Prevalence is a measure of the health status of a population for a given disease.
It is measured by the number of individuals suffering from a disease for a given population, grouping together new and existing cases.
Prevalence is to be distinguished from the prevalence rate, which represents the proportion of the population suffering from a disease.
Prevalence can also be expressed per population slice, usually per 100,000 persons.
Types of prevalence measures
There are 3 types of prevalence:
Instant prevalence: the number of cases of a disease at a given time
Prevalence over a given period: the number of cases of a disease over a given period (year, month, etc.)
Lifetime prevalence: the number of cases of a disease over a lifetime
Example of instantaneous prevalence
In a territory, 2,000 cases are identified at a time t for a population of 200,000.
The prevalence is 2,000 cases
The prevalence rate is 1.00%: ( 2,000 / 200,000 )
Expressed per 100,000 population, the prevalence is 1,000 cases per 100,000 population: ( 2,000 / 200,000 ) x 100,000
Example of prevalence over a given period
The point to consider over a period is that the population is changing over time. Over the study period, then, you need to take the average population during your study period.
In one territory, 5,000 cases are counted over a year for an average population of 205,472 over that period.
The prevalence is 5,000 cases
The prevalence rate is 2.43%: ( 5,000 / 205,472 )
Expressed per 100,000 population, the prevalence is 2,433 cases per 100,000 population: ( 5,000 / 205,472 ) x 100,000