had

The ribavirin story is a case study in how a drug achieves clinical entrenchment without the evidence base to justify it. The foundational 1986 McCormick study—still the most cited justification for ribavirin’s use in Lassa fever—was conducted without the controls or sample sizes that modern trial methodology would require. A 2022 systematic review commissioned by the Bristol Biomedical Research Centre and covering all published and unpublished data through March 8, 2022, concluded that “robust evidence supporting the use of ribavirin in Lassa fever is lacking.” Four decades of clinical practice, resting on a foundation the field’s own researchers have called into question.

This is the context into which the favipiravir Phase 2 trial drops its HR of 0.50. Favipiravir—originally discovered by Toyama Chemical Co., Ltd.—works as a prodrug that undergoes intracellular phosphoribosylation into its active form, favipiravir-RTP, a direct inhibitor of the RNA-dependent RNA polymerase (RdRp) of RNA viruses. The mechanism is well characterized. What was missing until now was a prospective, randomized head-to-head trial against the existing standard of care in the specific population where the disease actually kills people.

Running any randomized controlled trial in an endemic hemorrhagic fever zone is operationally brutal. Patient enrollment windows are narrow because the disease progresses within days. Informed consent in acute febrile illness requires cultural sensitivity and real-time interpreter infrastructure. Supply chain integrity for investigational product must be maintained in settings where cold chain logistics can fail overnight. The fact that this trial was completed at all—open-label design notwithstanding—is a methodological achievement that deserves explicit recognition before any critique of its limitations begins.

What an Open-Label Design Actually Costs You

Here is where the regulatory stress-test begins. Open-label RCTs are not inherently weak—the RECOVERY trial used an open-label design to generate the dexamethasone evidence that changed COVID-19 treatment globally. But in a mortality trial where the primary endpoint is objective and binary, the open-label limitation is manageable. What matters more is whether the randomization was preserved, whether allocation concealment was maintained pre-assignment, and whether the comparator arm received standard-of-care ribavirin at doses consistent with current WHO protocols.

The WHO’s September 2024 Guidance for Best Practices for Clinical Trials explicitly addresses the design requirements for trials in endemic settings, emphasizing the need for adaptive infrastructure that remains “continuously functional and active for endemic conditions.” The favipiravir Phase 2 design appears to take this seriously. An open-label randomized controlled trial with mortality as the primary endpoint, conducted against an active comparator in an endemic zone, is exactly the kind of pragmatic design the WHO framework envisions—and the kind of evidence that a pathway to approval in rare tropical diseases must be built on.

But the FDA’s Accelerated Approval and Tropical Disease Priority Review pathways—both theoretically available to a drug in this space—carry requirements that a single Phase 2 trial cannot fully satisfy. The Tropical Disease Priority Review Voucher program, established under the FDA Amendments Act, rewards sponsors who complete trials for qualifying tropical diseases. Lassa fever is on that list. What the voucher program does not do is lower the evidentiary bar for approval itself. A sponsor taking favipiravir forward will need to answer the FDA’s fundamental question: is Phase 2 mortality data, from an open-label trial in a West African endemic setting, sufficient to support a New Drug Application? Or does the agency require a Phase 3 confirmatory trial—which, given Lassa fever’s seasonal and geographic concentration, could take a decade to enroll?

The Enrollment Arithmetic Nobody Wants to Do

Consider what Phase 3 enrollment actually looks like in this disease space. IAVI’s Phase 2 Lassa fever vaccine trial—designated IAVI C105/PREVAIL15 and described as the first-ever Phase 2 trial for any Lassa fever vaccine candidate when it dosed its first Nigerian participants in April 2024—represents the current frontier of Lassa clinical development infrastructure. That is a vaccine trial. A therapeutic trial carries additional operational weight: patients must be actively infected, symptomatic, and enrolled within a narrow treatment window. The endemic zone is concentrated in Nigeria, Sierra Leone, Guinea, and Liberia. Lassa fever’s seasonality peaks in the dry season, roughly November through April. A Phase 3 trial adequately powered to detect a survival benefit would require multi-site enrollment across several of these countries, sustained over multiple seasonal cycles.

