Prevalence

    Prevalence is the proportion of people in a specific population who have a specific condition at a given time.

    Summary

    What is prevalence?

    Prevalence is a measure of disease frequency assessed at a single point in time. It refers to the proportion of people in a defined population who have a specific condition at a given time, including right now.[1][2][3] Measuring prevalence helps researchers understand the burden of disease, which is used for health care planning.[1][2]

    Prevalence is calculated as a proportion: existing cases divided by the number of people assessed, either at one time point (point prevalence) or across a time period (period prevalence).[2] Prevalence is also reported as an average prevalence across time, places, or groups.

    To summarize:

    • Point prevalence: the number of people having the condition at a specific time ÷ the total number of people in the population being assessed.
    • Period prevalence: (the number of people having the condition at the start of the time period + the number of people who develop the condition over the time period) ÷ the total number of people in the population being assessed.
    • Average prevalence: a descriptive summary of several prevalence estimates (across time, places, or groups) that answers the question, “On average, what is the typical prevalence across these times/places/groups?”

    Here are some examples:

    1. BMI data of U.S. adults were extracted from the National Health and Nutrition Examination Survey (NHANES).[4] The point prevalence of obesity increased from approximately 39% in 2017 to approximately 41% in 2019. These data show a snapshot of obesity prevalence at two different points (times of measurement) within the U.S. health care system.

    2. A community-based cohort study followed 408 participants with dementia for about 5 years, with repeated assessments for neuropsychiatric symptoms like delusions, aggression, depression, anxiety, and irritability at 1.5 years, 3 years, 4 years, and 5 years.[5] The authors reported that 5-year period prevalence was greatest for depression (77%). These data show that 77% of people in the cohort had depression symptoms at any time during the 5-year period.

    3. BMI data from NHANES showed that the average prevalence of obesity in U.S. adults during the period of August 2021 to August 2023 was approximately 40%.[6] These data show a snapshot of the average obesity prevalence in the U.S. health care system during the defined time period.

    Is prevalence different from incidence?

    Yes. Prevalence is the proportion of people who have the condition at a given time, whereas incidence refers to the number of people who develop a specific condition over a defined time period.[1][2]

    Although incidence and prevalence provide different information, 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]

    Prevalence counts people living with a specific condition at a given time, so it also increases when people live a long time with a condition. This means that prevalence is roughly the product of incidence and the average duration of the condition. If the condition lasts a long time, then prevalence rises, even if incidence stays low or falls. HIV is the textbook example. The introduction of antiretroviral therapy extended survival for people with HIV, but there was a persistently high prevalence, while new infections (incidence) weren’t rising. Why? Because antiretroviral therapy helped people live many more years with HIV.[7]

    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. ^Setia MSMethodology Series Module 3: Cross-sectional Studies.Indian J Dermatol.(2016 May-Jun)
    4. ^Henderson J, Ehlers AP, Lee JM, Kraftson AT, Piehl K, Richardson CR, Griauzde DHWeight Loss Treatment and Longitudinal Weight Change Among Primary Care Patients With Obesity.JAMA Netw Open.(2024 Feb 5)
    5. ^Steinberg M, Shao H, Zandi P, Lyketsos CG, Welsh-Bohmer KA, Norton MC, Breitner JC, Steffens DC, Tschanz JT, Cache County InvestigatorsPoint and 5-year period prevalence of neuropsychiatric symptoms in dementia: the Cache County Study.Int J Geriatr Psychiatry.(2008 Feb)
    6. ^Emmerich SD, Fryar CD, Stierman B, Ogden CLObesity and Severe Obesity Prevalence in Adults: United States, August 2021-August 2023.NCHS Data Brief.(2024 Sep)
    7. ^Zaidi J, Grapsa E, Tanser F, Newell ML, Bärnighausen TDramatic increase in HIV prevalence after scale-up of antiretroviral treatment.AIDS.(2013 Sep 10)