Demographic approaches to the estimation of incidence of HIV-1 infection among adults from age-specific prevalence data in stable endemic conditions

Abstract

Objective: To develop methods for estimating the incidence of HIV-1 infection among adults from age-specific prevalence data derived in stable endemic conditions. Methods: Two methods are proposed. The first method is the Cumulative Incidence and Survival Method which treats HIV-1 prevalence at any given age as the cumulative incidence of new infections at each preceding age, adjusted for mortality. A model for age-specific incidence is fitted to the data using maximum likelihood techniques. The other method is the Constant Prevalence Method whereby the incidence of new infections within a time interval (t- r, t) is calculated as the difference, after adjusting for mortality, between observed prevalence levels at two successive age intervals, whose mean ages are r years apart. The two methods were applied to data from Kampala, Uganda. Results: Plausible estimates of age-specific and cumulative HIV-1 incidence were obtained from each of the methods. Estimates of HIV-1 incidence are sensitive to assumptions regarding the length of the survival period after infection and the stability of the epidemic. Conclusions: Reasonable estimates of HIV-1 incidence can be obtained from prevalence data derived in near-stable conditions. With the Constant Prevalence Method, these conditions may be relaxed if large sample sizes are available and age-reporting is good. The methods proposed could be used in the design and implementation of HIV-1 prevention trials. Cumulative incidence is a better indication of demographic impact than average age-specific incidence.