This is where the regulatory and operational realities collide in a way that the HR of 0.50 cannot resolve on its own.

The most viable regulatory pathway, given these constraints, may be a Biologics or Drug application submitted under the FDA’s Accelerated Approval mechanism, using the Phase 2 mortality data as reasonably likely to predict clinical benefit, with a post-market confirmatory commitment. This is not without precedent in rare infectious disease—the FDA has accepted surrogate endpoints and limited datasets for tropical disease approvals before. But mortality in a Phase 2 RCT occupies an unusual category: it is a direct clinical endpoint, not a surrogate, yet it comes from a trial not designed to meet Phase 3 statistical powering conventions. Sponsors and FDA reviewers will need to negotiate exactly what that means before a Type B meeting request gets filed.

There is also the question of who sponsors the next step. Favipiravir’s originating company, Toyama Chemical, licensed the drug to Fujifilm after acquisition. Fujifilm pursued emergency-use designations in Japan during COVID-19. Neither entity has a visible commercial pathway into the Lassa fever endemic zone that would justify the cost of a Phase 3 program at commercial risk. The more realistic model is a product development partnership—CEPI, Wellcome Trust, or BARDA—absorbing the development cost against a public health mandate. BARDA has funded Lassa fever therapeutic development through its emerging infectious disease portfolio before, and an HR of 0.50 from a published Phase 2 RCT is exactly the kind of inflection point that activates that conversation.

What Sponsors Running Rare Pathogen Programs Should Do Now

If you are a sponsor or a product development partnership managing an emerging pathogen therapeutic program, the favipiravir Phase 2 result reshapes your regulatory strategy in two immediate ways. First, the ribavirin comparator is no longer a safe harbor. Any future Lassa fever trial that uses ribavirin as its active control must now contend with a published, randomized dataset showing favipiravir’s superiority—which means your trial’s ethics committee will ask whether continuing to assign patients to ribavirin is defensible. The equipoise argument that justified ribavirin-controlled trials has a new and inconvenient data point sitting against it.

Second, the Phase 2 trial’s open-label design creates a template, but also a ceiling. If you intend to take a Lassa fever therapeutic to the FDA, request a Type B pre-NDA meeting before committing to Phase 3 enrollment. Bring the favipiravir Phase 2 dataset as a comparator benchmark. Ask the agency explicitly whether a confirmatory Phase 3 is required or whether an adaptive, seamlessly designed Phase 2/3 with pre-specified mortality endpoints would satisfy the evidentiary standard under Accelerated Approval. The WHO’s 2024 guidance on adaptive endemic trial infrastructure gives you the methodological language to make that argument. Use it.

Build your statistical analysis plan around the mortality endpoint from enrollment day one. Do not treat survival as a secondary endpoint you will “explore.” In a disease with a case fatality rate above 16%, mortality is your primary endpoint and your regulatory anchor—and any FDA reviewer who has read the 2022 ribavirin systematic review knows that the field has been operating on borrowed evidence for thirty-eight years.

That clinical trial coordinator in Kenema who watched the favipiravir mortality data emerge now has something she has never had in Lassa fever therapeutics: a result that holds up to methodological scrutiny. The harder question—whether the regulatory and funding architecture surrounding this disease can move as fast as the evidence just did—remains dangerously open.

References

1. Nature Medicine — “Favipiravir for Lassa fever: an open-label, randomized controlled phase 2 trial”
2. World Health Organization — Lassa Fever Fact Sheet (case fatality rate data, 2024)
3. Bristol Biomedical Research Centre / NIHR — “Using ribavirin to treat Lassa fever might not be backed up by evidence” (2022 systematic review)
4. PMC / NCBI — Favipiravir mechanism of action: RNA-dependent RNA polymerase inhibition and intracellular phosphoribosylation
5. World Health Organization — “Guidance for Best Practices for Clinical Trials” (September 2024)
6. IAVI — IAVI C105/PREVAIL15 Phase 2 Lassa Fever Vaccine Trial, first participants dosed April 2024