Incidence rates measure new cases over a period; prevalence measures existing cases at a given time. This statement describes their relationship. Which option is correct?

• A. Incidence rates measure new cases over a period; prevalence measures existing cases at a given time
• B. Incidence rates measure prevalence; prevalence rates measure incidence
• C. They are the same concept
• D. Incidence rates measure existing cases; prevalence measures new cases

Answer: (A)

Incidence vs prevalence hinge on time: incidence rates count new cases that develop during a defined period, usually as a rate per person-time, while prevalence measures the total number of people who have the condition at a specific point in time (point prevalence) or over a period (period prevalence). The statement describes this distinction by saying incidence captures new cases over time and prevalence captures existing cases at a given moment. This difference explains why they serve different public health purposes: incidence indicates risk and the impact of prevention, whereas prevalence shows the burden of disease at a particular time. Prevalence increases when the disease lasts longer or incidence stays high, and decreases when people recover or die quickly.