prevalence and incidence

1. Incidence and Prevalence Rate
Measures of disease frequency

2. OBJECTIVES:
 Importance of measures of disease
 Prevalence
 Incidence
 Relationship between prevalence and incidence
 Mathematical Relationship Among Prevalence,
Incidence, and Average Duration of Disease
 Stratification of disease frequency by Person, Place and
Time

3. Importance of measures of disease
 Early Detection:
Helps identify diseases in their initial stages, improving treatment outcomes.
 Public Health Planning:
Guides resource allocation, focusing on prevalent health issues in the community.
 Monitoring Trends:
Tracks disease patterns over time, aiding in outbreak detection and control.
 Assessing Interventions:
Evaluates the effectiveness of public health strategies and healthcare initiatives.

4. Prevalence defined:
 Prevalence refers to the total number of individuals in a population who have a disease
or health condition at a specific period of time.
 It has 3 main components:
• Existing cases
• Population at risk to have the condition
• Point in time to which the prevalence applies.

5. Prevalence measures helps to describe the current burden of a disease
in a population in order to facilitate planning and resource allocation.

6. Incidence defined:
 Incidence refers to the number of individuals who develop a specific
disease or experience a specific health-related event during a particular
time period (such as a month or year).
Two types of incidence are commonly used: Incidence proportion,
Incidence rate.

7. Incidence measures help to provide clue as to the cause or development of a disease.

8. Relationship between prevalence and
incidence
 Prevalence and incidence measure different phenomena, but they are related.
Prevalence vs. Incidence: Prevalence shows how many people have a disease at a certain
time, while Incidence reveals how fast new cases develop.
Influences on Prevalence: Prevalence is affected by both incidence (new cases) and the
average duration of the disease in the population.
Duration Matters: A disease's average duration is vital because people can stop being
prevalent cases by being cured or leaving the population.
Cured, Moved, or Deceased: If someone is cured, moves away, or passes away, they no
longer contribute to prevalence in the population.

9. Sink Analogy: Think of a sink fullness as similar to disease prevalence, showing the
percentage of people with the disease at a given time.
Water Flow : The water flowing into the sink represents disease incidence, with each drop being a
new case. Incidence adds to prevalence.
Drain Effect: Water can leave via the drain, symbolizing people being cured or dying, reducing
prevalence.

10. Constant Prevalence: If inflow (incidence) and outflow (cure or death) are equal, the
water level stays constant, meaning prevalence doesn't change.
Increasing Prevalence: If inflow increases and outflow remains constant, water level rises,
indicating an increase in prevalence.
Decreasing Prevalence: If outflow is constant and inflow decreases, the water level falls,
signifying a decrease in prevalence.
Changing Outflow: If incidence is constant, but cure or death rates increase, prevalence
decreases.
Prolonged Cases: If incidence is steady but cases last longer without cure, prevalence
rises.

11. Mathematical Relationship Among Prevalence, Incidence,
and Average Duration of Disease
• Steady State: When prevalence is stable, and incidence equals cure and death, it's like a
balanced equation:
P = Proportion with disease
IR = Incidence rate
“Average duration" = Time with the disease (until cure or death)
• Low Prevalence: If disease affects <10% of the population, the equation is simpler: P ≈ IR x
"Average duration“
• Prevention Impact: Reducing disease incidence lowers prevalence when "average duration"
remains the same.
• Cure Effect: Keeping incidence steady while developing a cure reduces both "average
duration" and prevalence.
• HIV Example: Anti-retroviral therapy improved HIV survival but didn't cure, increasing
"average duration" and raising HIV prevalence in the late 1990s.

12. Stratification
Stratification is defined as the act of sorting data, people, and objects into distinct groups or
layers. It is a technique used in combination with other data analysis tools.
Stratification can be by:
• Personal characteristic: Age, race/ethnicity, sex
• Geography
• Time period

13. Thank you