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Glossary · Epidemiology

Human-to-human transmission

Passage of an infectious agent from one human to another. For hantaviruses, only documented with Andes virus; requires close, prolonged contact with a symptomatic person.

Also called : person-to-person transmission, interhuman transmission, human-to-human transmission, human to human transmission Epidemiology

Human-to-human transmission refers to the passage of an infectious agent from one human to another, without an animal intermediary or vector. Among hantaviruses, only Andes virus shows this characteristic in a documented way. This particularity, which represents only 2 to 5 percent of all Andes virus cases, is the main reason why the MV Hondius episode is mobilising the health authorities of 10 countries.

Transmission mechanisms

Possible routes

Human-to-human transmission can occur through several routes: respiratory droplets (coughing, sneezing, speaking), aerosols (finer particles, with prolonged suspension), direct contact (skin, mucous membranes), biological fluids (blood, secretions), contaminated objects. For Andes virus, studies of the Epuyén outbreak suggest the main route is short-range respiratory, during close and prolonged contact.

Distinction from other modes

Human-to-human transmission differs from vector-borne transmission (by mosquito, tick, flea — malaria, dengue, Lyme), from environmental transmission (water, soil, food — cholera, salmonella), and from direct zoonotic transmission from an animal reservoir (rabies, "classic" hantaviruses).

The special case of Andes virus

An exception among hantaviruses

Andes virus is the only hantavirus for which human-to-human transmission has been demonstrated by formal epidemiological studies. The other hantaviruses (Sin Nombre, Hantaan, Seoul, Puumala, Dobrava) are transmitted only from their rodent reservoir, never from human to human. This singularity of Andes virus has been recognised since the late 1990s and confirmed several times since.

The Epuyén outbreak as reference

The Epuyén outbreak (Argentina, 2018-2019), published in the New England Journal of Medicine, is the reference study on human-to-human transmission of Andes virus. It documented 18 confirmed cases and 11 deaths in a rural village of 2,800 inhabitants, spread from 3 symptomatic patients who attended enclosed social events. The reproduction number R fell from 2.12 (before quarantine measures) to 0.96 (afterwards) — showing both the transmission potential and the effectiveness of control.

Virological characterisation

Argentine researchers (CONICET) characterised the Epuyén/18-19 strain and identified mutations in the envelope glycoproteins, likely associated with increased replication efficiency in the human respiratory tract. Precise understanding of the molecular determinants of human-to-human transmission remains an active area of research.

Public health consequences

For the general public

The WHO's assessment of the risk to the general population as "low" rests on three observations: (1) human-to-human transmission accounts for only 2 to 5 percent of cases, (2) it requires close, prolonged contact that is absent from most common urban situations, (3) it does not occur during the asymptomatic incubation phase. No barrier measures (masks, distancing) are required of the general population in connection with the MV Hondius episode.

For identified contacts

Identified passengers and contacts are subject to active medical surveillance for 42 days (the maximum incubation period), with self-reporting of symptoms, direct access to a referring physician and immediate isolation at the onset of any suggestive sign. This system, rather than blanket barrier measures, is the main control strategy.

For healthcare workers

Healthcare workers caring for a suspected or confirmed case wear personal protective equipment (FFP2 or N95, fluid-resistant gown, gloves, goggles or face shield). This precaution is consistent with WHO/ECDC recommendations for respiratory pathogens with documented human-to-human transmission, even when such transmission is not dominant.

Key figures

Standards & references

  • WHO — DON 2026 (MV Hondius) — WHO international outbreak notification confirming the possibility of human-to-human transmission on the vessel.

Frequently asked questions

Which hantaviruses are transmissible between humans?

Only one is documented: Andes virus (Argentina, Chile). The other New World hantaviruses (Sin Nombre, Laguna Negra) and Old World hantaviruses (Hantaan, Seoul, Puumala) are transmitted only from rodent reservoirs, through inhalation of aerosols from their excretions.

How does this transmission actually occur?

It requires close, prolonged contact with a symptomatic person: same household, direct care without protective equipment, prolonged contact in an enclosed space. Studies of the Epuyén outbreak suggest a short transmission window around superspreader events. It is not large-scale airborne transmission like influenza or COVID-19.

Why Andes virus and not the others?

Comparative analyses suggest that the envelope glycoproteins of Andes virus allow efficient replication in the human respiratory tract from very low doses. Specific mutations of the Epuyén/18-19 strain have been characterised by Argentina's CONICET. This genetic peculiarity likely explains why Andes virus is the only hantavirus capable of documented human-to-human transmission.

Is the risk to the general population high?

No. Human-to-human transmission accounts for only 2 to 5 percent of Andes virus cases. It requires prolonged and close contact with a symptomatic person, which excludes most everyday situations (occasional public transport, public spaces, brief contacts). The WHO assesses the risk to the general population as low, including in the MV Hondius context.

Further reading