Question

An investigator initiated a clinic-based cohort study in 1997 to investigate factors associated with HIV seroconversion in a population-based sample of n=489 adults between 35-60 years old. The study was launched on January 1, 1997 and participants were asked to return for follow-up visits on a yearly basis. - at the baseline visit, n=43 participants tested HIV-seropositive among the sample of 489. - at the first follow-up visit, beginning in January 1, 1998, among those who were HIV-seronegative at baseline, n=36 individuals were found to have seroconverted and tested positive for HIV. - at the second follow-up visit, beginning in January 1, 1999, n=24 among those who were HIV-seronegative (at baseline and the first follow-up visit) were found to have seroconverted and tested positive for HIV. - Over the three-year study period, no participants dropped out of the study. Based on this information, answer the following questions.

What is the population at risk for HIV seroconversion after the first follow-up visit (i.e. at the end of 1998)? What is the incidence of HIV after the second follow-up visit? What is prevalence of HIV after the second follow-up visit (i.e. at the end of 1999)?

Answer 1

The population at risk for HIV seroconversion after the first follow-up visit (i.e. at the end of 1998) is 36 / (489-43)*100 = 8.07%

Prevalence of HIV after the second follow-up visit (i.e. at the end of 1999) is 24 / (489-43-36) *100 = 5.85%

As we know that  prevalence = incidence × duration so here prevalence = incidence as duration is 1 year.