Andes Virus Person-to-Person Transmission: How It Spreads & 2026 Cruise Ship Risk

For 30 years infectious disease textbooks have carried a single asterisk against an entire viral family. Of the more than two dozen hantaviruses identified in mammals, only one — the Andes virus (ANDV) — has firm evidence of moving directly from one human being to another. That asterisk is the entire reason the 2026 MV Hondius cluster has been treated as a public-health priority rather than a run-of-the-mill rodent-borne illness.

This article walks through that evidence: where it came from, how strong it is, what it implies for the current outbreak, and the few things public-health agencies are watching that could change the picture quickly.

The 1996 El Bolsón cluster — the original evidence

In late 1996 a respiratory illness with a roughly 50% case-fatality rate broke out in El Bolsón, a small town in the Río Negro province of Argentine Patagonia. The first patient was a wilderness guide. Over the following weeks, two of the physicians who treated him, a nurse, three of his family contacts and several other patients in the local hospital fell ill with the same clinical picture. Twenty patients in total were ultimately identified, and the cluster crossed three provinces.

The defining feature was geographical: cases appeared in Buenos Aires, more than 1,500 kilometres from the only known reservoir (the long-tailed pygmy rice rat, Oligoryzomys longicaudatus). A traveller who had passed through El Bolsón fell ill back in Buenos Aires; later, two intimate contacts of that traveller also developed Andes virus disease, with no other plausible exposure.

Wells et al published the analysis in the journal Emerging Infectious Diseases in 1997 and concluded — cautiously, and only after exhausting environmental hypotheses — that direct person-to-person transmission was the most parsimonious explanation. Since then more than a dozen further clusters with the same epidemiological signature have been documented across Argentina and Chile.

How strong is the evidence, really?

Three independent lines of evidence converge on the same conclusion:

• Epidemiologic: Cases occur in tight household and healthcare-worker clusters, with secondary cases appearing on a biologically plausible incubation timeline (median ~22 days from presumed exposure to symptom onset).
• Geographic: Cases occur far from the rodent reservoir — sometimes thousands of kilometres away — but always downstream of contact with a primary case.
• Genomic: Where viral genome sequencing has been performed (Iglesias et al 2017; Martinez et al 2020), secondary cases share viral genomes that are nearly identical to those of the index case, and distinct from contemporaneous rodent isolates.

No single line is fully airtight on its own. Together they make a strong case.

What is the actual transmission rate?

The key number for any pathogen is the basic reproduction number R₀ — the average number of secondary cases an index case generates in a fully susceptible population. For Andes virus this number depends almost entirely on the contact setting.

For comparison, the early-2020 ancestral SARS-CoV-2 had an R₀ around 2.5–3.0 across all contact settings combined. The clinical-management lesson: Andes virus needs intimate or household-level contact to chain. It does not need a global population.

Why a cruise ship is the worst environment we have tested

The MV Hondius is not a worst-case scenario by accident. It combines three risk factors that don't overlap in any prior Andes virus cluster:

1. Cabins shared by intimate contacts. Households aboard ship are physically denser than households on land, with significantly less ventilation per occupant.
2. Shared medical staff with limited PPE inventory. Cruise medical units generally stock PPE for routine respiratory isolation, not for a Biosafety Level 3 pathogen they didn't plan for.
3. Crew turnover at port. Each port call adds new susceptible contacts — and removes potentially infectious crew before symptoms emerge.

This is exactly the environment in which a R₀ of ≈1 can chain just enough to make the cluster grow before any one person notices.

What public-health agencies are watching

Three signals would meaningfully change the assessment of the MV Hondius cluster from “serious but contained” to “materially worse than expected”:

• A clearly tertiary case. Someone infected with no link to the ship and no link to a previously identified patient. As of 2026-05-12 no such case has been reported in any official document.
• A healthcare-worker case despite intact PPE. Would imply higher infectiousness than the Patagonia clusters — and possibly a small change in the viral genome.
• Genomic divergence from historical ANDV. An aerosol-tropic mutation would be the worst plausible scenario. The WHO Collaborating Centre in Buenos Aires is performing this analysis on every confirmed isolate.

What this means for travellers and clinicians

For travellers who have not been in close contact with an MV Hondius passenger or crew member, the practical risk is extraordinarily low — Andes virus simply does not transmit casually. The reasonable precaution is what you'd already take: wash your hands, watch for fever and shortness of breath for 6 weeks if you were on the ship or in close contact with someone who was, and seek medical attention promptly if you develop symptoms. Our symptoms guide walks through what to monitor.

For clinicians: Andes virus is a Biosafety Level 3 pathogen. Patients should be cared for under contact and droplet precautions with N95-or-better respiratory protection until further characterisation. Notify your public-health authority promptly; clinical management is supportive (oxygenation, careful fluid management, ECMO consideration in severe cardiopulmonary syndrome).

Frequently asked questions

Can Andes virus spread by sneezing or coughing?

Probably not via casual respiratory aerosols. The current evidence suggests transmission requires close, sustained contact — consistent with droplet-range exposure, not airborne aerosol spread. This distinction matters: it is why R₀ drops sharply outside very close contact.

Is sexual transmission documented?

Yes. Several of the household clusters documented since 1996 had secondary cases in spouses with no other plausible exposure. The viral RNA has been identified in saliva and seminal fluid in some cases. This is also why a household contact threshold is the primary one in current guidance.

Can a recovered patient transmit?

The communicable period appears to be roughly the prodromal phase plus the first week of cardiopulmonary syndrome. Recovered patients beyond 4 weeks have not been documented to transmit.

Does prior hantavirus exposure provide cross-protection?

Limited. Antibodies to other New World hantaviruses (Sin Nombre, Black Creek Canal) appear to be partially cross-reactive in vitro but their clinical protective effect against Andes virus is not established. Plan accordingly.

Sources and further reading

• Wells RM et al. An unusual hantavirus outbreak in southern Argentina: person-to-person transmission? Emerging Infectious Diseases, 1997. (Open access)
• Iglesias A et al. Genetic characterization of Andes virus from a fatal case of person-to-person transmission. Journal of Clinical Virology, 2017.
• Martinez VP et al. “Super-spreaders” and person-to-person transmission of Andes virus in Argentina. New England Journal of Medicine, 2020.
• Pan American Health Organization. Hantavirus pulmonary syndrome in the Americas. Technical reference document, 2022.
• World Health Organization. Disease Outbreak News (DON-599) — Hantavirus, multi-country. May 2026.

Read next

• Andes virus: full strain profile
• Hantavirus vs. COVID — a sober comparison
• MV Hondius outbreak — full timeline