Incidence (Meaning and Explanation)

We explain what incidence is, its differences with prevalence and what the incidence rate is. Also, what is an incident.

What is incidence?

With the term incidence we generally refer to a event or happening that occurs in the middle of an issue and that has some relationship with him. This general meaning comes from the very origin of the word, which dates back to Latin incidentiacomposed of in- (“inwards”) and the verb cadere (“fall”), so that figuratively it is understood as something that “falls” or happens within the framework (“within”) of an issue.

In this sense, an incident is understood as something fortuitous, sudden, unexpected, that occurs in the middle of a matter or event. That is why we use the expression “by incidence” as a synonym for “by chance”, and the verb incidir as a synonym for “affect”, that is, to have an unpredictable effect on something (“the rains affected the climate”).

However, one of the most common uses of incidence is the one that has to do with medical terminology, that is, with the world of health and medicine, and that usually refers to the number of new cases of a disease that occur within a population determined in a set period of time. That is, how many infections there have been within a population within a period of time.

The incidence of a disease can also address specific conditions, for example:

Recidivism: The reappearance of the disease in patients who already suffered from it before.
Cumulative incidence: The percentage chance that a healthy individual will develop the disease within a set period of time.

Incidence rate

The incidence rate or incidence density is, within the framework of the study of a disease, the statistical relationship between the number of new cases in a specific period of time, and the sum of all observation periods. This means that it is an indicator expressed in cases-time, to express how frequent the appearance of a disease is in a given population over time.

The incidence rate is calculated by dividing the total number of new cases by the sum of the time periods. Let's look at a couple of examples of this:

In the midst of a study on the reappearance of the flu, the health of 10 people of the same age range is monitored for two years. Of them, 6 contracted the flu on more than one occasion. Therefore, the recidivism rate reported in the study will be 6/2, that is, 3 person-years.
Another study focuses on the appearance of a certain sexually transmitted disease in adult males, studying a population of 5 volunteers for a maximum of five years. Among them, two were studied for all five years, another two for four years, and the remaining one for only one. At the end of the study, the disease had occurred in only 2 of the individuals studied. Thus, the incidence rate would be 2 / (5+5+4+4+1) = 2 / 19 = 0.10526 person-years.

Prevalence and incidence

Prevalence and incidence are different, but similar, statistical factors. Prevalence refers to the persistence of a disease over a period of time ; incidence, on the other hand, always refers to new cases.