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Glossary · Statistics

What are prevalence and incidence?

Prevalence is the proportion of a population with a disease at one moment (point prevalence) or over a period (period prevalence), counting new and existing cases. Incidence counts only new cases: the incidence proportion is the risk over a period, the incidence rate uses person-time as denominator.

According to the CDC, prevalence is the share of a population that has a given disease or attribute, counting both new and previously existing cases. Point prevalence is measured at a particular time, period prevalence at any time during an interval. StatPearls (Tenny and Hoffman) defines the same two measures. The CDC notes that for chronic diseases whose onset is hard to date, prevalence is often measured instead of incidence.

Incidence counts new cases arising in a specified period, and the CDC describes two forms. The incidence proportion (cumulative incidence, risk, attack rate) is the share of people who start disease-free and become cases within the period. The incidence rate (person-time rate) puts observation time into the denominator: the total time all participants were followed.

According to the CDC, prevalence depends on how many new cases arise and how long the illness lasts: a high prevalence can come from many new cases, from long survival without cure, or from both, and a low prevalence from few new cases, rapid death or fast recovery. Freeman and Hutchison (1980) state that in a steady-state population two of prevalence, incidence and duration can be used to obtain the third. According to StatPearls, if the disease lasts long and both incidence and the cure or death rates stay fairly constant (a steady state), prevalence is approximately incidence × disease duration; the approximation holds only while prevalence is low.

Example

In a hypothetical town, 200 of 10,000 residents have diabetes on 1 January, so the point prevalence is 200 / 10,000 = 0.02, or 2%. If 49 of the 9,800 residents without diabetes receive a new diagnosis during the year, the cumulative incidence is 49 / 9,800 = 0.005, or 0.5%.

A point to watch

According to the CDC, prevalence and incidence are frequently confused, and the main difference lies in the numerators: incidence counts only new cases, prevalence all cases present in the period. On the CDC’s account that prevalence depends on incidence and duration, a long-lasting chronic disease can have high prevalence even when incidence is low.

Related terms

Guides and tools

Sources

  1. Principles of Epidemiology in Public Health Practice, Third Edition, Lesson 3, Section 2: Morbidity Frequency Measures — U.S. Centers for Disease Control and Prevention, 2012
  2. Measures of disease frequency: prevalence and incidence — Nephron Clinical Practice (Karger), 2010
  3. Prevalence, incidence and duration — American Journal of Epidemiology (Oxford University Press), 1980
  4. Prevalence — StatPearls Publishing (NCBI Bookshelf), 2023