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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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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