Trends in HIV-1 prevalence may not reflect trends in incidence in mature epidemics: data from the Rakai population-based cohort, Uganda

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Objectives:To assess whether trends in serial HIV-1 prevalence reflect trends in HIV incidence, and to decompose the effects of HIV-1 incidence, mortality, mobility and compliance on HIV-1 prevalence in a population-based cohort.Design:Two-year follow up (1990-1992) of an open cohort of all adults aged 15-59 years, resident in a sample of 31 representative community clusters in rural Rakai District, Uganda.Methods:A detailed household enumeration was conducted at baseline and in each subsequent year. All household residents were listed, and all deaths and in- and out-migrations that occurred in the intersurvey year were recorded. In each year, all consenting adults were interviewed and provided a serological sample; 2591 adults aged 15-59 years were enrolled at baseline.Results:HIV prevalence among adults declined significantly between 1990 and 1992 (23.4% at baseline, 21.8% in 1991, 20.9% in 1992; P < 0.05). Declining prevalence was also observed in subgroups, including young adults aged 15-24 years (from 20.6 to 16.2% over 3 years; P < 0.02), women of reproductive age (from 27.1 to 23.5%; P < 0.05), and pregnant women (from 25.4 to 20.0%; not significant). However, HIV incidence did not change significantly among all adults aged 15-59 years [2.1 ± 0.4 per 100 person-years of observation (PYO) in 1990-1991 and 2.0 ± 0.3 per 100 PYO in 1991-1992], nor in population subgroups. HIV-related mortality was high (13.5 per 100 PYO among the HIVpositive), removing more infected persons than were added by seroconversion. Net out-migration also removed substantial numbers of HIV-positive individuals.Conclusions:In this mature HIV epidemic, HIV prevalence declined in the presence of stable and high incidence. HIV-related mortality contributed most to the prevalence decline. Prevalence was not an adequate surrogate measure of incidence, limiting the utility of serial prevalence measures in assessing the dynamics of the HIV epidemic and in evaluating the impact of current preventive strategies.

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