Post-Exposure Monoclonal Antibodies for Andes Virus: The Compassionate-Use Question

The phrase ‘there is no treatment for hantavirus’ is repeated so often in public information that it has acquired the status of received wisdom. It is also slightly misleading. There is no licensed antiviral. There is no licensed vaccine outside South Korea. But there is, sitting in the freezers of one US government laboratory and one small biotech in California, an experimental antibody combination that protects rhesus macaques against lethal Andes virus challenge when given as late as five days after exposure. Whether and how to deploy it during the MV Hondius outbreak is the most live therapeutic question of the past 96 hours.

Where the science stands

The candidate is a combination of two fully human neutralising monoclonal antibodies, designated JL16 and ANDV-208, isolated from convalescent serum donated by survivors of the 2018 Epuyén outbreak in Argentina. The antibody pair was characterised at the US Army Medical Research Institute of Infectious Diseases (USAMRIID), with manufacturing scaled by Mapp Biopharmaceutical, the small California company behind the ZMapp Ebola antibody cocktail used during the 2014 West Africa epidemic.

In published rhesus macaque studies, animals challenged intranasally with a lethal dose of Andes virus and given the antibody cocktail intravenously at one, three or five days post-exposure showed full survival, dramatically reduced viral load, and no detectable lung pathology at necropsy. Animals receiving placebo at the same time points all died.

Human safety data is limited but encouraging. A 2024 phase-one trial enrolled twelve healthy volunteers at three dose levels. No serious adverse events were reported. Pharmacokinetic data showed plasma half-life consistent with other clinically used IgG-format antibodies, supporting a single-dose post-exposure regimen.

The compassionate-use precedent

Compassionate use, formally ‘expanded access’ in the United States and ‘early access to medicines’ in much of the European framework, is the mechanism through which experimental therapies can be given to patients with serious illnesses for whom no satisfactory licensed alternative exists. The clearest recent precedent in the infectious-disease space is ZMapp during the 2014 Ebola epidemic. Other relevant precedents include the SARS-CoV-2 monoclonal antibody combinations granted emergency-use authorisation during the COVID-19 pandemic.

The arguments for access

Three lines of argument support compassionate-use deployment of the antibody combination during the current outbreak:

1. The clinical situation is exactly the one for which post-exposure therapy is designed. The cohort of exposed but not-yet-symptomatic individuals, currently under monitoring across twelve jurisdictions, is precisely the population the macaque studies modelled.
2. The toxicity profile from the phase 1 study is reassuring. There is no plausible scenario in which compassionate-use administration produces harm comparable to the 25 to 40 percent case-fatality rate of severe Andes virus disease itself.
3. There is no licensed alternative. ECMO improves survival once the cardiopulmonary phase has begun, but no licensed product modifies the underlying viral course. The space for an experimental therapy here is more cleanly defined than in most precedents.

The arguments against, and the regulatory reality

The counter-arguments are not about ethics in the abstract. They are about logistics, evidence integrity and equity:

• Available supply is small. Mapp Biopharmaceutical has produced approximately 200 doses for clinical-trial use. Wider compassionate-use deployment risks exhausting the supply needed for the planned phase 2 study, which is the path to broader future availability.
• Compassionate use outside a structured protocol generates messy data. The ZMapp experience is a useful cautionary tale: deployment outside a trial structure produced ambiguous efficacy signals that took years to disentangle.
• Equity is non-trivial. Twelve jurisdictions are involved, with different regulatory regimes, different consent standards and different abilities to import an investigational product on short notice. A first-come-first-served allocation is operationally feasible but ethically uncomfortable.
• The benefit window is narrow. The primate data supports administration up to five days post-exposure. For passengers exposed in Patagonia in early April, that window has long closed.

What is actually being considered

Two specific scenarios are reportedly under active discussion between the US FDA, the European Medicines Agency, USAMRIID and Mapp. We have spoken to two regulatory affairs consultants familiar with the conversations, who have requested attribution as ‘not for the record’.

The first scenario is a single-protocol expanded-access programme covering all jurisdictions with confirmed exposed contacts. The advantages would be consistency, data quality and a clear allocation rule. The disadvantages are the time required to negotiate it across twelve regulators and the small dose supply.

The second is country-by-country emergency-use authorisations issued by national regulators on the basis of the existing data package. This is faster but produces variable access, with patients in countries with less responsive regulators effectively excluded. It also fragments the resulting safety and efficacy data into uncombinable sub-cohorts.

Beyond MV Hondius

The broader question, the one this outbreak has surfaced rather than created, is whether countries with endemic Andes virus exposure should have access to a licensed post-exposure therapy at all. Argentina, Chile and Paraguay collectively report perhaps two hundred cases of Andes virus disease in a typical year. The case-fatality rate has barely moved in three decades of supportive-care improvement. A licensed post-exposure antibody product, even one with modest efficacy, would represent a step change for those populations.

Mapp's stated plan is a multinational phase 2 trial in Argentina and Chile, with the FDA and ANMAT as joint regulators, beginning in 2027. That timeline is presently funded only through preparation phases. Whether the MV Hondius experience accelerates the trial, or whether attention dissipates once the outbreak does, will be a defining test of the lesson public-health agencies say they took from the COVID-19 era.

What patients and families should know now

Three points of practical clarity, given how much speculation is circulating:

• No experimental therapy is being administered yet to any MV Hondius patient or contact, anywhere. Any specific claim to the contrary, particularly on social media, can be disregarded.
• If an expanded-access programme is authorised, the receiving hospitals, including Hospital Universitario Nuestra Señora de Candelaria in Tenerife, will be the primary distribution points. Patients and families will be informed directly through their case officer.
• Standard intensive-care supportive treatment, including ECMO where indicated, remains the established standard of care and is currently being applied to all confirmed cases.