Lecture on Incidence and Prevalence

Importance

• Crucial to understand the difference between incidence and prevalence.
• Incidence: Measure of disease risk.
• Prevalence: Measure of disease burden.

Incidence

• Definition: Number of new cases of a disease during a specific time period divided by the number of persons at risk during the same period.
• Calculation: Resulting proportion can be multiplied by 1000 or 100 to get new cases per 1000 or per 100 population.
  • Example: In a population of 10 women, 4 develop uterine cancer in a year.
    • Incidence = (4/10) * 1000 = 400 cases per 1000 population per year.
• Denominator: Everyone in the denominator must have the potential to become part of the numerator (disease group).
  • Adjusted Example: 3 women have hysterectomies (cannot develop uterine cancer).
    • Incidence = (4/7) * 1000 = 570 cases per 1000 population per year.
• Key Point: Incidence is a measure of events and risk.

Prevalence

• Definition: Number of affected persons in the population divided by the number of all persons in the population at a specific point in time.
  • Example: In a population of 10 with 3 affected, prevalence = (3/10) * 1000 = 300 per 1000 population.

Difference Between Incidence and Prevalence

• Incidence: Measures new cases over time.
• Prevalence: Measures total cases at a specific point in time.
• Example with Tuberculosis (TB):
  • 5 people develop TB over 6 months.
    • Incidence: Number of new cases each month.
    • Prevalence: Total cases each month (takes into account duration of disease).
      • Prevalence changes as people are cured or die (leave the prevalence pool).
  • Visual Aid: Vessel filled with pebbles (cases).
    • Incidence: Adds pebbles (new cases).
    • Death/Cure: Removes pebbles.

Factors Affecting Prevalence

• Increase in Incidence: Prevalence goes up.
• Decrease in Death/Cure Rates: Prevalence goes up.
• Increase in Death/Cure Rates: Prevalence goes down.
• Overall Impact: Prevalence = Incidence * Duration
  • Duration shortened by higher death/cure rates, lengthened by lower rates.
• Example: Coronary Heart Disease (CHD) in Hollywood vs. the Bronx
  • Hollywood: 50 cases per 1000 (better care, longer duration).
  • Bronx: 10 cases per 1000 (shorter duration, higher death rate).
  • Conclusion: Higher prevalence in Hollywood due to better care, not higher risk.

Summary

• Incidence: Risk of developing a disease.
• Prevalence: Current burden of the disease.
• Incidence = total new cases/time.
• Prevalence = total cases at a specific time.
• Prevalence influenced by incidence and duration (death/cure rates).