Incidence

In epidemiology, incidence refers to new cases of disease or new occurrences of medical conditions such as becoming infected with a virus; it is contrasted with prevalence, which includes both new and existing cases or occurrences. While historically the term incidence was limited to disease or death, it is increasingly being used more broadly to quantify the occurrence of events, not just a new disease. Examples of incident cases or events include when a person develops diabetes, becomes infected with HIV, starts [Page 529]smoking, or is admitted to the hospital. In each of these situations, individuals go from an occurrencefree state to having the occurrence.

It is important for epidemiologists to make a clear distinction between incidence and prevalence. While incidence refers to a new disease or event, prevalence means an existing disease or events. The following example will help refine the distinction between incidence and prevalence. A person who is newly diagnosed with diabetes has an incident case of diabetes; a person who has had diabetes for 10 years has a prevalent case of diabetes. Prevalence includes incident cases or events as well as existing cases or events. For chronic diseases, a person can have an incident case just once in his or her lifetime. For diseases and occurrences that can be fully resolved, a person can have multiple incident cases of a disease (e.g., common cold).

The study of incident cases informs us about the etiology (or cause) of a disease and its outcome. In research, the study of incident cases allows the epidemiologist to determine the risk factors for a disease or another event. The study of prevalent cases combines the study of new and surviving cases, making it unclear as to if risk factors are causes of a new disease or causes of survival on getting a disease.

To compute incidence, three elements must be defined: (1) the number of new cases of disease or occurrence, (2) a population at risk, and (3) the passage of time. Incidence can be measured as a proportion or a rate. Measured as a proportion, incidence quantifies the risk of an occurrence. Measured as a rate, incidence quantifies the speed of a disease or occurrence in a population.

Incidence proportion measures the probability that a person will develop a disease or condition within a given period of time. Accurate measurement of incidence proportion requires that all the individuals ‘at risk’ for the outcome under study be followed during the entire study period (or until getting the disease or event). Because complete follow-up is required to directly compute incidence proportion, it is usually only calculated for studies with a short follow-up period. For incidence proportion, the numerator is the number of new cases of a disease during a given time period. The denominator is the total population at risk during the defined study period.

• Example: On a recent 7-day cruise, 84 of 2,318 passengers reported to the ship's infirmary with gastrointestinal illness. The incidence of disease on this ship equals 84 new cases of illness divided by 2,318 total passengers at risk, resulting in an incidence proportion of 4 % during a 7-day period.

The incidence rate numerator is likewise the number of new cases. The denominator, however, is the total person-time of observation at risk for the disease or occurrence.

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