New HIV prevalence figures released on Tuesday suggest the global AIDS epidemic may be waning in many countries, but that UNAIDS also overestimated the number of people living with HIV in its earlier reports.

The 2007 AIDS epidemic update, jointly published by UNAIDS and the World Health Organisation (WHO), puts the number of people living with the virus at 33.2 million, a significant decrease from the 2006 estimate of 39.5 million. However, applying an "improved methodology", UNAIDS has also revised the 2006 figure to 32.7 million.

The overall percentage of people living with HIV appears to have levelled off, but because infected people are generally living longer, the global number continues to rise.

Based on "improvements in country data collection and analysis, as well as a better understanding of the natural history and distribution of HIV infection", the new report puts the annual number of new infections at 2.5 million, down from a peak of over 3 million a year in the late 1990s.

Critics have long accused UN officials of exaggerating the scale of the epidemic in order to secure more funding to combat the disease, but UNAIDS attributes the revisions mainly to the development of more sophisticated methodology for calculating HIV estimates, and more accurate and representative surveys for tracking infections.

Previously, epidemiologists relied mainly on data from antenatal clinics to calculate HIV prevalence rates, but the age groups of the women tested, and the fact they were clearly having unprotected sex, meant these numbers tended to overestimate HIV infections in the general population.

Many countries are now conducting more representative population-based household surveys, with the result that in India, for example, the estimated number of HIV infections has come down by more than half.

"The data for measuring the HIV epidemic ... has considerably expanded and improved in recent years," commented Ron Brookmeyer, Professor of Biostatistics at the US-based Johns Hopkins Bloomberg School of Public Health. "Nevertheless, there is a need to further improve the representativeness of the underlying data."

A better understanding of the natural dynamics of an epidemic has also helped to account for the lower estimates - HIV prevalence tends to reduce over time as the number of new infections in a population reaches a "saturation" point and deaths resulting from AIDS increase.

The report attributes at least some of the drop in prevalence to HIV-prevention efforts. Infections among young pregnant women attending antenatal clinics have declined over the last five years in 11 of the 15 most affected countries, and studies looking at risk behaviour among young people also show encouraging results in countries such as Zimbabwe, Kenya, Botswana, Malawi and Zambia.

"Prevalence is dropping in a lot of countries," noted Prof John Hargrove, director of the Centre of Excellence in Epidemiological Modelling and Analysis (SACEMA) at the University of Stellenbosch, South Africa. "There's absolutely no question, for example, that HIV prevalence in Zimbabwe has been dropping like a stone in recent years, but there is some resistance from people in the First World to accepting that."

UNAIDS officials were at pains to point out that the global AIDS epidemic still represented a public health crisis of devastating proportions, especially in the eight southern African countries where national adult HIV prevalence exceeded 15 percent.

"Unquestionably, we are beginning to see a return on investment," said UNAIDS head Dr Peter Piot in a statement. "But with more than 6,800 new infections and over 5,700 deaths each day due to AIDS, we must expand our efforts in order to significantly reduce the impact of AIDS worldwide."

Despite revised estimates for several sub-Saharan African countries, including Nigeria, Zimbabwe, Kenya and Angola, the region remains the one most affected by the epidemic, accounting for more than two-thirds of all people living with HIV and more than three-quarters of all AIDS deaths in 2007.

Unlike elsewhere in the world, the majority of people living with HIV in sub-Saharan Africa (61 percent) are women. Although access to life-prolonging antiretroviral treatment has improved, AIDS is still the region's number one cause of death.

According to Hargrove, many countries in the region still lack adequate HIV-prevalence data, and techniques for estimating the proportion of new infections in a cross-section of HIV-positive people are still not reliable enough. "The one thing we really need is to develop better methods for measuring the rate at which new infections are occurring."