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Estimating incidence from prevalence in generalised HIV epidemics: Methods and validation

Hallett TB, Zaba B, Todd J, Lopman B, Mwita W, Biraro S, Gregson S, Boerma T.

PLoS Med 5(4): e80.

Background: HIV surveillance primarily relies on prevalence at antenatal clinics but estimates of incidence in the general population would be more useful. Repeated cross-sectional measures of HIV prevalence are now becoming available for general populations in many countries and we aim to develop and validate methods that use these data to estimate HIV incidence.
Methods: Two methods were developed that decompose observed changes in prevalence between two sero-surveys into the contributions of new infections and mortality. Method 1 uses cohort mortality data and Method 2 uses information on survival after infection. Their performance was assessed using simulated data and actual data from three community-based cohort studies in Africa.
Results: Simulation studies show that these methods can work in a variety of epidemic conditions and accurately detect changes in incidence. Method 1 is simple to implement but assumes a selected standard age-schedule of cohort HIV mortality rates is locally appropriate. Method 2 assumes that a standard distribution of age-specific survival post infection times is universally applicable. The estimates from both methods are within the 95% confidence intervals of all actual measurements of HIV incidence for the broad age group 15-44. The patterns of incidence over age are also captured correctly although there were greater disparities at older ages that may be partly due to the random errors in the measurements themselves.
Conclusions: It is possible to estimate incidence from cross-sectional prevalence data with sufficient accuracy to monitor the epidemic. The choice of method will depend on the local availability of HIV mortality data.

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