# Case fatality rate

> Proportion of deaths among people diagnosed with a disease. For hantavirus pulmonary syndrome: ~36% on average, up to 40% for Andes virus.

Canonical source: https://hantatracker.fr/en/glossary/case-fatality-rate/

**Aliases**: case fatality rate, case fatality ratio, CFR

The **case fatality rate** (CFR) is a fundamental epidemiological indicator expressing the proportion of people diagnosed with a disease who die from it. For **hantavirus pulmonary syndrome**, it lies between 30% and 40% depending on the strain, making it one of the most severe acute viral infections in immunocompetent humans.

## Definition and calculation

### Formula

Case fatality is calculated as:

```
Case fatality (%) = (number of deaths / number of diagnosed cases) × 100
```

It is expressed as a percentage and applies to a **specific disease**, during a **given period**, in a **population of confirmed cases** (not in the general population).

### Not to be confused with

Case fatality differs from several other related indicators:
- **Mortality**: number of deaths relative to the total population (for example, deaths per 100,000 inhabitants per year).
- **Infection Fatality Rate (IFR)**: proportion of deaths among all infected people, including asymptomatic or undiagnosed cases. The IFR is always less than or equal to the CFR.
- **Individual risk**: for a given infected person, risk of death depends on age, comorbidities, strain, and access to care.

## Case fatality of hantavirus diseases

### Hantavirus pulmonary syndrome

| Agent | Region | Case fatality |
|-------|--------|---------------|
| Andes virus | Argentina, Chile | ≈ 40% |
| Sin Nombre virus | North America | 30–35% |
| Laguna Negra virus | Paraguay, Bolivia | ≈ 12% |

Across all US cases since 1993, the CDC uses an average case fatality of **36%**.

### Hemorrhagic fever with renal syndrome

HFRS shows more variable case fatality, strongly dependent on the strain:
- **Hantaan virus** (Asia): up to 15%
- **Dobrava virus** (Balkans): 5–15%
- **Seoul virus**: 1–2%
- **Puumala virus** (European nephropathia epidemica): less than 1%

## Case fatality observed on MV Hondius

As of **10 May 2026**, WHO recorded for the MV Hondius event **8 cases** (6 confirmed, 2 probable) and **3 deaths**, that is a crude case fatality of **37.5%**. This value is consistent with the historical range of Andes virus (40%).

### Interpretation caveats

During an ongoing event, case fatality must be interpreted carefully for several reasons: (1) the most severe cases are identified first, which overestimates the initial CFR; (2) some hospitalised patients may still survive, which will lower it after resolution; (3) mild cases may be missed and not appear in the denominator. WHO recommends consolidating outbreak case fatality only after all cases have resolved.

## Reducing case fatality

Three main levers have demonstrated impact:

### Early diagnosis

Identifying hantavirus disease from the flu-like prodromal phase (days 1 to 5) by RT-PCR enables rapid referral to intensive care before the critical phase. For passengers and contacts of MV Hondius, surveillance during the 42-day incubation window has precisely this objective.

### Experienced intensive care

Prognosis is better in centres accustomed to managing acute respiratory distress and cardiogenic shock: mechanical ventilation with lung-protective strategy, strict hemodynamic management, rapid use of extracorporeal membrane oxygenation (ECMO) for the most severe forms.

### International coordination

Early identification of a cluster (as on MV Hondius), identification of the pathogen by PCR, notification to international health authorities (WHO, ECDC, CDC) and coordination of contact surveillance are the conditions that allow avoidable deaths to be prevented and operational case fatality during an outbreak to be reduced.