A Guide to HIV/AIDS Epidemiological and Surveillance Terms

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Appendix
Incidence rate, cumulative incidence, incidence density

There are two different ways to describe the rate at which new infections are occurring.

Cumulative Incidence

The incidence rate can be described as the number of people who become infected during a specified period of time as a proportion of a specific population at risk of the disease. This is called cumulative incidence.

Cumulative Incidence

It is important to pay attention to the time period to which the cumulative incidence relates. A cumulative incidence of 6% among attendees of a detox program would be viewed very differently if it referred to a three-month period or a three-year program period.

Incidence Density

Unlike participants in a residential program who can be followed for the specified time period of the study, most participants in cohort studies enter the study over a period of time, often over several years. Others will become lost to contact during the follow-up period so that their information is not available at the end of the study. The length of time of the study or follow-up will therefore not be the same for each participant. This can be seen in Figure A below.

Incidence rate can therefore also describe the proportion of people who become infected during a specified period of time as the proportion of the total time at which individuals in a population are at risk. You may sometimes see this way of calculating the number of people who become infected with a specific disease written as incidence density. Incidence density accounts for the varying time periods of follow up. Incidence density is therefore a more precise estimate of the rate of occurrence of a particular disease.

Although the numerator is the same as in the calculation of cumulative incidence, the denominator is now the sum of each individual's time at risk or the sum of the time that each person remained under observation and free from disease.

Summary Example

Figure A, above, shows 5 people, 2 of whom developed HIV in the five-year follow-up period of the study. The cumulative incidence of HIV would thus be:

However, this measure of the development of HIV is misleading as it does not reflect the fact that only 1 of the 5 individuals (subject C) was in fact observed for the entire follow-up period. Subject A was observed for only 2 years before being lost to follow-up, while subjects B, D and E were followed for 3.0, 4.0, and 2.5 years respectively. The total time at risk for this population of five subjects, observed by adding their individual times, would be 16.5 person years.

Incidence density would be calculated as follows:

Incidence density is therefore a more accurate measure of the rate at which new infections are occurring.

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