# Favipiravir

> Broad-spectrum antiviral developed in Japan against influenza (Fujifilm Toyama, 2014). Now being explored against Andes hantavirus — efficacy not yet demonstrated.

Canonical source: https://hantatracker.fr/en/glossary/favipiravir/

**Aliases**: Avigan, T-705, RNA antiviral

**Favipiravir** (code T-705, trade name **Avigan**) is a **broad-spectrum antiviral** developed by Fujifilm Toyama Chemical and **approved in Japan in 2014** against influenza. Its mechanism: it inhibits viral RNA polymerase — the enzyme RNA viruses need to replicate. This biological target makes it potentially active against a broad family of viruses, which prompts its episodic exploration whenever a new threat emerges (Ebola, COVID-19, Lassa…).

## Why it resurfaces with hantavirus

On 24 May 2026, HHS Secretary **Robert F. Kennedy Jr.** signs a **targeted [PREP Act](/en/glossary/prep-act/) declaration** published in the Federal Register (document **2026-10539**). It legally protects the **investigational use** of favipiravir as a potential treatment for [hantavirus pulmonary syndrome](/en/glossary/hantavirus-pulmonary-syndrome/), within the scope of the [MV Hondius](/en/glossary/mv-hondius/) outbreak and its chain of transmission. The measure is in force until 18 July 2026.

## Efficacy not demonstrated against Andes

This is the core point to remember, and one that the PREP Act declaration itself implicitly recalls: legal protection does **not** prejudge clinical efficacy. The available data on favipiravir against the Andes virus are **limited to animal studies**. **No human phase 3 clinical trial** has demonstrated, to date, that it improves HPS prognosis.

This is the same logic seen for the USAMRIID DNA [vaccine](/en/glossary/vaccine/) candidate: the bottleneck is not the availability of a plausible molecule, but the difficulty of **proving its efficacy** when the disease is too rare to run a phase 3 trial.

## Availability: a drug without a producer

A central point, revealed at the **French Senate hearing of 20 May 2026** by Prof. **Karine Lacombe** (head of infectious diseases, Saint-Antoine, AP-HP): « *it is the only treatment with preclinical and animal-model efficacy. The problem is that there are currently no producers at the global scale. […] There is major European work to source this drug and above all to restart its manufacture.* »

In other words: a US **PREP Act** declaration lifting liability barriers for investigational use **has little to protect as long as the production chain has not been restored**. That is the blind spot the French Senate flagged — and the ongoing European industrial stake.

## Known adverse effects

Favipiravir is **teratogenic** in animals (contraindicated during pregnancy) and can raise liver enzymes and uric acid. Its use in Japan has been tightly regulated since approval in 2014.

For the broader picture of hantavirus treatment, see our articles on [symptoms and treatment](/en/articles/hantavirus-symptoms-treatment/) and [vaccine research status](/en/articles/andes-hantavirus-vaccine-research-status/).