Abstract
HIV prevalence has recently begun to decline in Zimbabwe, a result of both high levels of AIDS mortality and a reduction in incident infections. An important component in understanding the dynamics in HIV prevalence is knowledge of past trends in incidence, such as when incidence peaked and at what level. However, empirical measurements of incidence over an extended time period are not available from Zimbabwe or elsewhere in sub-Saharan Africa. Using mortality data, we use a back-calculation technique to reconstruct historic trends in incidence. From AIDS mortality data, extracted from death registration in Harare, together with an estimate of survival post-infection, HIV incidence trends were reconstructed that would give rise to the observed patterns of AIDS mortality. Models were fitted assuming three parametric forms of the incidence curve and under nine different assumptions regarding combinations of trends in non-AIDS mortality and patterns of survival post-infection with HIV. HIV prevalence was forward-projected from the fitted incidence and mortality curves. Models that constrained the incidence pattern to a cubic spline function were flexible and produced well-fitting, realistic patterns of incidence. In models assuming constant levels of non-AIDS mortality, annual incidence peaked between 4 and 5% between 1988 and 1990. Under other assumptions the peak level ranged from 3 to 8% per annum. However, scenarios assuming increasing levels of non-AIDS mortality resulted in implausibly low estimates of peak prevalence (11%), whereas models with decreasing underlying crude mortality could be consistent with the prevalence and mortality data. HIV incidence is most likely to have peaked in Harare between 1988 and 1990, which may have preceded the peak elsewhere in Zimbabwe. This finding, considered alongside the timing and location of HIV prevention activities, will give insight into the decline of HIV prevalence in Zimbabwe.
Highlights
The recently observed decline in HIV prevalence in Zimbabwe is a result of both high levels of AIDS mortality combined with a reduction in incident infections.[1,2] An important component in understanding these dynamics in HIV prevalence is knowledge of the historical trend in HIV incidence, including at which level and year incidence peaked
Knowledge of incidence trends combined with data on sexual behaviour change and prevention program implementation will aid in understanding the decline in HIV prevalence
The timing and level of the peak will be related to dynamics of spread which may be mediated by behaviour change such as reductions in casual partnerships or increased condom use.[4]
Summary
The recently observed decline in HIV prevalence in Zimbabwe is a result of both high levels of AIDS mortality combined with a reduction in incident infections.[1,2] An important component in understanding these dynamics in HIV prevalence is knowledge of the historical trend in HIV incidence, including at which level and year incidence peaked. Knowledge of incidence trends combined with data on sexual behaviour change and prevention program implementation will aid in understanding the decline in HIV prevalence. For Zimbabwe, most incidence estimates that do exist are not from population-based studies, so may be biased and/or are recent, and likely to be well after the peak in infections occurred.[5,6,7] In the absence of an extended time-series of empirical measurements, other approaches are needed to estimate incidence. 0.000626 0.000491 0.001011 0.000370 0.000306 0.000541 0.000313 0.000363 0.000229 0.009933 0.009917 0.014146 0.015465 0.013988 0.020026 0.008181 0.009149 0.013801 0.000640 0.000592 0.000511 0.000307 0.000237 0.000181 0.000724 0.000295 0.000189 doi:10.1371/journal.pone.0001711.t001
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