Improved plausibility bounds about the 2005 HIV and AIDS estimates.

BACKGROUND: Since 1998 the Joint United Nations Programme on HIV/AIDS and the World Health Organization has provided estimates on the magnitude of the HIV epidemic for individual countries. Starting with the 2003 estimates, plausibility bounds about the estimates were also reported. The bounds are intended to serve as a guide as to what reasonable or plausible ranges are for the uncertainty in HIV incidence, prevalence, and mortality. METHODS: Plausibility bounds were developed for three situations: for countries with generalised epidemics, for countries with low level or concentrated epidemics (LLC), and for regions. The techniques used build on those developed for the previous reporting round. However the current bounds are based on the available surveillance and survey data from each individual country rather than on data from a few prototypical countries. RESULTS: The uncertainty around the HIV estimates depends on the quality of the surveillance system in the country. Countries with population based HIV seroprevalence surveys have the tightest plausibility bounds (average relative range about the adult HIV prevalence (ARR) of -18% to +19%.) Generalised epidemic countries without a survey have the next tightest ranges (average ARR of -46% to +59%). Those LLC countries which have conducted multiple surveys over time for HIV among the populations most at risk have the bounds similar to those in generalised epidemic countries (ARR -40% to +67%). As the number and quality of the studies in LLC countries goes down, the plausibility bounds increase (ARR of -38% to +102% for countries with medium quality data and ARR of -53% to +183% for countries with poor quality data). The plausibility bounds for regions directly reflect the bounds for the countries in those regions. CONCLUSIONS: Although scientific, the plausibility bounds do not represent and should not be interpreted as formal statistical confidence intervals. However in order to make the bounds as meaningful as possible the authors have tried to apply reasonable statistical approaches and assumptions to their derivation. An understanding of the uncertainty in the HIV estimates may help policy makers take better informed decisions to address the epidemic in their respective countries.

Criteria for detecting and understanding changes in the risk of HIV infection at a national level in generalised epidemics.

Identification of causes of changes in prevalence and incidence of HIV at a national level is important for planning future prevention and intervention needs. However, the slow progression to disease and the sensitive and stigmatising nature of the associated behaviours can make this difficult. Changing rates of incidence are to be expected as an epidemic progresses, but separating background changes from those brought about by changes in behaviour and interventions requires careful analysis. This paper discusses the criteria required to determine whether observed changes in HIV prevalence are the result of changes in behaviour.

The UNAIDS Estimation and Projection Package: a software package to estimate and project national HIV epidemics.

This paper describes the Estimation and Projection Package (EPP) for estimating and projecting HIV prevalence levels in countries with generalised epidemics. The paper gives an overall summary of the software and interface. It describes the process of defining and modelling a national epidemic in terms of locally relevant sub-epidemics and the four epidemiological parameters used to fit a curve to produce the prevalence trends in the epidemic. It also provides an example of using the EPP in a country with a generalised epidemic. The paper discusses the strengths and weaknesses of the software and its envisaged future developments.

Uncertainty in estimates of HIV/AIDS: the estimation and application of plausibility bounds.

OBJECTIVES: To establish the accuracy of the country specific estimates of HIV prevalence, incidence, and AIDS mortality published every 2 years by UNAIDS and WHO. METHODS: We review sources of error in the data used to generate national HIV/AIDS and where possible estimate their statistical properties. We use numerical and approximate analytic methods to estimate the combined impact of these errors on HIV/AIDS estimates. Heuristic rules are then derived to produce plausible bounds about these estimates for countries with different types of epidemic and different qualities of surveillance system. RESULTS: Although 95% confidence intervals (CIs) can be estimated for some sources of error, the sizes of other sources of error must be based on expert judgment. We therefore produce plausible bounds about HIV/AIDS estimates rather than statistical CIs. The magnitude of these bounds depends on the stage of the epidemic and the quality and coverage of the sentinel HIV surveillance system. The bounds for adult estimates are narrower than those for children, and those for prevalence are narrower than those for new infections. CONCLUSIONS: This paper presents a first attempt at a rigorous description of the errors associated with estimation of global statistics of an infectious disease. The proposed methods work well in countries with generalised epidemics (>1% adult HIV prevalence) where the quality of surveillance is good. Although methods have also been derived for countries with low level or concentrated epidemics, more data on the biases in the estimation process are required.

Epidemiological analysis of the quality of HIV sero-surveillance in the world: how well do we track the epidemic?

OBJECTIVE: The objective of this paper was to analyse the quality of HIV/AIDS sentinel surveillance systems in countries and the resulting quality of the data used to make estimates of HIV/AIDS prevalence and mortality. METHODS: Available data on sero-surveillance of HIV/AIDS in countries were compiled in the process of making the end of 1999 estimates of HIV/AIDS. These data came primarily from the HIV/AIDS Surveillance Database developed by the United States Census Bureau, from a database maintained by the European Centre for the Epidemiological Monitoring of AIDS and all country reports on sentinel surveillance that had been provided to World Health Organization or UNAIDS. Procedures were developed to score quality of surveillance systems based on four dimensions of quality: timeliness and frequency; appropriateness of groups; consistency of sites over time; and coverage provided by the system. In total, the surveillance systems from 167 countries were analysed. RESULTS: Forty-seven of the 167 countries whose surveillance systems were rated were judged to have fully implemented sentinel surveillance systems; 51 were judged to have systems that had some or most aspects of a good HIV surveillance system in place and 69 were rated as having poorly functioning or non-existent surveillance systems. CONCLUSION: This analysis suggests that the quality of HIV surveillance varies considerably. The majority of countries most affected by HIV/AIDS have systems that are providing sufficient sero-prevalence data for tracking the epidemic and making reasonable estimates of HIV prevalence. However, many countries have poor systems and strengthening these is an urgent priority.

Country-specific estimates and models of HIV and AIDS: methods and limitations.

OBJECTIVE: This paper presents the methods used to calculate the end of 1997 country-specific estimates of HIV and AIDS produced by the UNAIDS/WHO Working Group on Global HIV/AIDS and STD Surveillance. The objective of this exercise was to improve estimates on HIV/AIDS by using country-specific models of HIV/AIDS epidemics. The paper describes and discusses the processes and obstacles that were encountered in this multi-partner collaboration including national and international experts. METHODS: The 1997 estimates required two basic steps. First, point prevalence estimates for 1994 and 1997 were carried out and the starting year of the epidemic was determined for each country. The procedures used to calculate the estimates of prevalence differed according to the assumed type of the epidemic and the available data. The second step involved using these estimates of prevalence over time and the starting date of the epidemic to determine the epidemic curve that best described the spread of HIV in each particular country. A simple epidemiological program (EPIMODEL) was used for the calculation of estimates on incidence and mortality from this epidemic curve. RESULTS: Regional models that were used in previous estimation exercises were not able to capture the diversity of HIV epidemics between countries and regions. The result of this first country-specific estimation process yielded higher estimates of HIV infection than previously thought likely, with over 30 million people estimated to be living with HIV/AIDS. The application of survival times that are specific to countries and regions also resulted in higher estimates of mortality, which more accurately describe the impact of the epidemics. At the end of 1997, it was estimated that 11.7 million people worldwide had died as a result of HIV/AIDS since the beginning of the epidemic. CONCLUSION: This exercise is an important step in improving understanding of the spread of HIV in different parts of the world. There are, however, shortcomings in the current systems of monitoring the epidemic. Improvements in HIV surveillance systems are needed in many parts of the world. In addition, further research is needed to understand fully the effects of the fertility reduction as a result of HIV, differing sex ratios in HIV infection and other factors influencing the course and measurement of the epidemic.