# Relationship between incidence and prevalence in epidemiology

### Measures of disease frequency and disease burden | Health Knowledge

This course covers basic epidemiology principles, concepts, and procedures useful in who become ill (incidence) or are ill at a given time (prevalence). Incidence refers to the occurrence of new cases of disease or injury in a document the difference between community transmission of illness versus. Calculation of standardized incidence rate · PAMCOMP Programme for calculating standardized incidence rates (SIRs). e) The relationships between incidence and prevalence The essence of epidemiology is to measure disease occurrence and make comparisons between .

At 5 yrs you found 20 new cases, so you assume they developed HIV at 2.

## Measures of disease frequency and disease burden

At 10 years you found 30 new cases. These people did not have HIV at 5 years, but did at 10, so you assume they were infected at 7.

Now take the 50 new cases of HIV, and divide by to get 0. In other words, if you were to follow people for one year, you would see 28 new cases of HIV.

This is a much more accurate measure of risk than prevalence.

Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence indicates how widespread the disease is. Prevalence is the proportion of the total number of cases to the total population and is more a measure of the burden of the disease on society with no regard to time at risk or when subjects may have been exposed to a possible risk factor. Prevalence can also be measured with respect to a specific subgroup of a population see: Incidence is usually more useful than prevalence in understanding the disease etiology: For example, consider a disease that takes a long time to cure and was widespread in but dissipated in Incidence reflects the rate at which new cases of disease are being added to the population and becoming prevalent cases.

### Incidence (epidemiology) - Wikipedia

Average duration of disease is also important, because the only way you can stop being a prevalent case is to be cured or to move out of the population or die. For example, about a decade ago the average duration of lung cancer was about six months.

Therapy was ineffective and almost all lung cancer cases died. From the time of diagnosis, the average survival was only about six months. So, the prevalence of lung cancer was fairly low.

### Epidemiology, Prevalence and Incidence - Physiopedia

In contrast, diabetes has a long average duration, since it can't be cured, but it can be controlled with medications, so the average duration of diabetes is long, and the prevalence is fairly high. If the population is initially in a "steady state," meaning that prevalence is fairly constant and incidence and outflow [cure and death] are about equalthen the relationship among these three parameters can be described mathematically as: Duration is the average time that people have the disease from diagnosis until they are either cured or die.

If the frequency of disease is rare i. Similarly, if the incidence remained constant, then developing a cure would reduce the average duration of disease, and this would also reduce the prevalence of disease.

In the late s anti-retroviral therapy was introduced and greatly improved the survival of people with HIV. However, they weren't cured of their disease, meaning that the average duration of disease increased. As a result, the prevalence of HIV increased during this period.

The relationship can be visualized by thinking of inflow and outflow from a reservoir. The fullness of the reservoir can be thought of as analogous to prevalence. Raindrops might represent incidence or the rate at which new cases of a disease are being added to the population, thus becoming prevalent cases.