M09.01.02 Incidence and Prevalence

Understanding incidence and prevalence rates is essential in epidemiology as these measures provide insights into disease occurrence, burden, and trends. These indicators help in tracking health events, planning healthcare resources, and evaluating preventive measures.

1. Incidence Rate (IR)

Incidence Rate (IR) refers to the occurrence rate of new cases of a disease within a specified population and period. It is crucial for assessing the risk of developing a new health event and is particularly useful in tracking outbreaks or the spread of infectious diseases.

Formula

Incidence Rate (IR)=Number of new events in a specified period of persons at risk during that period×10n

Example Calculation

For instance, if 5 men are newly diagnosed with prostate cancer in a study of 200 men (all cancer-free at the start of the study), the incidence rate would be:

Attack Rate

Attack Rate is a type of cumulative incidence used to describe the proportion of people who become ill in a given population during an epidemic. This measure is commonly applied in foodborne outbreaks and is calculated from the onset to the end of the outbreak.

Formula

Attack Rate=Number of exposed people infectedTotal number of exposed people×100%

Example Calculation

If an outbreak of the Norwalk virus affects 18 people in a community of 1,000, the attack rate is:\

2. Prevalence

Prevalence measures the total number of existing cases (both new and pre-existing) of a disease in a population at a specific time. It helps in understanding the disease’s burden, especially for chronic illnesses.

Types of Prevalence

• Point Prevalence: Measures the proportion of a population with the disease at a single point in time (snapshot).
• Period Prevalence: Measures the proportion of a population with the disease over a specified period.

Formula

Prevalence=All cases (new and existing) at a given time total population at risk at that time×10n

Example

For example, if the prevalence of obesity among American adults is estimated at 20%, this includes all cases of obesity at a single point in time.

3. Differences Between Incidence and Prevalence

Points to Remember

• Incidence only counts new cases within a set time frame, making it suitable for assessing disease transmission and outbreak trends.
• Prevalence includes both new and ongoing cases, which is useful for understanding chronic disease burdens.
• Attack Rate is a type of incidence used in epidemic studies to gauge infection spread within a group.
• The “Prevalence Pot” concept illustrates how prevalent cases may leave the pool due to recovery or death.