# Viral variant

> A strain of a virus carrying one or more mutations in its genome compared with reference strains. Most are neutral; some change transmissibility, virulence or treatment resistance.

Canonical source: https://hantatracker.fr/en/glossary/variant-viral/

**Aliases**: variant, variants, variant strain, viral strain, variant viral

A **viral variant** is a version of a virus whose genome carries one or more **mutations** compared with a reference strain. The vast majority of mutations are neutral and go unnoticed. A few can change transmissibility, virulence, immune escape or treatment sensitivity — only in those cases does a variant become notable. For the [MV Hondius](/en/glossary/mv-hondius/) cluster, **no variant of concern has been documented** to date.

## Mutation, variant, strain

### Three levels

- **Mutation**: a single change in viral DNA or RNA. RNA viruses, including hantaviruses, mutate continuously (their polymerase has a high error rate).
- **Variant**: a virus carrying one or more distinctive mutations that differentiate it from a reference strain.
- **Strain**: a variant sufficiently characterised and stable to be named and used as reference (for example "Sin Nombre strain", "Andes strain").

### Most mutations are silent

The genetic code is redundant: several nucleotide triplets code for the same amino acid. A mutation can therefore change one nucleotide without changing the produced protein. It is then **silent**. Mutations that change the protein are **non-synonymous**; only those can potentially alter the virus's behaviour.

## Variant surveillance

### WHO framework

WHO classifies variants in three categories, under a framework initially developed for SARS-CoV-2 but applicable to other emerging viruses:
- **Variant of Interest (VOI)**: presents genetic changes likely to affect viral behaviour, without solid clinical evidence yet.
- **Variant of Concern (VOC)**: demonstrated impact on transmissibility, virulence, treatment efficacy or immune evasion.
- **Variant of High Consequence (VOHC)**: impact such that it compromises the ongoing public health strategy.

### Hantavirus surveillance

Hantaviruses are relatively **conserved** genetically within a strain: diversity within Andes or Sin Nombre is low, unlike viruses such as SARS-CoV-2 or influenza. This facilitates detection of any atypical variant. Routine sequencing, still recent for these viruses, relies on National Reference Centres (Swiss NRZ, US NIH/CDC, French Institut Pasteur).

## Application to the MV Hondius

### Swiss sequencing of 5 May

On 5 May 2026, the Swiss National Reference Centre for Emerging Viral Infections (NRZ Geneva + Zurich) published on Virological.org a preliminary phylogenetic analysis of Andes virus sequences from the MV Hondius cluster. Main findings:
- the strain is **close** to the **Epuyén** outbreak strain (Argentina, 2018-2019),
- **no unusual mutation** has been documented,
- the analysis covers the first samples; complete sequencing of all cases is in progress.

### Precautionary hypothesis of 12 May

On 12 May 2026 at 17:54, Prof. **Xavier Lescure** (infectious disease specialist, Bichat AP-HP) raised in a press conference the hypothesis of a possible variant that may have mutated, to explain the severity of forms observed in the French patient and several foreign cases. This hypothesis was presented as **precautionary**, pending complete sequencing, and is not confirmed by the Swiss data of 5 May. Media coverage has sometimes turned this hypothesis into a statement of fact, which it is not.

## Limits and reading caveats

### Do not confuse hypothesis and confirmation

Until exhaustive sequencing has compared the genomes of all cases, the variant hypothesis remains open but unproven. Health authorities (WHO, ECDC, Santé publique France) communicate cautiously on this point.

### No variant ≠ no severity

Absence of a variant does not exclude severe disease. The "ordinary" Andes virus has historically a high case-fatality rate (~30-40 %) in hospitalised patients. The severity observed in the MV Hondius cluster can be explained by host factors (age, comorbidities), high viral load, or simply the intrinsic nature of the virus, without necessarily involving a variant.