Incidence & Prevalence: Incidence | EUPATI Open Classroom

4. Incidence

Prevalence looks at existing cases, while incidence looks at new cases. Reading in a newspaper that a certain country has a prevalence of 2% of a certain disease will not give you any information about when those patients got the disease, or for how long they have had it.

For example, HIV is nowadays a treatable infection with a normal life expectancy. This means that with stable numbers of new cases, prevalence numbers will increase. Looking at the new cases (incidence) provides a deeper understanding of what is going on.

Depending on how incidence is expressed, either as a percentage or per person-year there are two different formulas.

To express incidence as a percentage:

Incidence is equal to new cases during period of observation divided by total population at risk during the same period

To express incidence per person-year at risk:

Incidence is equal to new cases during period of observation divided by time at risk

Often you can read about the incidence of a heath event as a certain number of cases per 100 or 1,000 person-years.

In such a case, the epidemiologist would count the total years the (healthy) population was at risk and divide the new cases by this amount of person-years at risk. This is described in more detail below.