3. Incidence: Points to consider | EUPATI Open Classroom

Incidence rate measures are useful when describing how quickly disease occurs in a population. This information can be used to monitor the effectiveness of vaccination schemes or changes in the applied preventative measures to see if they cause a knock-on reduction in disease incidence rate. Equally, if incidence rates rise, it can act as a warning that existing preventative measures are failing and therefore guide remedial action.

Since for incidence rate person-time is calculated for each individual it can include persons entering and leaving study populations, whereas incidence proportion assumes that those individuals were disease-free; therefore, incidence rate generally is more accurate than incidence proportion for long-term studies.

·An accurate measure of incidence, whether incidence proportion or incidence rate, requires a precise definition of the denominator. The denominator accounts for study participants who are lost to follow-up or who die during the study period. Because incidence is a measure of new cases during a given time period, it is important that those persons in the denominator be at risk (see text box on denominators earlier in the lesson).

An additional consideration for the use of incidence rate is that it assumes a constant probability of disease, which may not reflect actual probability, particularly for conditions whose risk increases with age.

When communicating about incidence rates, refrain from an epidemiologist’s jargon. Reporting the incidence rate of, for example, stroke in country X for 2010 as 117 per 100,000 person-years, most people will not understand. To convert this jargon to something understandable, simply replace “person-years” with “persons per year” and convert the numbers to a plausible format. Reporting the results as 1,17 new cases of stroke per 1,000 persons per year sounds like plain language rather than jargon.