Cumulative Incidence

To define the cumulative incidence, we must first understand what incidence is. Incidence refers to a measure of probability. It measures how probable is the occurrence of a particular medical condition within a given time period. Cumulative incidence definition refers to incidence proportion. So we can define cumulative incidence as the probability that a specific event has already taken place prior to the given time. 

On this page, we will try to understand various aspects of this definition such as cumulative incidence rate, cumulative incidence calculation, cumulative incidence formula, cumulative incidence epidemiology, the cumulative incidence of death or cumulative incidence of mortality, etc. 

Cumulative Incidence Epidemiology

In epidemiology, which studies the pattern, distribution and determinants of disease and health conditions within specified populations, the cumulative incidence is frequently used. It is used for estimating the risks that an individual is likely to be affected by a disease or experience an event within the time period specified. It is used to identify the proportion of a population that was initially disease-free but has developed diseases, gotten injured or died within the specified time period. 

Cumulative incidence takes into account two particular factors: 

• Individuals in the population considered are not dependent on the outcome at the onset of the study period, 

• The individuals possess the potential to develop or exhibit the outcome of interest during the time period when the study is carried out.

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Cumulative Incidence Calculation

Cumulative incidence calculation is done by using the cumulative incidence formula. The cumulative incidence rate/formula is determined by dividing the number of new disease cases or new events by the total no. of individuals within a population who stand at risk for a particular time period. 

For example, if in a population of 1000 individuals originally, 38 people exhibit a condition from the incidence of the disease up to a defined point in time, then the cumulative incidence proportion is 38 cases per 1000 individuals or 3.8%.

The cumulative incidence formula, mathematically, is given as follows: 

IP(t) = 1 -  e -IR(t).D

The cumulative incidence rate formula is often used by researchers to predict the risks associated with an event or disease outbreak over shorter or longer time periods. 

Cumulative Incidence Calculation Numerical

Q. In a population of 1000 infants newly-born, 100 infants had severe birth defects when born and 20 among the100 infants died within the first year of life while 90 of the remaining 900 infants who did not have any birth defects also died within the first year of life. Calculate the total cumulative incidence of mortality in this neonatal population?          

 a.  110/1,000 = 11%

 b.  10/900 =1.1%

 c. 20/1000 = 2%

 d.  90/1000 = 9%

Ans. (a) 

The cumulative incidence of death in this population is 11%. The overall cumulative incidence is equal to the absolute no. of infant deaths (irrespective of whether they had defects or not) divided by the overall number of infants that were born.

Cumulative Incidence Examples

A few examples of cumulative incidence can be listed as under: 

• Senior citizens are at risk of developing influenza despite being vaccinated against the disease. 

• The proportion of passengers on  cruise ship developing gastroenteritis 

• The proportion of patients that develop complications within the first month of having undergone surgery. 

In the case of influenza, the senior citizens in a study are vaccinated at the start of the flu season, before the incidence of any cases of influenza in a particular region. Investigators define the flu season as a time period either by considering particular months (say November to April) or by combining a time period and events observed. 

For example, in the USA, the flu season is defined as the time period from the first case of influenza in the area to the last case in the area within one continuous period of time, usually from September to June. Irrespective of how the study period is considered, it is uniform for all the participants included in the study. All the participants possess the same opportunity to be identified as influenza-infected in the scenario that they develop the disease.

Cumulative incidence can be directly calculated in studies, where a particular group is assessed for a short period of time. On the other hand, studies, where there is a need for longer follow-up periods, such as the risk of developing diabetes mellitus or cohort diet studies, a direct estimation of cumulative incidence, cannot be achieved. This is because, in the case of studies with long follow-up periods such as this, patient follow-up is often lost. In such cases, the incidence rates are computed to address the question at hand and then the cumulative incidence is estimated from the rate. However, the rates in such studies must be constant throughout the duration of the study. If this criterion is not met, then distinct rates are to be calculated for specific time periods and then must be aggregated in order to obtain the best possible cumulative incidence rate estimate. 

It is important to note that while rates characterize the incidence of disease for a particular population, cumulative incidence allows for the characterization of risk accumulated over time. 

Cumulative Incidence Significance

Cumulative incidence is of great help to clinicians and public health professionals from a clinical standpoint. It can allow for the estimation of risks of the development of a condition or contracting a disease over a certain period of time. Health care professionals such as paediatricians can make use of cumulative incidence for predicting the onset of type 2 diabetes over the next ten years or by adolescence, in a child who is overweight. 

While cumulative incidence cannot be computed directly in studies with long follow-up periods due to losses in patient follow-up, it can be estimated in such studies by first calculating the incidence rate and then estimating the cumulative incidence from the rate. In this case, rates should be constant throughout the course of the study, and if they are not, distinct rates must be calculated for discrete-time periods and then aggregated to obtain the best estimate of the cumulative incidence.