Andes Hantavirus: What We Know, the Risks, and Open Questions

Andes virus is the only hantavirus for which sustained human-to-human transmission has been documented. That distinction sets it apart from all other members of the hantavirus family and explains why the outbreak recorded in April 2026 aboard the ship Hondius triggered a coordinated response from the World Health Organization (WHO), the European Centre for Disease Prevention and Control (ECDC), and the Italian Ministry of Health.

We spoke about Andes hantavirus with Antonella Castagna, Full Professor of Infectious Diseases at Vita-Salute San Raffaele University and Director of the Medical Residency in Infectious Diseases.

What Hantavirus is, and Why Andes Virus is Different

Hantaviruses are traditionally divided into Old World strains, predominant in Europe and Asia, and New World strains, found mainly in the Americas, each associated with distinct clinical presentations.

Old World strains cause haemorrhagic fever with renal syndrome (HFRS), while New World strains are linked to hantavirus pulmonary syndrome (HPS), characterised by acute respiratory failure and a case-fatality rate that, in some reported series, exceeds 40%.

Andes virus belongs to the New World group. As with all hantaviruses, the natural reservoir consists of wild rodents: each strain is associated with specific host species, a factor that contributes to the relatively contained geographic distribution of these infections.

Andes virus is a significant exception to that pattern. «Andes is the only hantavirus for which human-to-human transmission has been documented» says Professor Castagna. In other strains, the human being is generally considered an epidemiological dead end: infection occurs through exposure to excreta or contaminated aerosols from infected rodents, without documented onward human spread.

With Andes virus, under conditions of close and prolonged contact, human-to-human transmission can occur.

The Hondius Outbreak: the Numbers and the Genetic Evidence

According to data available on 18 May 2026, the outbreak comprised 12 cases in total: 9 confirmed, 2 probable, and 1 still under evaluation. Three people died.

For context: the last documented Andes virus epidemic, which occurred in Argentina between 2018 and 2019, involved 34 people, caused 11 deaths, and was contained over approximately four months.

At the genomic level, sequencing of the strain isolated from a patient hospitalised in Zurich, carried out by the Swiss National Centre for Emerging Viral Infections together with the Geneva University Hospitals and integrating data deposited on GenBank (US National Institutes of Health), reportedly showed 99% similarity to the strain responsible for the 2018 Argentine epidemic.

According to the ECDC, this finding suggests that the Hondius outbreak may be linked to a single, relatively stable viral lineage, and that the confirmed positive samples are likely connected to a common initial source of infection.

The couple of photographers who had stayed in Patagonia, an area where the virus is endemic in wild rodents, before boarding the Hondius represent, based on the available epidemiological reconstructions, the most probable point of introduction of the virus on board.

The confined environment of the ship may have facilitated human-to-human transmission. «It is a very particular setting,» Professor Castagna observes, «a closed situation in which people are in close contact for extended periods.» She adds, however, that «Andes hantavirus does not have a transmissibility comparable to that of SARS-CoV-2.»

[Antonella Castagna, Full Professor of Infectious Diseases at Vita-Salute San Raffaele University and Director of the School of Specialization in Infectious Diseases]

How Andes Virus Spreads: Questions that Remain Open

Several aspects of Andes virus transmission are still under investigation.

The efficiency of airborne transmission has not been fully defined, nor has the possible role of mildly symptomatic or asymptomatic individuals in spreading the infection. «The evidence available so far suggests that transmission occurs mainly in the presence of symptoms,» Castagna explains, «but this outbreak could help clarify the virus’s epidemiology and its real clinical risk.»

In documented secondary cases, the incubation period has generally been estimated at between 18 and 23 days, with reports extending to 42 days. Monitoring close contacts therefore requires a prolonged follow-up window.

Late June 2026 has been identified as a possible timeframe to assess whether the outbreak has run its course.

How Hantavirus is Treated: Supportive Care and Early Diagnosis

There are currently no approved antivirals against Andes virus. Treatment is therefore primarily supportive and includes respiratory assistance, cardiocirculatory support, and intensive management of complications.

«The aim is to sustain the patient through the critical phase of the infection» Castagna says.

In this context, early diagnosis is essential not so much to initiate targeted antiviral therapy but to allow timely activation of clinical support measures and appropriate isolation procedures.

The Italian Ministry of Health has published guidelines for the safe management of biological samples, including blood, urine, faeces, and respiratory secretions, from suspected or confirmed patients, in coordination with WHO and ECDC.

Andes virus is classified in Group 3 of biological agents under Italian Legislative Decree 81/08, a category covering microorganisms capable of causing serious disease in humans and potentially transmissible to exposed workers.

The Italian National Institute of Health (Istituto Superiore di Sanità) has documented, in previous outbreaks, rare cases of secondary transmission in healthcare settings. The risk appears concentrated mainly during aerosol-generating procedures such as intubation, bronchial suctioning, or nebulisation.

For these situations, ministerial guidelines require appropriate personal protective equipment, including FFP2 or FFP3 respirators, eye protection, double gloves, and mucosal protection.

Correct management of biological samples is essential to reduce occupational risk throughout the entire diagnostic chain.

What COVID Changed in the Response to Emerging Viruses

The SARS-CoV-2 pandemic accelerated the development of surveillance, isolation, and international coordination protocols for managing emerging pathogens.

«Today we have greater speed in coordinating between institutions and in managing both patients and biological material» Professor Castagna notes.

On the pharmacological research front, antiviral development timelines could shorten compared to the past. Remdesivir is frequently cited as an example of a molecule developed and clinically validated within a timeframe compatible with a global health emergency.

According to Castagna, artificial intelligence could become a significant accelerator in selecting the most promising drug candidates, helping to optimise the early stages of research and reduce the number of molecules destined to fail in advanced trials.