UN:United Nation's Aids scientists have acknowledged that they have long overestimated the size and the course of the epidemic, which they now believe has been slowing for nearly a decade.
Aids remains a devastating public health crisis in the most heavily affected areas of sub-Saharan Africa. But the far-reaching revisions amount to at least a partial acknowledgment of criticisms long levelled by outside researchers who disputed the UN portrayal of an ever-expanding global epidemic.
Latest estimates put the number of new HIV infections at 2½ million annually, a reduction of more than 40 per cent from last year's estimate, documents show.
The worldwide total of people infected with HIV - estimated a year ago at nearly 40 million and rising - will now be reported as 33 million.
Some researchers contend that persistent overestimates in the widely quoted UN reports have long skewed funding decisions and obscured potential lessons about how to slow the spread of HIV.
Critics have also said that UN officials overstated the extent of the epidemic to help gather political and financial support for combating Aids.
"There was a tendency toward alarmism, and that fitted perhaps a certain fundraising agenda," said Helen Epstein, author of The Invisible Cure: Africa, the West, and the Fight Against Aids. "I hope these new numbers will help refocus the response in a more pragmatic way."
The revision of the figures stemmed mainly from better ways of quantifying numbers infected, than fundamental shifts in the epidemic.
Among the reasons for the overestimate is methodology. UN officials traditionally based their national HIV estimates on infection rates among pregnant women receiving prenatal care.
Such women were younger, more urban, wealthier and likely to be more sexually active than populations as a whole, according to recent studies.
The UN Aids agency, known as Unaids and led by Belgian scientist Peter Piot since its establishment in 1995, has been a major advocate for increasing spending to combat the epidemic. Over the past decade, global spending on Aids has grown by a factor of 30, reaching as much as $10 billion (€6.76 billion) a year.
But Unaids has drawn criticism in recent years from Ms Epstein and others who have accused it of being politicised and not scientifically rigorous.
For years, Unaids's reports have portrayed an epidemic that threatened to burst beyond its epicentre in southern Africa to generate widespread illness and death in other countries.
In China alone, one report warned, there would be 10 million infections - up from one million in 2002 - by the end of the decade.
Mr Piot often wrote personal prefaces to those reports warning of the dangers of inaction, saying in 2006 that "the pandemic and its toll are outstripping the worst predictions". But by then, several years' worth of newer, more accurate studies already offered substantial evidence that the agency's tools for measuring and predicting the course of the epidemic were flawed.
Newer studies commissioned by governments and relying on random, census-style sampling techniques found consistently lower infection rates in dozens of countries.
For example, the UN has cut its estimate of HIV cases in India by more than 50 per cent because of a study completed this year.
This week's report also includes significant reductions to UN estimates for Nigeria, Mozambique and Zimbabwe.