Hantaviruses have been making headlines for several days. Some describe the virus as having a "40% fatality rate," while others speak of a "low risk to the population." These two statements may seem contradictory, but they are based on two very different concepts: lethality and mortality.

To better understand the risks posed by hantaviruses, it's essential to first understand the difference between lethality and mortality. Experts emphasize that these terms are not interchangeable. Understanding this distinction is crucial for accurately assessing the true danger of an infectious disease.

Case fatality rate measures the severity of a disease in infected individuals. Specifically, it is the percentage of people who die after contracting the infection. Conversely, mortality measures the impact of a disease on an entire population, whether infected or not. In other words, a disease can be highly lethal without causing high mortality if it remains rare or has low transmissibility.

TWO OFTEN CONFUSED INDICATORS

Hantaviruses are viruses transmitted primarily by infected rodents, notably through the inhalation of particles from urine, saliva, or feces. In Europe and Asia, they mainly cause hemorrhagic fevers with kidney involvement, while in the Americas, certain strains lead to much more severe pulmonary syndromes.

The World Health Organization (WHO) indicates that some American variants of the virus can have a fatality rate of up to 50%. This alarming figure does not, however, mean that one in two infected individuals in the general population will die. It means that approximately one in two severely infected people may die in certain specific outbreaks. Overall mortality remains low because infections are still rare. In metropolitan France, around one hundred cases are detected each year on average.

WHY DOES THIS NUANCE CHANGE EVERYTHING?

This distinction explains why very different diseases can be misunderstood. Covid-19, for example, had a significantly lower lethality rate than severe forms of hantavirus. Yet, its global mortality rate was much higher because the virus spread extremely easily and affected hundreds of millions of people.

Conversely, Ebola and certain hantaviruses have very high fatality rates, but their spread generally remains limited, particularly because their transmission often requires close or specific contact. Health authorities also emphasize that the risk of a hantavirus pandemic remains low, especially because human-to-human transmission is rare. Only the Andes strain, found in South America, has shown limited human-to-human transmission capabilities.

A COMMON CONFUSION

In the media and on social networks, the two concepts are often used interchangeably, sometimes fueling disproportionate anxiety. Saying that a virus is "40% fatal" without specifying that this refers to lethality can lead people to believe that 40% of the population is at risk, which is false. For epidemiologists, however, these indicators answer two different questions:

The lethality question asks: "What are the risks for a sick person?"

Mortality raises the question: "What impact does the disease have on society as a whole?"

An essential nuance to avoid alarmist interpretations or, conversely, minimizing a real danger.