The HIV pandemic which started 28 years ago is officially in decline, two of the world's leading health organisations said yesterday.

The number of new HIV infections peaked in the mid-1990s and has since declined by almost a third, according to the annual update on the pandemic for 2009, published yesterday by the Joint United Nations programme on HIV/Aids (UNAids) and the World Health Organisation.

It is the first time that UNAids and the WHO have confirmed that the pandemic is on a downward trend and represents a landmark in the history of the disease. In their 2008 report, they said suggestions the epidemic had peaked were "speculation" and that it was "difficult to predict the epidemic's future course".

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That report warned: "The HIV epidemic has repeatedly defied predictions... HIV is likely to have additional surprises in store that the world must be prepared to address."

But the 2009 update throws scientific caution to the winds and instead states clearly that the pandemic has passed its zenith: "The latest epidemiological data indicate that globally the spread of HIV appears to have peaked in 1996 when 3.5 million new infections occurred. In 2008 the estimated number of new HIV infections was approximately 30 per cent lower than at the epidemic's peak 12 years earlier."

It says that, in sub-Saharan Africa – the worst-affected region – new infections in 2008 were "approximately 25 per cent lower than at the epidemic's peak in the region in 1995".

It adds: "Asia's epidemic peaked in the mid-1990s and annual HIV incidence has subsequently declined by more than half. Regionally, the epidemic has remained somewhat stable since 2000." The annual report from UNAids and the WHO is the official record of the progress of HIV/Aids, and confirmation that the worst disease of modern times is in decline will be widely welcomed. Two years ago the organisations admitted that they had overestimated the numbers affected and revised the total down from 40 million to 33 million.

Despite the fall in new infections, the number living with HIV increased last year to 33.4 million as people are surviving longer with the roll-out of antiretroviral drug treatment. Greater access to drugs has helped cut the death toll by 10 per cent over the past five years.

There are now 4 million people on the drugs worldwide, a 10-fold increase in five years. The report says 2.9 million lives have been saved since effective treatment became available in 1996 but less than half the patients who need them are currently getting them.

The reasons for the decline in new infections are disputed. UNAids said prevention programmes involving sex education, HIV awareness campaigns and distributing condoms had had an impact. Critics said the pandemic was already in decline before prevention programmes were widely implemented and the disease was burning itself out. Ties Boerma, a WHO statistics expert, said countries whose HIV prevalence declined dramatically, like Zimbabwe, were not always those that got the most HIV cash.

Experts at UNAids said new infections had fallen 17 per cent since 2001, when the UN Declaration of Commitment on HIV/Aids was signed, triggering a global push to deliver anti-retroviral drugs and prevention programmes to the hardest hit parts of the world. Michel Sidibe, executive director of UNAids, said: "We have evidence that the declines we are seeing are due, at least in part, to HIV prevention. However, the findings also show that prevention programming is often off the mark and that, if we do a better job of getting resources and programmes to where they will make most impact quicker, progress can be made and more lives saved."

But Philip Stevens of International Policy Network, the London-based think-tank, said with HIV declining it was time to rethink global spending priorities and switch funds currently being spent on HIV to other conditions that kill more people. Globally, HIV causes about 4 per cent of all deaths, but gets 23 pence in every pound spent on development aid for health ($21.7bn in 2007, or £13.1bn).

Mr Stevens said: "In most countries HIV is a relatively minor problem compared with other conditions such as malaria and diarrhoeal disease. The exception is sub-Saharan Africa. South Africa has a 23 per cent prevalence but in many other countries [in the region] it's 3 to 5 per cent. They have a problem but it is not the huge problem that UNAids is claiming. We shouldn't let this single disease continue to distort overall global funding, especially when bigger killers like pneumonia and diarrhoea in developing countries are far easier and cheaper to treat."

Mr Stevens said the "single issue advocacy" by UNAids, which existed solely to draw money to the disease, had distorted global health priorities. "Governments are now talking about placing a bigger emphasis on primary care and building up public health systems."

Dr Karen Stanecki, senior adviser to UNAids, said repeated studies in different parts of the world, comparing the reduction in new infections with what happened where there was no intervention, had demonstrated the effectiveness of prevention programmes.

"The decline was over and above the natural decline in the epidemic. They showed it could only have been explained by behavioural change."

She denied that too much was being spent fighting HIV/Aids. "We are facing a great many challenges. There are still 7,400 new infections a day. For every five people who become infected, two start on treatment. So we still have a long way to go."

Global killer: The history of HIV

1981 Aids is detected in California and New York. The first cases are among gay men, then drug users

1982 Aids is reported among haemophiliacs and Haitians in the US; cases in some European countries

1984 Scientists identify HIV as the cause of Aids

1985 An HIV test is licensed for screening blood supplies

1987 AZT is the first drug approved for treating Aids

1990 Around 8 million people are living with HIV worldwide.

1996 Combination anti-retroviral treatment is shown to be highly effective against HIV

1997 Aids deaths begin to decline in developed countries, due to the new drugs

2001 At a UN special session, world leaders set long-term targets on HIV/Aids

2002 The Global Fund is established to boost the response to Aids, TB and malaria

2003 The WHO launches the "three by five" campaign, to get three million people on drug treatment by 2005

2004 After much hesitancy, South Africa begins to provide free antiretroviral treatment

2007 Around 33 million people are estimated to be living with HIV

2009 UNAids and WHO confirm Aids pandemic is in decline