Incidence

    Incidence refers to the number of people in a particular population who develop a specific condition over a defined time period. It is reported as a risk (the proportion who develop the condition over the time period) or an incidence rate (new cases divided by total person-time).

    Summary

    What is incidence?

    Incidence is a measure of disease frequency in a defined population. It refers to the number of people who develop a specific condition over a defined time period.[1][2] Measuring incidence helps researchers assess possible causes of disease and identify risk factors.[1][2]

    Incidence is reported as a risk (the proportion of the studied population who develop the condition over the time period, or cumulative incidence) or an incidence rate (new cases divided by total person-time).[1][2] These are calculated as follows:

    • Risk: the number of people developing the condition over a specific time period ÷ the total number of people followed over that time period.
    • Incidence rate: the number of people developing the condition ÷ the total time people were at risk for the condition.

    Here are some examples:

    1. A cohort study enrolled about 45,000 U.S. adults under 65 years old without diabetes.[3] Participants in the cohort were followed over about 6.8 years on average, and about 4,000 participants were diagnosed with type 2 diabetes. This means that the risk, or cumulative incidence, for developing diabetes was approximately 9% over 6.8 years, with the risk calculated as: (4,000 ÷ 45,000) × 100%. The authors then used this number to calculate a 10-year cumulative incidence, which was about 13%. This figure suggests that over a 10-year period, 13% of U.S. adults under 65 are likely to be diagnosed with diabetes.

    2. The Veterans Administration Diabetes Risk (VADR) cohort includes all 6 million veterans in the U.S. who did not have diabetes when they enrolled in the Veterans Administration (VA) primary care system.[4] Participants in the cohort were followed over about 5.5 years (about 35 million person-years) on average, and about 900,000 of them were diagnosed with type 2 diabetes. This equates to an incidence rate of about 26 new diabetes cases per 1,000 person-years (1,000 × 900,000 ÷ 35 million), which provides an estimate of diabetes incidence in U.S. veterans.

    Is incidence different from prevalence?

    Yes. Incidence is the proportion of people in a defined population who develop a specific condition over a defined time period, whereas prevalence is the proportion of people in a defined population who have a specific condition at a given time, including right now.[1][2][5]

    Although incidence and prevalence provide different information about disease frequency, they are related. For example, over time, there will be new cases (incidence) of a specific condition in an at-risk population. As new cases arise (i.e., as incidence increases), the number of existing cases (prevalence) will also increase. However, the number of existing cases (prevalence) will later decrease because, typically, people die from the condition or recover from it.[1]

    References

    1. ^Noordzij M, Dekker FW, Zoccali C, Jager KJMeasures of disease frequency: prevalence and incidence.Nephron Clin Pract.(2010)
    2. ^Jager KJ, Zoccali C, Kramar R, Dekker FWMeasuring disease occurrence.Kidney Int.(2007 Aug)
    3. ^Egan AM, Wood-Wentz CM, Mohan S, Bailey KR, Vella ABaseline Fasting Glucose Level, Age, Sex, and Body Mass Index and the Development of Diabetes in US Adults.JAMA Netw Open.(2025 Jan 2)
    4. ^Avramovic S, Alemi F, Kanchi R, Lopez PM, Hayes RB, Thorpe LE, Schwartz MDUS veterans administration diabetes risk (VADR) national cohort: cohort profile.BMJ Open.(2020 Dec 4)
    5. ^Setia MSMethodology Series Module 3: Cross-sectional Studies.Indian J Dermatol.(2016 May-Jun)