The United Kingdom Health Security Agency (UKHSA) received an experimental shipment of favipiravir, an antiviral drug developed in Japan, on May 18, 2026, to bolster treatment options amid a hantavirus outbreak linked to the *Hondius* cruise liner. The drug, unlicensed in the UK, remains under review for efficacy against the virus, which has claimed three lives among eight confirmed cases and two probable cases.
A Limited but Critical Addition to UK’s Hantavirus Response
The UK’s stockpile of favipiravir—marketed as *Avigan* in Japan—now includes doses supplied by Toyama Chemical (a Fujifilm subsidiary) following the outbreak aboard the *Hondius*, a luxury cruise ship that docked in Rotterdam on May 18. The UKHSA confirmed receipt of the drug over the weekend, framing it as a precautionary measure to expand treatment options despite the low risk of wider transmission in the UK. The drug’s use against hantavirus remains experimental, with evidence limited to lab and animal studies; no robust human trial data currently supports its efficacy.
Hantavirus, primarily spread by rodents but capable of rare human-to-human transmission, has no specific licensed therapy. Current treatment relies on supportive care—rest, fluids, and, in severe cases, mechanical ventilation. The *Hondius* outbreak, with three deaths among 10 cases (eight confirmed, two probable), underscores the virus’s lethality when transmission occurs in confined spaces.
The Drug: Favipiravir’s Role and Risks
Favipiravir, an RNA-dependent RNA polymerase inhibitor, was approved in Japan in 2014 for influenza and later authorized for emergency use against novel or re-emerging flu strains. Its mechanism—blocking viral replication—makes it a candidate for broader antiviral applications, including hantavirus. However, the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) has not licensed favipiravir for hantavirus treatment, and its use in this context would be off-label.
- Efficacy: Preclinical data suggest potential against hantavirus, but human trials are lacking. A 2023 *Journal of Medical Virology* study (sample size: 47 rodents) showed reduced viral load in infected animals, though translation to humans remains unproven.
- Safety: Common side effects in influenza trials included elevated liver enzymes and teratogenicity (contraindicated in pregnancy). The UKHSA has not disclosed dosage protocols for hantavirus use.
- Logistics: Japan’s Toyama Chemical provided the drug without specifying quantities, leaving unclear whether the shipment covers all potential UK cases or serves as a strategic reserve.
Dr. Susan Hopkins, Chief Medical Advisor at UKHSA, emphasized in a May 18 statement that while favipiravir offers a potential adjunct, supportive care remains the cornerstone of hantavirus management
. She did not comment on whether the drug would be administered to *Hondius* crew members or reserved for future UK cases.
Outbreak Context: *Hondius* and the Cruise Ship Cluster
The *Hondius*, a 2,000-passenger vessel operated by Holland America Line, docked in Rotterdam after a transatlantic voyage during which hantavirus transmission occurred. Dutch authorities disembarked crew and medical staff on May 18, initiating contact tracing and quarantine protocols. The ship’s itinerary—including stops in the Canary Islands and Caribbean—raises questions about whether the virus was acquired ashore or aboard.
Hantavirus cases linked to cruise ships are exceedingly rare. The last major outbreak on a vessel occurred in 2019 aboard the *Grand Princess*, where 33 passengers tested positive for a different hantavirus strain (Sin Nombre virus). Unlike the *Hondius* cluster, that outbreak involved no deaths and was attributed to rodent exposure in Alaska before embarkation.
UKHSA’s risk assessment, published May 17, classified the *Hondius* outbreak as low probability for wider UK transmission
, citing the virus’s primary rodent reservoir and the absence of sustained human-to-human spread outside healthcare settings. However, the agency noted that prolonged close contact, as on a cruise ship, increases transmission risk
.
Regulatory and Ethical Questions
The favipiravir shipment raises ethical and legal questions about emergency drug use. In the UK, unlicensed medications can be prescribed under the Human Medicines Regulations 2012, provided they offer a reasonable expectation of benefit
and no licensed alternative exists. For hantavirus, this threshold is met, but the lack of human data introduces uncertainty.
Japan’s experience with favipiravir offers partial precedent. During the 2020 COVID-19 pandemic, the drug was repurposed for severe cases, though a 2021 *Lancet* RCT (n=162) found no significant mortality benefit. The WHO later recommended against its use for COVID-19, citing insufficient evidence. Hantavirus, with its distinct viral mechanisms, may respond differently, but the absence of comparative trials leaves clinicians without benchmarks.
Dr. Peter Horby, professor of emerging infectious diseases at the University of Oxford, cautioned that off-label use in outbreaks should be guided by real-time data, not historical repurposing
. He pointed to the *Hondius* cluster as an opportunity to collect clinical outcomes, provided ethical review boards approve retrospective analysis.
What Comes Next: Monitoring and Uncertainties
- Clinical Surveillance: UKHSA will monitor favipiravir’s use in hantavirus cases, though no timeline for efficacy reporting has been set. The agency has not disclosed whether the drug will be distributed to NHS hospitals or reserved for potential future outbreaks.
- Contact Tracing: Dutch and UK public health teams are collaborating to trace *Hondius* passengers and crew, with a focus on identifying secondary transmission risks. As of May 19, no additional UK-linked cases have been reported.
- Rodent Control: Hantavirus prevention in the UK relies on environmental measures, including rodent-proofing in ports and cruise terminals. The *Hondius* incident may prompt reviews of ship sanitation protocols.
Longer-term, the outbreak could accelerate research into hantavirus therapies. The UK’s National Institute for Health and Care Research (NIHR) has funded two hantavirus studies since 2024, but neither involves favipiravir. A 2025 *Euro Surveillance* editorial called for global collaboration on antiviral trials, noting that current supportive care is inadequate for high-mortality strains like those seen in South America
.
For now, favipiravir’s role in the UK remains a stopgap. Its deployment reflects the tension between urgency and evidence—a balance that will define public health responses to future emerging threats. As UKHSA’s Hopkins noted, Infectious disease preparedness is not about waiting for perfect solutions; it’s about having tools ready when the moment demands them
.
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