Since 1997, The Joint United Nations Programme on AIDS (UNAIDS) and the World Health Organisation (WHO) have provided estimates for the number of people living with HIV in different countries every two years.  As new methods are developed for calculating existing and future trends in the disease, figures can be startlingly different.  How can accurate decisions be made on health spending if policy-makers are unclear about how many people are HIV positive?

Researchers review the process, methods and procedures that have been used in the past and current round of estimates of HIV/AIDS burden. There are several major differences between previous estimates and those made in 2004. First, in the past, a single percentage figure, such as 16.5 percent, was given as an estimate of the number of people globally or nationally with HIV. This presented a false sense of accuracy.  However, now a range is given, for example, between 13.5 percent and 20.1 percent of a population is HIV positive to represent the uncertainty about the estimates.

Second, individual countries have become more involved in preparing the estimates and the use of local knowledge has meant that the figures have become more accurate.  The countries no longer just comment on figures produced international organisations. They receive training in estimation methods and are supplied with specialist computer software for the work. Rather than just plotting a single disease trend for a whole country, government agencies now often calculate the trends for different regions within a country.

Third, since the 2002 round of estimates, new data from nationally representative household surveys have become in available in many countries. These data have been used to refine the assumptions used in making estimates and have been the basis of calibrating data from sentinel surveillance sites.

The study found that the following methods can reduce levels of uncertainty when calculating how many people are HIV positive in a country:

• Other sources can be used to verify the data, for example, national censuses could be used to look at the changes in mortality over time.
• Data sources are being checked.  In Zambia several urban clinics were categorised as rural clinics by mistake.  HIV is often higher in cities and the estimate for the whole of Zambia was brought down once the mistake was discovered.
• It has become acceptable to undertake HIV testing during household surveys of other kinds of data.  This gives a more accurate reading of the whole population than the usual method of testing pregnant women who attend antenatal clinic.

In Kenya, the national estimate was adjusted considerably following a household survey because it emerged that twice as many women as men were infected with the disease.  Improving ways of collecting data and analysing future trends means that HIV figures can change quite dramatically from one year to the next.  Such a change can be confusing.  It is not that the epidemic has taken a down-turn but that the processing of information has been refined.  Medical bodies and government agencies have to be open to new methods to make their predictions as accurate as possible.  The study suggests that:

• It would help to include several years using the new method to give people a better understanding of the information provided. 
• The use of a range of possibilities rather than a single figure is a more appropriate way of providing the information. 
• The information is accurate enough to make sound policy decisions in health spending, particularly if estimates are made for different social groups, such as sex worker, and for different parts of a country.

All of these suggestions were implemented in the 2004 round of estimates released by UNAIDS and WHO.

Source(s):
'Estimating the global burden of HIV/AIDS:  what do we really know about the HIV pandemic?', The Lancet 363, by N. Walker, N. Grassly, G.P. Garnett, KA. Stanecki and P.D.Ghys, 2004
'Evidence and causes of declines in HIV prevalence and incidence in countries with generalised epidemics', Report of a meeting of the UNAIDS Reference Group, November 2004
'Report on the global AIDS epidemic', UNAIDS, 2004

Funded by: UNAIDS; The Royal Society, UK; UK Medical Research Council

id21 Research Highlight: 31 March 2005

Further Information:
Neff Walker
UNICEF
3 UN Plaza
New York
New York 10017
USA

Contact the contributor: nwalker@unicef.org

UNAIDS, Reference Group on Estimates, Modelling and Projections

Other related links:
UNAIDS, Reference Group on Estimates, Modelling and Projections

'Development of the software packages, EPP v2 and Spectrum, and measuring and tracking the epidemic in countries where HIV is concentrated among populations at high risk of HIV', Report of a meeting of the UNAIDS Reference Group, December 2004

'Responding to surveillance: methods and software to produce HIV/AIDS estimates in the era of population-based prevalence surveys', Report of a meeting of the UNAIDS Reference Group, May 2004

UNAIDS Estimation and Projection Package (EPP) software

'Predicting health care needs and costs at HIV/AIDS clinics in India'

'A matter of time - predicting the HIV/AIDS epidemic in Ukraine'

'Stronger health systems for more effective HIV/AIDS prevention and care'