Review Question - QID 214978

The drug's overall effect on disease prevalence is indeterminate because it both increases the cure rate (which decreases the proportion of the population with the infection) and decreases the infection fatality rate (which increases the proportion of the population with the infection). Since the number of new cases per day is constant while the at-risk population declines each day (due to new infections), disease incidence increases.

Prevalence and incidence are measures of disease frequency in a population. Prevalence is the proportion of the population with the disease at a specific point in time (i.e., proportion of the population with NoV today). Prevalence is a function of disease incidence, mortality, cure rate, and detection ability. For example, diseases with low mortality and/or cure rates can increase in prevalence over time in spite of a low incidence. Incidence is defined as the number of new cases in a population divided by the number of at-risk, or susceptible, people in that population over a specified time period (e.g., 100 cases of NoV infection per 10,000 people per day). The size of the at-risk population is a function of infection and vaccination rates (i.e., those who are already infected or vaccinated are no longer at risk for infection).

Illustration A depicts a scheme of NoV transmission in this population. The relative magnitudes of x (new cases per day), y (cured cases per day), and z (deaths from infection per day) determine the prevalence of NoV. NoV incidence is a function of both x and the size of the uninfected (at-risk) population. When x > y + z, prevalence increases; when x < y + z, prevalence decreases.

Incorrect Answers:
Answer 1: Prevalence decreases and incidence decreases could be observed if x < y + z and the number of new cases per day (x) declines. In other words, this occurs if the drug’s effect on the cure rate is greater than its effect on the infection fatality rate, and the net effect results in more people leaving the infected state (y + z) than entering (x).

Answer 2: Prevalence decreases and incidence increases could be observed if the drug’s effect on the cure rate is greater than its effect on the infection fatality rate, and the net effect results in more people leaving the infected state (y + z) than entering (x). As long as the number of new cases per day (x) remains constant, the at-risk population will decrease, resulting in increasing incidence.

Answer 3: Prevalence increases and incidence increases could be observed if the drug’s effect on infection fatality rate (i.e., the reduction in z) is greater than its effect on the cure rate, and the net effect results in more people remaining in the infected state (due to new infections and reduced infection fatality) than leaving (i.e., x > y + z). As long as the number of new cases per day (x) remains constant, the at-risk population will decrease, resulting in increasing incidence.

Answer 5: Prevalence indeterminate and incidence unchanged could be observed if the number of new cases (x) decreases in proportion with the at-risk population, or if both x and the size of the at-risk population remain constant over time (e.g., migration of uninfected individuals into the population).

Bullet Summary:
Prevalence is the proportion of a population with the disease at a specific point in time (total number of cases/total number of people in the population) whereas incidence is the proportion of an at-risk population that newly develops the disease over a specified time period (number of new cases in the population/number of at-risk individuals per unit time).