The 5 differences between mortality and lethality - medical - 2023



What do you think is more deadly? The flu or Ebola? Surely, if you are not familiar with statistics within epidemiology and public health, you will say that Ebola. And unfortunately, you will be wrong. The flu is more deadly than Ebola.

Now, what do you think is more lethal? The flu or Ebola? Now you can say Ebola and you will be right. Indeed, Ebola is a disease that has a very high fatality rate but a very low mortality rate. Influenza, on the other hand, has a very low fatality rate and a higher mortality rate.

Mortality and fatality are not synonymous. Both are related to deaths associated with infectious or non-infectious diseases, but they are calculated in quite different ways. While the mortality rate is calculated with respect to the total population, the fatality rate is calculated with respect to the sick population..

And in today's article, in order for you to see answered all the questions you may have about this topic, we will see the main differences between mortality and lethality, in addition to, obviously, defining exactly what the mortality rate is and what it is. the fatality rate. Let us begin.

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What is the death rate? And the fatality rate?

Before detailing exactly the differences between both concepts, it is essential that we define them individually. And it is so, understanding what the mortality rate and the fatality rate consist of, that we can see their points in common but also those of disagreement.

Mortality rate: what is it?

The death rate from a disease is a statistical measure that indicates the proportion of people who die from a specific pathology with respect to the total population of a set of inhabitants studied.

In this sense, mortality is a mathematical ratio between the number of deaths due to a specific disease during a certain period of time and the total population in that same period. Therefore, the specific mortality rate of an infectious disease or not is the proportion of deaths related to this pathology within a specific population, taking into account both healthy and sick inhabitants.

How much does a disease kill within a healthy and diseased population? This is the question that the death rate calculation seeks to answer. For this reason, those diseases that cause the most deaths (despite the low fatality rate that we will analyze later) will have the highest mortality rate.

Depending on the needs of the epidemiological study and how small (or large) this rate is, it will be expressed in deaths per 1,000, 10,000, 100,000 or 1,000,000 inhabitants of a specific geographic area or population.

As of this writing (April 6, 2021), in Spain 75,783 people have died from COVID-19. If we take into account the total population of that country (not the total number of coronavirus cases that have occurred), which is about 47 million people, we can find the death rate for, for example, 10,000 inhabitants.

We multiply the number of deaths (75,783) by 10,000 (which is how we want the proportion to show us) and divide the result of this multiplication by the total population (47,000,000). Conclution? The death rate of COVID-19 in Spain is 16 deaths per 10,000 inhabitants. Or, if you want to show as a percentage, a mortality of 0.16%. This is the mortality rate: deaths with respect to the total population.

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Case fatality rate - what is it?

The fatality rate of a disease is a statistical measure that indicates the proportion of people who die from this disease with respect to the population that has been infected (or that has developed, if not infectious) with this pathology.

In this sense, fatality results from the ratio of deaths in relation to people who, within a population, have suffered from this disease. Therefore, the fatality rate refers to the proportion of sick people (we stop taking into account the healthy population) who have died as a result of an infectious disease or not.

How much does a disease kill people who get sick from it? This is the question that the mathematical calculation of the fatality rate seeks to answer. It is the proportion of people who die from a disease among those affected by it. Thus, if we talk about a disease that has a fatality rate of 10%, it means that of every 100 people who contract the disease, 10 die.

The fatality rate is normally calculated as a percentage, since it is no longer of much interest to see the deaths with respect to a population, but we want to see the proportion of deaths among those affected.Continuing with our example from before, we continue with the 75,783 deaths from COVID-19 in Spain, but now our reference is not the total population of the country, but how many cases of coronavirus there have been since the start of the pandemic.

Reviewing the data, we see that 3,300,000 cases have been diagnosed. Therefore, now we repeat the process of multiplying deaths (75,783) by 100 (because we want to obtain a percentage), but now we do not divide it by 47,000,000 (inhabitants of Spain), but by 3,300,000 (people who have sick with COVID-19 in Spain). Conclution? In Spain, the coronavirus has a fatality rate of 2.29%. As we can see, while its mortality rate is 0.16%, the fatality rate is 2.29%. This is fatality: deaths among the sick population.

