Incidence and Prevalence
Measuring disease frequency happens in various means. The most commonly used measures in epidemiology are prevalence and incidence. Incidence refers to the occurrence of a new disease. Prevalence provides a measure for existing disease. The two measures are important providing measures for natural disease. Incidence focuses on the transition of disease from health to disease. Prevalence deals with the period that an individual lives with the disease.
Incidence
Incidence refers to the occurrence of a new case of a disease that builds up in a candidate population over a specified time. Three ideas arise from this definition. Incidence measures new disease events. In cases of diseases that occur more than once, incidence provides a measure of the first occurrence of the disease. Candidate population provides a measure of the new cases. The candidate population is the population of people who are at risk of contracting the disease. The person is at risk because he or she has the appropriate body organ that has no immunity. A good example is a child who has not received immunization against the virus is at risk of getting the disease. This child is a candidate for the disease. It is possible to determine to measure and define the incidence of disease in a population that faces no risk of getting the disease. Incidences take into consideration the specific amount of time that members of the population receive following until they develop the disease. Time must pass for a person to transition from a healthy state to a state of disease. The changes must occur in a person. There are two types of incidence measures. They include the incidence rate and cumulative incidence. They are closely related, but each of the measures has its own strengths and weaknesses. Their use is on different settings. Cumulative incidence refers to the proportion of the candidate population that experiences a disease of a specified period. Its expression mathematically is achieved by dividing the number of new cases with a number in the candidate population over a specified time. Cumulative incidence may be taken as an average risk of contracting a disease over a specified period. It refers to the probability of getting the disease. The most commonly cited measure of cumulative incidence is the risk of breast cancer in women. It is estimated that every one woman among eight is at a risk of getting cancer. It means that around 14% of women will develop cancer at some time in their lives. The length of time that applies influences cumulative incidence. A cumulative incidence over a long time is greater in comparison to cumulative incidence over a short time. Cumulative incidence is commonly used in fixed populations that have no losses that require follow up (Aschengrau & Seage, 2014). It means that every member in the population is covered and followed in the cumulative incidence.
Incidence rate refers to the occurrence of a new case of a disease that arises during the individual time of observation. Obtaining the incidence rate happens by dividing the number of new cases of the disease by a person-time of observation in a candidate population. The person time is accrued among the candidates for the disease. The incidence rate can work in both fixed and dynamic population. In all the populations, it takes into account the changes that occur in the population. The changes include birth, migration and death. The changes are very useful in providing transition between the health state and disease state in a dynamic population.
Prevalence refers to the proportion of the total population that experiences a disease. Two types of prevalence exist. They include the period prevalence and point prevalence. Point prevalence is the proportion of the population that has a disease at a specific period. They can be taken as a snapshot of the population at a specific time (Friis & Sellers, 2014). The point can refer to a particular date such as January 1, 2014 or a point in a person’s life like during the wedding of a person. Period prevalence refers to the proportion of the population that has a disease over duration of time such as the year 2013. Period prevalence provides the number of cases that existed at the start of the period and the numbers that developed during the specified period. Prevalence depends on the rate of development of new cases as well as the length of time that the individuals suffer from the disease. The duration of the disease begins at the time of diagnosis and ends when a person receives cure of dies from the disease. There is a relationship between incidence and prevalence and it is as follows:
P/ (1-P) =IR x D
Where P refers to the prevalence, which is the proportion of the total population that has the disease
(1-P) refers to the proportion of the population that has no disease
IR refers to the incidence rate
D refers to the average duration that a person has the disease.
The above equation provides a relationship for fixed population and a duration that is also fixed (Rothman et al., 2008).
Incidence and prevalence measures have uses in epidemiology. Incidence is useful in providing evaluation of the effectiveness of programs aimed at preventing diseases from occurring. Researchers like studying new cases of diseases, which refer to the incidence than studying the existing cases, which is the prevalence. Prevalence is very important in providing estimates of medical facilities and allocation of resources used for treating people that already have the disease.
References
Aschengrau, A., & Seage, G. R. (2014). Essentials of epidemiology in public health.
Friis, R. H., & Sellers, T. A. (2014). Epidemiology for public health practice. Burlington, Mass: Jones & Bartlett Learning.
Rothman, K. J., Greenland, S., & Lash, T. L. (2008). Modern epidemiology. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
