The UN and the World Health Organisation significantly cut their estimates of the number of people with the HIV virus today, after a large downward revision of HIV cases in India.
In their latest report, UNAids and the WHO said the new estimate of 33.2 million people living with HIV replaced last year's estimate of 39.5 million.
The international bodies said the estimates reflected improvements in country data collection and analysis, as well as a better understanding of the natural history and distribution of HIV infection.
Even with the revised figures, "the numbers are probably still on the high side", Daniel Halperin, an Aids epidemiologist at the Harvard school of public health, told the Associated Press.
Halperin attended the WHO/UNAids meeting last week that reviewed the figures, and said the estimates were getting closer.
But a US group, Aids Healthcare Foundation (AHF), questioned whether the new lower estimate was any more accurate than the previous one. Calling for greater transparency among international organisations that track the disease, AHF said all the numbers, be they higher or lower, were guesswork at best.
"These figures are rough numbers based upon extrapolations gleaned from unreliable data, since so few people are being tested. Let's stop guessing and make routine testing worldwide a priority," said Michael Weinstein, the AHF's president.
UN experts acknowledged the difficulties in gathering reliable data, despite recent improvements.
"There is a need to further improve the representativeness of the underlying data. There is a need to expand disease surveillance systems to better track the sub-epidemics in risk populations within each county," said Ron Brookmeyer, professor of biostatistics at the Johns Hopkins Bloomberg school of public health.
Previous estimates were largely based on the numbers of infected pregnant women at clinics, as well as projecting the Aids rates of certain high-risk groups, such as drug users, to the entire population at risk. Officials said those numbers were flawed, and they are now incorporating other data such as national household surveys.
The US Agency for International Development has started funding surveys that choose thousands of households at random in both urban and rural areas. Health workers then take detailed medical and lifestyle histories and blood samples. Although more expensive, such surveys produce more reliable results.
The single biggest reason for the reduction in global HIV figures in the past year was the recent revision for India, UNAids said. The infection rate for India was cut to 3 million from 6 million after an intensive reassessment of the epidemic there.
UNAids and WHO officials pointed out that the new estimates did not change the need for immediate action and increased funding to reach the goal of universal access to HIV prevention, treatment and care.
Charities, including the Gates Foundation, have donated more than $1.1bn to fighting HIV/Aids since 1995, on top of the $7bn committed by the Global Fund to Fight Aids, tuberculosis and malaria. The fund has provided Aids treatment for 1.1 million people.
Aids remains the fourth biggest killer worldwide, causing an estimated 2.1 million deaths this year alone. Sub-Saharan Africa remains the region most severely hit. An estimated 22.5 million living with HIV - or 68% of the global total - are in the region, with eight countries accounting for almost one-third of all new HIV infections and Aids deaths worldwide.
Other regions have seen a big increase in HIV cases. The number of HIV cases in eastern Europe and central Asia has increased by more than 150% from 630,000 to 1.6 million. The estimated number of people living with HIV in Vietnam has more than doubled between 2000 and 2005, and Indonesia has the fastest growing epidemic.
