Incidence

Incidence is the frequency with which new cases of a disease or condition arise in a population over a period of time. Unlike prevalence, which counts all existing cases, incidence counts only new events among people who are at risk, making it the fundamental measure of how quickly disease occurs and the basis for estimating risk.

Definition

Incidence is the occurrence of new cases of a disease or condition in a defined at-risk population over a specified period, expressed either as a proportion of the population developing the condition (cumulative incidence) or as a rate per unit of person-time (incidence density).

Scope

This entry covers incidence as a measure of new disease occurrence: the requirement of a population at risk followed over time, the count of new cases as the numerator, and the two principal forms incidence takes — a proportion (cumulative incidence) and a rate (incidence density). It is methodological and does not provide clinical guidance.

Key concepts

• New cases (numerator)
• Population at risk
• Follow-up over time
• Cumulative incidence (proportion)
• Incidence density (rate)
• Distinction from prevalence

Mechanisms

Incidence is measured by following a population that is initially free of the condition and counting those who newly develop it during a defined period. Only people who are at risk — that is, who could still develop the condition — belong in the denominator, and individuals already affected are excluded. Because it captures the transition from non-diseased to diseased status, incidence reflects the force with which a disease arises, independent of how long affected people survive with it. This is what makes incidence, not prevalence, the appropriate frequency measure for studying causes and estimating risk: it ties new events to the population and time from which they arose.

Clinical relevance

Incidence quantifies how rapidly a condition develops in a population and is the frequency measure from which risk ratios and rate ratios are constructed, so it is central to appraising evidence about causes and prevention. It describes population-level occurrence and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Measuring incidence requires follow-up of an at-risk population over time, which is why it is the natural output of cohort studies and surveillance systems rather than one-time surveys. The need to track who is at risk, and for how long, distinguishes incidence from the more easily measured prevalence and underlies the two ways of expressing it as a proportion or a rate.

History

The conceptual separation of new-case occurrence from existing-case burden — incidence from prevalence — was consolidated in twentieth-century epidemiology, alongside the formal definitions of the at-risk denominator and person-time accounting. Standard textbooks and dictionaries of the field codified the proportion and rate forms of incidence that are in routine use today.

Seminal works

• grimes-cohort-2002
• rothman-2008
• porta-2014

Frequently asked questions

How does incidence differ from prevalence?

Incidence counts only new cases arising over a period among people at risk, whereas prevalence counts all existing cases at a point or interval. Incidence measures how fast disease occurs; prevalence measures how much disease is present.

Why must the denominator be a population at risk?

People who already have the condition, or who cannot develop it, cannot become new cases, so including them would dilute the measure. Restricting the denominator to those at risk keeps incidence a valid estimate of how often new disease arises.