What is the difference between prevalence and incidence rate of disease?

Question

A timely question -

In epidemiology, what is the difference in the information provided by the prevalence of a disease vs. the incidence of the disease? (Hint: one is a proportion and is therefore bounded by 1, while the other one is a rate and bounded by infinity. Your denominator and numerator matter!)

Answer 1

There are different measures of disease frequency. They are used to measure the occurrence of disease in a population. Typically, we are studying the first occurrences of disease, although diseases can re-occur and the subsequent occurrences can be impacted by the primary occurrence.

Measures of disease frequency can incorporate person (definition of the population), time and the disease state. Two (2) frequently mentioned measures of disease frequency are prevalence and incidence. Incidence can refer to cumulative incidence (simple or actuarial), incidence rate (or incidence density). We;ll focus on prevalence vs. incidence rate for now.

Prevalence: Is the proportion of people in a population with a disease a particular time point. For example, if there are 10 cases of cancer in a population of 100 people, the prevalence is 10/100 or 0.1 (10%). The prevalence provides a snapshot of the disease at 1 time (it is a cross-sectional measure of disease frequency). Remember that prevalence includes an enumeration of people in the denominator.

Incidence rate (or incidence density): Is the number of newly developed cases in a time period divided by the total amount of time contributed by each person in the population. For example, let's say you are interested in the incidence rate of a disease over 1 year. You follow 100 people for 1 year. The total person-time would be 100*1 year = 100 person-years. In that year, 5 people develop the disease. The incidence rate would be 5 cases/100 person-years. Remember that a rate includes a measure of time in the denominator.