# 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.

Canonical source: https://hantatracker.fr/en/glossary/person-to-person-transmission/

**Aliases**: person-to-person transmission, interhuman transmission, human-to-human transmission, human to human transmission

**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.