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A Guide to HIV/AIDS Epidemiological and Surveillance Terms

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Frequently Used Terms in HIV/AIDS Monitoring and Epidemiological Research in Canada

I

INCIDENCE
(See also INCIDENCE RATE and FAQs 18-21)

Incidence is the number of new events of a specific disease during a specified period of time in a specified population.

HIV incidence is the number of new HIV infections occurring in a specified period of time in a specified population.

Knowing or estimating the number of new events of a specific disease during a specified period of time gives a sense of how quickly a disease is spreading.  In HIV/AIDS research, incidence gives us a sense of how fast the HIV epidemic is spreading.

Incidence may be expressed as a number or a rate (see below).

Incidence is different from prevalence; however, the two terms are frequently confused.

INCIDENCE DENSITY
(See Appendix)

INCIDENCE RATE

The incidence rate is the rate at which new events, or new cases, occur in a specified time in a defined population that is "at risk" of experiencing the condition or event.

Incidence rate =
    Number of new events in a specified period   
Number of people exposed to risk in this period

It is very important to know the number of new people infected with a disease (incidence) in relation to the total population at risk of contracting that disease.

For example, a report of HIV infection diagnosed in 5 new people over 12 months only tells us the number of new HIV infections that are occurring during a specific time period. However, 5 new cases of HIV infection reported over 12 months in a population of 10 people at risk (example 1) has a different meaning from 5 new people diagnosed with HIV infection over 12 months in a population of 100 people at risk (example 2). To take account of this, incidence is expressed as a rate.

Using this example, we would calculate incidence rate as either of the following:

Example 1

Incidence rate =
5 new people with a diagnosis of HIV
= 0.50
Population of 10 people

Example 2:

Incidence rate =
5 new people with a diagnosis of HIV
= 0.05
Population of 100 people

In the first example, the incidence rate is 0.50 (× 100 = 50%). This means that in a group of 100 people, we would expect 50% of them, or 50 of 100, to be newly diagnosed with HIV over the 12 months. In the second example, we would expect 5%, or 5 of 100 people.

Incidence rate can also be calculated using person years:

Incidence rate =
Number of new events in a specified period
Number of person years


For example, in a study investigating the number of new cases of HIV among young gay and bisexual men, an overall incidence rate of 1.3 per 100 person years was reported. (Martindale et al., 2001)

To understand what this 1.3 means, consider this example:
If there were 1,000 men in the study and they were followed for two years, 26 of them would have gone from being HIV negative to HIV positive over the study time period of two years. This is calculated by:

1. Calculating the person years of the study:
(1,000 men × 2 years = 2000 person years)

2. Multiplying this result with the incidence rate in person years:
(1.3 new infections/100 person years) × (2000 person years) = 26 new infections

A more detailed explanation of incidence rate and the related terms "cumulative incidence" and "incidence density" appears in the Appendix.


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Date Modified: 2006-09-27

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