M09.01.002 Incidence and Prevalence

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Learning Objective

By the end of this topic, medical students should be able to:

Differentiate incidence and prevalence.
Calculate incidence rate, attack rate, and prevalence.
Understand how these measures reflect disease risk vs burden in both acute and chronic conditions.

Incidence Rate (IR)Incidence rate measures how quickly new cases of a disease occur in a population over a defined period. It excludes pre-existing cases and focuses only on those at risk.

Formula:

$\text{Incidence Rate (IR)} = \frac{\text{Number of new cases during a specified period}}{\text{Population at risk during the same period}}$

The denominator includes only individuals at risk.
Often expressed per 100,000 population for comparison.
Useful for infectious diseases (e.g., TB, malaria).

Example:
Study population: 200 men without prostate cancer
New cases in 1 year: 5

$\text{IR} = \frac{5}{200} = 0.025 \text{ (or 2,500 per 100,000 men-year)}$

Attack Rate

Cumulative incidence during an epidemic; commonly used in outbreaks (e.g., foodborne illnesses).

Formula:
$\text{Attack Rate} = \frac{\text{Number of exposed people infected}}{\text{Total number of exposed people}} \times 100$

Example:
Norwalk virus outbreak: 18 infected out of 1,000 exposed

$\text{Attack Rate} = \frac{18}{1000} \times 100 = 1.8\%$

Prevalence

Prevalence measures all cases (new + existing) of a disease in a population at a specific point or over a period. It reflects disease burden, not risk.

Formula:
$\text{Prevalence} = \frac{\text{All existing cases at a point/period}}{\text{Total population at risk at that point/period}}$

Types of Prevalence

Type	Definition	Use
Point prevalence	Proportion of cases at a specific date	“Snapshot” of disease in population
Period prevalence	Proportion of cases during a specified time period	Useful for chronic diseases

Example:
TB prevalence in a community on 1/1/2025:
TB patients: 50
Total population: 1,000

$\text{Point prevalence} = \frac{50}{1000} \times 100 = 5\%$

Key Differences: Incidence vs Prevalence

Feature	Incidence	Prevalence
Definition	New cases over time	All cases (new + existing)
Formula	New cases ÷ population at risk	Total cases ÷ total population
Use	Measures risk or rate of developing disease	Measures disease burden
Affected by	Minimal disease duration	Strongly influenced by disease duration
Example	New TB cases in 2025	Total TB cases on 1/1/2025

Practical Notes

Prevalence decreases if: recovery increases or mortality increases.
Incidence decreases if: effective vaccination is implemented or transmission is reduced.
Chronic diseases (e.g., obesity, HIV, TB) → prevalence is more meaningful.
Acute diseases or outbreaks → incidence or attack rate is more informative.