The United Nations has lowered its estimate of the number of people infected with the HIV virus, from 40 million to 33 million. Revised figures for India accounted for half the decrease, which the Joint UN Progamme on HIV and Aids (UNAids) said was due to improved monitoring and changes in the way that figures for 2007 were calculated.

HIV remains a devastating epidemic, with an estimated 2.5 million people infected this year and more than 2.1 million deaths. However, the number of new infections is continuing to fall as the prevalence of the disease levels off, according to the UNAids report, 2007 Aids Epidemic Update, published yesterday.

Vast sums have been spent on HIV/Aids projects over the past decade and UNAids admits its estimate of the resources needed in future will have to be revised downwards in light of the new figures.

Some glimmers of hope in the fight against HIV are being seen. The number of cases among pregnant women aged 15 to 24 attending antenatal clinics has fallen since 2001 in 11 of the 15 worst-affected countries. Preliminary evidence also suggests young people are practising safer sex in nine of the worst-hit countries.

About 22.5 million people in sub-Saharan Africa have HIV but the number of new cases – 1.7 million in the past year – is a smaller increase than in previous years. UNAids said there were clear signs of a decline in infections in Kenya and Zimbabwe, as well as more modest progress in Ivory Coast, Mali and Burkina Faso. But the decline in Burundi since the late 1990s halted in 2005 and the infection rate is rising again, it reported.

The fall in annual deaths to 2.1 million is being attributed by UNAids to wider access to antiretroviral drugs. A growing number of prevention campaigns also contributed to the overall fall, said Peter Piot, the agency's executive director.

"Unquestionably, we are beginning to see a return on investment," he added. "New HIV infections and mortality are declining and the prevalence of HIV levelling. But with more than more than 6,800 new infections and 5,700 deaths each day due to Aids, we must expand our efforts."

Across the world, the picture is mixed. Infection rates are falling in Thailand, Cambodia, Burma and Haiti but rising in Vietnam, Indonesia and eastern Europe. Rachel Baggaley, head of the HIV unit at the charity Christian Aid, said: "Complacency is one of the greatest dangers we face. Many countries have made huge strides in containing HIV yet, just at the time when the prevention programme needs even more support, there are threats of funding cuts."

Despite a general downward trend in HIV infections, the number of people in Europe and central Asia living with the virus has doubled in six years, from 1.25 million to 2.4 million.

In western Europe, the number of new cases has been rising since 2002 and an estimated 760,000 EU citizens now live with HIV. In North America, however, the spread of the disease has stabilised.

More than half of those infected are believed to be unaware of it. They are three times as likely to pass on the virus as those who know they have HIV.

In Britain, more than 8,000 new HIV cases were diagnosed last year, bringing the total living with the disease to about 63,500. Of these, a third have not been diagnosed.

At a meeting in Brussels next week, 300 specialists will consider how to standardise HIV testing and improve diagnosis rates throughout Europe.