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Incidence – Definition & Meaning in Epidemiology

Incidence refers to the number of new cases of a disease occurring in a defined population over a specified period of time. It is a key measure in epidemiology.

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Things worth knowing about "Incidence"

Incidence refers to the number of new cases of a disease occurring in a defined population over a specified period of time. It is a key measure in epidemiology.

What Is Incidence?

Incidence is an epidemiological measure that describes how many new cases of a disease or health condition occur in a defined population during a specific time period. It is typically expressed as an incidence rate or incidence proportion and serves as a fundamental tool in epidemiology – the science that studies the distribution and determinants of disease in populations.

Unlike prevalence, which captures all existing cases of a condition at a given point in time, incidence focuses exclusively on newly occurring cases. It therefore provides insight into how quickly a disease is spreading within a population.

Calculating Incidence

Incidence is usually expressed in relation to a defined reference population – for example, per 100,000 people – over a specified period, typically one year. The basic formula is:

Incidence rate = (Number of new cases / Number of people at risk) × 10,000 or 100,000

A distinction is made between cumulative incidence (also called incidence proportion), which describes the proportion of new cases in a fixed time period, and incidence density (also known as the incidence rate in the narrow sense), which calculates the event rate per person-time at risk.

Importance in Medicine and Public Health

Incidence is an indispensable tool for:

Monitoring infectious diseases: Public health authorities use incidence data to detect outbreaks early and implement countermeasures.
Assessing risk factors: Comparing incidence rates across different groups helps identify risk factors for diseases.
Healthcare planning: Rising incidence rates signal an increased need for medical resources and infrastructure.
Evaluating preventive measures: A decline in incidence following a vaccination campaign or public health intervention demonstrates its effectiveness.

Incidence vs. Prevalence

Although incidence and prevalence are closely related, they measure different aspects of a disease:

Incidence: Newly occurring cases within a time period – reflects the risk of developing a disease.
Prevalence: All existing cases (new and old) at a point in time or over a period – reflects the overall burden of disease.

For a disease with a long duration (e.g., diabetes mellitus), prevalence is typically much higher than incidence. For an acute, rapidly resolving illness (e.g., seasonal flu), incidence and prevalence are closer in value.

Practical Examples

A well-known example of incidence in practice is the 7-day incidence, which was used as a key control parameter during the COVID-19 pandemic. It indicated how many new infections were reported per 100,000 inhabitants within a seven-day period.

Further examples:

The annual incidence of breast cancer in Germany is approximately 130 new cases per 100,000 women.
The incidence of myocardial infarction (heart attack) in Germany is approximately 300 cases per 100,000 inhabitants per year.

References

Rothman KJ, Greenland S, Lash TL: Modern Epidemiology. 3rd edition. Lippincott Williams & Wilkins, Philadelphia, 2008.
World Health Organization (WHO): Introduction to Epidemiology. WHO Press, Geneva, 2020. Available at: https://www.who.int
Centers for Disease Control and Prevention (CDC): Principles of Epidemiology in Public Health Practice. 3rd edition. Atlanta, 2012.

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