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Glossary · Epidemiology

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.

Also called : case fatality rate, case fatality ratio, CFR Epidemiology

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.

Key figures

Standards & references

Frequently asked questions

What is the difference between case fatality and mortality?

The case fatality rate (CFR) is the proportion of deaths among people diagnosed with a disease: (deaths / confirmed cases) × 100. Mortality is the number of deaths relative to the total population, over a given period, often expressed per 100,000 inhabitants. For a rare disease like hantavirus, mortality in the general population remains negligible despite a high case fatality rate in infected individuals.

Why is the case fatality of Andes virus so high?

Andes virus causes massive pulmonary oedema through capillary leak and rapidly progressing cardiogenic shock. In the absence of an approved specific antiviral, survival depends on intensive care: early mechanical ventilation, strict hemodynamic management, ECMO in the most severe forms. Prognosis is better in experienced centres with rapid access to ECMO.

Is the case fatality observed on MV Hondius representative?

Not necessarily. During an ongoing outbreak, case fatality (CFR) is calculated on confirmed cases and deaths at a given moment. It can overestimate the real risk because the most severe cases are identified first, and some patients survive cases that have not yet been counted. WHO recommends calculating a reliable case fatality only after all cases have resolved. The 37.5% observed as of 10 May 2026 (3 out of 8) should be interpreted with caution.

How can hantavirus case fatality be reduced?

Three main levers: (1) early diagnosis, from the first flu-like symptoms, by PCR; (2) immediate transfer to intensive care at the pulmonary phase, before cardiogenic shock; (3) access to ECMO for the most severe forms. For passengers and contacts of MV Hondius, rapid identification through surveillance during the 42-day incubation window is designed precisely to enable this early management.

Further reading