How are mortality and fatality different?

Having defined the concepts of mortality rate and fatality rate, surely the differences have become more than clear. Even so, in case you want to have the most synthesized information, we have prepared a selection of the most important differences in the form of key points.

1. Mortality is calculated with respect to the total population; the lethality, with respect to the sick

Without a doubt, the most important difference and the mainstay of everything. As we have seen, the mortality rate is a statistical measure obtained by calculating the proportion of deaths due to a disease with respect to the total population, both sick and healthy.

The fatality rate, on the other hand, is obtained by calculating the proportion of death due to a disease, but not with respect to the total population, but rather to the one that has become ill due to a pathology. In fatality, we see how many sick people are killed by a specific pathology.

2. Mortality is easier to calculate than lethality

Both are statistical measures whose calculation is complicated. But within this inevitable complexity, the mortality rate is easier to calculate than the fatality rate. And it is that knowing the number of deaths from a disease and the total population of the geographic area that is being studied, you already have it.

In the case fatality rate, on the other hand, you need a factor that can be very difficult to obtain precisely: the sick population. A logistical problem that becomes even greater for those diseases that present asymptomatic cases, that is, people who, despite suffering from the disease, do not have symptoms. For this reason, lethality is a statistical measure that, in certain cases, may not be entirely representative.

3. The fatality rate does not take into account the healthy population

As we have seen, while the mortality rate seeks to answer the question of how many people die within a healthy and diseased population Due to a disease, the fatality rate only seeks to know how many of the sick people end up dying. In our example, for the mortality rate we took into account the total population in Spain (47 million people), but for the fatality rate we only used the cases of COVID-19 that there have been (3.3 million).

4. The diseases that kill the most are those with high mortality

A high mortality rate does not necessarily imply a high fatality rate. And the diseases that kill the most in the world are those with high mortality, not high fatality. And it is that a high mortality rate translates into more deaths within a population.

Ischemic heart disease, respiratory tract infections, obstructive pulmonary disease, lung cancer, diabetes, dementia, diarrheal diseases, tuberculosis, AIDS, etc., are the main causes of death in the world not because have a great lethality (which some do), but because their mortality is very high. They affect many people, which translates into more deaths.

A very common disease with a low fatality rate can lead to more deaths than a less common disease with a higher fatality rate.

5. The most lethal diseases are usually rare

Most seasonal flu have a fatality rate of 0.1%. "Only" 1 in 1,000 people with the flu dies. However, with about 25% of the world's population sick with the flu each year, it should come as no surprise that this low fatality translates into between 300,000 and 600,000 deaths annually worldwide.

However, true lethal diseases are, fortunately, very rare. In nature, a high fatality rate generally translates into a low incidence, fortunately. Thus, anthrax has a lethality of 85%, Ebola of 87%, rabies of 99% and Creutzfeldt-Jakob disease, the most lethal disease in the world, of 100%.

But of course, let's take a very lethal disease, such as primary amoebic meningoencephalitis, caused by an infection of the brain by an amoeba that inhabits lakes and rivers called Naegleria fowleri (better known as brain-eating amoeba). This pathology has a lethality of 97%. Of every 100 people who develop the disease, 97 die.

Even so, every year between 0 and 8 cases are registered worldwide. Let's put ourselves in the worst case scenario: 8 cases and 8 deaths. If we take into account that the world population is about 7,700 million people and that among all of them there are only 8 deaths due to the infection. The mortality rate for amoebic meningoencephalitis is 0.0000001%.

These deadly diseases are also so strange that while its fatality rate can reach 97%, it affects so few people that the mortality rate can be as low as 0.000001%. That is why it is so important not to confuse, especially in the news, news and press, between these two concepts.

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