M09.01.002 Incidence and Prevalence

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Learning Objectives

By the end of this module, the medical student will:
  • Differentiate between Incidence (new cases) and Prevalence (all cases).
  • Calculate Attack Rates during outbreak scenarios.
  • Predict how interventions (vaccines, treatments) impact epidemiologic metrics.
  • Distinguish between Morbidity and Mortality rates.

I. Incidence: The “New Events” Rate

Incidence Rate (IR) measures the flow of new disease into a healthy population.

Formula:

IR = (Number of NEW cases in a period) / (Population AT RISK during that period)

Special Case: Attack Rate
Used typically used for food-borne illness or short-term outbreaks. It is the cumulative incidence from the start to the end of an outbreak.

Example: 18 people ill with Norwalk virus in a community of 1,000.
Attack Rate = 18 / 1,000 = 1.8%

II. Prevalence: The “Total Burden”

Prevalence is a snapshot of all individuals affected by a condition (New + Old cases).

Type Description
Point Prevalence Disease present at a single point in time (e.g., “Today”).
Period Prevalence Disease present over a defined span (e.g., “The year 2021”).

The “Prevalence Pot” Concept

1. Incidence (New cases) flows INTO the pot.
2. Death or Recovery flows OUT of the pot.

III. Dynamic Changes in Population Metrics

How do specific events change the data? This is a high-yield exam topic.

Scenario Incidence Prevalence
New effective Vaccine Decrease Decrease
New effective Treatment (Cure) No Change Decrease
Increased Mortality (Death) No Change Decrease
Improved Survival (but no cure) No Change Increase
Reduced Risk Factors Decrease Decrease

IV. Morbidity vs. Mortality

Morbidity Rate

The rate of disease (sickness) in a population. Includes both incident and prevalent cases.

Mortality Rate

The rate of death in a population. Usually calculated as incident cases only (death is a one-time event).

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