Question

explain the relationship between the incidence and prevalence while highlighting the importance of each.

Answer 1

Prevalence is the proportion of a population that has a condition at a specific time, but the prevalence will be influenced by both the rate at which new cases are occurring and the average duration of the disease. Incidence reflects the rate at which new cases of disease are being added to the population (and becoming prevalent cases). 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. 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 equal), then the relationship among these three parameters can be described mathematically as:

P/(1-P) = IR x Avg. Duration,

where P= proportion of the population with the disease and (1-P) is the proportion without it, IR is the incidence rate, and Avg. 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.e., <10% of the population has it), then the relationship can be expressed as follow:

Prevalence = (Incidence Rate) x (Average Duration of Disease)

• If the average duration of disease remains constant, then preventive measures that reduce the incidence of disease would be expected to result in a decreased prevalence.
• 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.