Incidence – Concept, prevalence and incidence rate

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

In medicine, incidence is the number of new cases of a disease.

What is incidence?

By the term incidence we generally refer to a event or occurrence that occurs in the middle of a matter and that you have a relationship with him. This general meaning comes from the very origin of the word, which dates from Latin incidentia, composed by in- (“Inward”) and the verb cadere (“Fall”), so that it is figuratively understood as something that “falls” or occurs 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 an issue or event. That is why we use the expression “by incidence” as a synonym for “by chance”, and the verb influence as a synonym of “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 that which 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 are registered within a population determined within a set period of time. In other words, how many infections have there been within a population within a certain time frame.

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 previously.
  • Cumulative incidence: The percentage probability that a healthy individual will develop the disease within a set period of time.

This information is always useful when designing public health policies or tackling a particularly contagious disease in the context of an epidemic.

Incidence rate

The incidence rate or incidence density is, in the framework of the study of a disease, the statistical relationship between the number of new cases in a given 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 in time the appearance of a disease is in a given population.

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 middle 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 these, 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 five years, another two for four years, and the remaining only for 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.

In other words, prevalence refers to the proportion: how many affected individuals are there in relation to the total population. Therefore, it is expressed as a fraction, a percentage or a specific number of cases per 10,000 or 100,000 inhabitants.

Unlike incidence, which expresses the rate of onset of the disease (that is why it considers the measurement time), prevalence can be understood as how common the disease is within the population, that is, what percentage of the population gets it. The higher the prevalence, the more common the disease; the lower the prevalence, the rarer it is.