Incidence
What is incidence?
Incidence is an epidemiological measure used to quantify the occurrence of new cases of a disease in a given population over a given period of time.
There are two main ways of measuring incidence:
- Incidence rate (IR): The frequency with which new cases of a disease occur in a population at risk, taking into account the exposure time of each individual. It is calculated by dividing the number of new cases by the amount of time each person was observed.
Example:
During 2010–2018, a group of 8 women aged 35-50 who had recovered from a urinary tract infection were followed to measure disease recurrence. Three participants were followed for 5 years, three for 4 years, and the remaining two for 3 years. At the end of the study, 3 of the 8 women had a recurrence of the infection.
The incidence rate (IR) would be calculated by dividing the number of new cases by the total person-years of follow-up:
IR = 3 / (5 + 5 + 5 + 4 + 4 + 4 + 3 + 3) = 3 / 33 = 0.0909 cases per person-year.
This means that there were approximately 9 cases per 100 person-years of follow-up.
- Cumulative incidence (absolute risk): The proportion of people who develop the disease within a given population and time interval. It is calculated by dividing the number of new cases by the total number of people at risk at the start of the period. This measure assumes that all individuals are followed for the entire period, which is why it is often used in cohort studies.
Example:
In a one-year study, a group of 1,050 people was observed, 50 of whom had the flu. During that year, 20 more people got the flu.
The cumulative incidence (CI) would be calculated by dividing the number of new cases during the year by the number of susceptible people at the start of the period. Only new cases among those who were healthy at the start (i.e., susceptible) are counted.
CI = 20 / (1,050 – 50)
CI = 0.02
The cumulative incidence would be 2%.
Why should we calculate it?
Knowing the incidence of a disease is essential in epidemiology and public health because it allows us to assess the risk of a person in a given population developing the disease in a given period of time. This indicator is crucial for the early detection of outbreaks and epidemics, facilitating the timely implementation of control and prevention measures. It also provides valuable information for monitoring trends, evaluating the impact of prevention and treatment programmes, and allocating resources efficiently. It also helps to identify groups at risk and to develop strategies to reduce transmission.
Incidence should not be confused with prevalence. While incidence refers to the number of new cases that occur during a given period, prevalence measures all existing cases of a disease at a given time.
A rapidly spreading disease of short duration may have a low prevalence and a high incidence. In this way, incidence is useful for assessing parameters such as the speed of transmission or the effectiveness of preventive measures, as in the case of the COVID-19 pandemic.
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