3. Incidence
4. Points to consider
- 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.