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The prevalence and incidence rates are calculated as follows:

Prevalence = number of individuals who are infected with HIV at a specific time / number of individuals in the population at that point in time

Incidence rates = number of new cases of HIV during a certain time period / number of individuals who are infected with HIV at a specific time

Prevalence reflects the situation at one point in time, while incidence measure the rate at which the infection is spreading. The two measures are complementary ans together provide an understanding of the dynamic changes in an epidemic.