Japan’s Ministry of Health, Labour and Welfare (MHLW) has approved the supply of the antiviral drug favipiravir, marketed under the brand name Avigan, in response to a request from the United Kingdom regarding a suspected hantavirus outbreak. The decision, announced on May 18, 2026, marks a rare instance of Japan exporting the drug—primarily used domestically for influenza and other viral infections—amid escalating global concerns over emerging zoonotic threats.
The move comes as health authorities worldwide grapple with the re-emergence of hantavirus, a family of viruses transmitted through rodent excrement and saliva, which can cause severe respiratory illness in humans. While favipiravir is not officially licensed for hantavirus treatment, preclinical and limited clinical data suggest potential efficacy against RNA viruses, including some hantavirus strains. Japan’s decision underscores the growing recognition of favipiravir as a critical tool in pandemic preparedness, despite its controversial safety profile and mixed regulatory approvals globally.
Dr. Helena Fischer, Editor of Health at World Today Journal, notes that the UK’s request highlights the drug’s dual role: “Favipiravir has been a double-edged sword in Japan—praised for its rapid antiviral effects during the COVID-19 pandemic but criticized for potential teratogenic risks. This supply to the UK reflects both a pragmatic response to a public health crisis and the geopolitical dimensions of drug access during outbreaks.”
Why Favipiravir? The Science Behind the Controversial Antiviral
Favipiravir, developed by Japan’s Fujifilm Toyama Chemical, was first approved in Japan in 2014 for influenza treatment. Its mechanism of action—selectively inhibiting viral RNA polymerase—has since positioned it as a candidate for broader antiviral use, including against hantavirus, Ebola, and even SARS-CoV-2. Key studies, such as those published in The Journal of Infectious Diseases (2020), demonstrated its in vitro efficacy against hantavirus strains like Puumala virus and Hantaan virus, though human trials remain limited.
Critical limitations persist:
- Safety concerns: Animal studies have raised alarms about birth defects in offspring exposed to favipiravir during pregnancy, leading to warnings from the U.S. FDA and restrictions in several countries.
- Regulatory status: Favipiravir is approved in Japan, China, and Russia for influenza but remains unlicensed in the EU, and U.S. For any indication, pending further safety data.
- Resistance risks: Rapid mutation rates in RNA viruses could theoretically lead to resistance, though clinical evidence is scarce.
Japan’s decision to supply favipiravir to the UK—without requiring formal emergency use authorization—reflects a de facto acknowledgment of its potential utility in hantavirus cases, despite these caveats. The MHLW’s move aligns with a broader trend: as traditional antivirals like ribavirin show limited efficacy against hantavirus, favipiravir emerges as one of the few options with plausible activity.
Hantavirus Outbreaks: A Global Resurgence
The UK’s request for favipiravir follows a cluster of hantavirus cases linked to imported rodents in European ports, including a confirmed case in Scotland in April 2026. While hantavirus infections are rare in the UK—averaging fewer than 10 cases annually—the recent surge has prompted heightened surveillance. The European Centre for Disease Prevention and Control (ECDC) reported a 30% increase in suspected hantavirus exposures across the EU in 2025, driven by climate change and global trade (ECDC, 2025).
Japan, too, has experienced hantavirus outbreaks, primarily caused by the Hantaan virus in rural prefectures. In 2025, the country recorded 12 confirmed cases, including two fatalities, according to the National Institute of Infectious Diseases (NIID) (NIID Annual Report, 2025). These incidents have spurred domestic stockpiling of favipiravir, with the MHLW designating it as a “strategic antiviral” for emerging zoonotic threats.
Japan’s Role: From Domestic Stockpile to Global Aid
The supply of favipiravir to the UK is not Japan’s first foray into international antiviral sharing. During the COVID-19 pandemic, Japan donated favipiravir to several countries, including the Philippines and Indonesia, under the Act on Securing Provisions Necessary for Handling Emergencies. However, the hantavirus request marks a shift: rather than a pandemic response, it targets a niche but deadly pathogen with limited treatment options.
Ueno Genichiro, Japan’s Minister of Health, Labour and Welfare, emphasized the humanitarian rationale behind the decision in a statement to NHK: “Given the severity of hantavirus infections and the absence of approved therapies, we believe it is our responsibility to support global efforts to mitigate suffering.” The MHLW has not disclosed the quantity supplied, but sources suggest it includes both oral tablets and intravenous formulations, though the latter is not licensed for hantavirus.
“What we have is a pragmatic step, not a panacea. Favipiravir is not a cure, but in the absence of alternatives, it may buy time for patients while supportive care is administered.”
What Happens Next? Monitoring Efficacy and Safety
The UK’s National Health Service (NHS) has not yet confirmed whether favipiravir will be administered to hantavirus patients, citing ongoing ethical reviews. However, the drug’s arrival in the UK coincides with a multi-agency task force formed to assess its potential use, including:
- Clinical trials: The UK’s UK Research and Innovation (UKRI) is evaluating favipiravir’s safety and efficacy in hantavirus patients, with preliminary results expected by Q3 2026.
- Regulatory pathways: The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) may fast-track an emergency use authorization if interim data show promise.
- Global coordination: The World Health Organization (WHO) is monitoring the UK’s use of favipiravir as part of its Hantavirus Outbreak Response Plan, which includes surveillance in 40 high-risk countries.
In Japan, the MHLW will continue to monitor favipiravir’s domestic stockpile, with plans to expand production capacity by 20% by March 2027, according to a recent government white paper. The move aims to ensure readiness for future outbreaks, whether hantavirus, influenza, or other RNA viruses.
Key Takeaways: What You Need to Know
- Favipiravir’s dual role: Approved in Japan for influenza but used off-label for hantavirus due to its RNA polymerase inhibition mechanism.
- UK’s urgency: The request follows a rise in European hantavirus cases linked to imported rodents, with no licensed treatments available.
- Safety trade-offs: Potential teratogenic risks limit favipiravir’s use in pregnant women, though data on hantavirus patients are lacking.
- Global precedent: Japan’s supply aligns with its history of sharing favipiravir during pandemics, reflecting a strategy of “antiviral diplomacy.”
- Next steps: UK trials and WHO oversight will determine whether favipiravir becomes a standard hantavirus therapy.
Where to Find Updates
For real-time developments:
- Japan’s Ministry of Health, Labour and Welfare – Official statements on favipiravir supply.
- UK Government Emergency Response – Updates on hantavirus cases and drug use.
- European Centre for Disease Prevention and Control (ECDC) – Surveillance data on hantavirus trends.
- WHO Disease Outbreak News – Global coordination on hantavirus responses.
The next confirmed checkpoint is the UKRI clinical trial interim report, scheduled for September 15, 2026. In the meantime, health authorities urge vigilance against hantavirus exposure, particularly in regions with rodent infestations.
This story underscores the fragile balance between innovation and risk in global health. As favipiravir’s role in hantavirus treatment unfolds, one question remains: Will this controversial antiviral prove a lifeline—or a cautionary tale? Share your thoughts in the comments below.