# Incidence
> Disease incidence is the occurrence of new cases in a defined population over a specified period. It can be measured as cumulative incidence or as a rate based on person-time at risk.
Source: https://www.visfo.health/glossary/incidence
Updated: 2026-08-16T12:10:28.730366+00:00

What is disease incidence?

Disease incidence is the occurrence of new cases of a disease in a defined population over a specified period. It describes how quickly people who are initially at risk develop the disease, rather than how many people are living with it at a particular time.

An incidence estimate must therefore define the disease or event, the population at risk, the geographical setting and the observation period. People who already have the disease at the start are normally excluded when the first occurrence of disease is being measured.

Why does incidence matter in market access?

Incidence helps [market access](/glossary/market-access) teams estimate how many patients may become newly eligible for diagnosis, treatment or another intervention. This informs population forecasts, budget impact models, service planning and the assessment of unmet need.

It can also affect evidence submitted for [health technology assessment](/glossary/health-technology-assessment-hta). An incidence assumption that is poorly matched to the treatment indication, geography or time horizon can distort estimates of eligible patients and expected expenditure. Medical affairs teams may use the same evidence to describe disease burden, identify evidence gaps and plan scientific communication.

How is incidence measured?

Incidence can be expressed in two closely related ways:

- Cumulative incidence is the proportion of an initially disease-free population that develops the disease during a defined period. The numerator is the number of new cases, and the denominator is the number of people at risk at the start.
- Incidence rate, also called incidence density, divides new cases by the total person-time contributed by people while they remain at risk. This is useful when follow-up time differs between individuals or the population changes during observation.

The case definition, start and end dates, eligibility criteria and handling of repeat events should be set before calculation. Results may be presented as crude incidence or stratified by characteristics such as age, sex, disease subtype, risk group or region. Standardisation may be needed when populations have different demographic structures.

Where do incidence estimates come from?

Common sources include disease registries, surveillance systems, electronic health records, claims data, cohort studies and administrative datasets. The appropriate source depends on whether cases can be identified consistently and whether the population at risk and observation time can be measured.

Recorded incidence may differ from the underlying occurrence of disease. Diagnostic access, screening, coding practices, reporting requirements and changes in the clinical definition can all alter the number of detected cases. Teams should check whether the source captures first diagnoses, recurrent events or both, and whether patients can move into or out of the dataset.

What makes an incidence estimate suitable for decision-making?

A useful estimate is aligned with the decision being made. The disease definition should match the indication, the population should reflect the relevant setting, and the observation period should be recent enough to represent current diagnostic and treatment practice.

Teams should report the numerator, denominator, unit of time, data source and method used to identify new cases. Important limitations include incomplete case capture, short follow-up, duplicate records, changes in testing and failure to exclude existing cases. Sensitivity analyses can show how alternative credible incidence assumptions affect forecasts or economic models.

How does incidence differ from prevalence and related measures?

Incidence counts new cases arising over time, whereas [prevalence](/glossary/prevalence) measures the proportion of a population living with a disease at a point in time or during a period. A disease can have low incidence but high prevalence if people live with it for a long time; it can have high incidence but lower prevalence if its duration is short.

Incidence should also be kept separate from mortality, which measures deaths, and from diagnosis volume, which may include existing or repeat cases. In disease-burden work, incidence shows the flow of new cases, prevalence shows the existing patient pool, and mortality and remission help explain how that pool changes.