NEW DELHI, July 4 (Reuters) - India has fewer than 2.5 million people living with HIV/AIDS, a senior health official said on Wednesday, nearly 60 percent lower than the 5.7 million estimated by the United Nations.

India has been ranked with the world's biggest HIV-positive caseload, but the dramatically lower new figure based on an updated survey would put it behind South Africa and Nigeria.

The health official was speaking before an official announcement due on Friday of the new total, which was calculated with the help of international agencies including the United Nations and the United States Agency for International Development (USAID).

"The new number is just under 2.5 million," the health official, who is closely involved in India's AIDS control programme, told Reuters on condition of anonymity.

"A fall of 3.2 million affects the global picture of HIV," said the official, referring to nearly 40 million people living with the deadly virus across the world.

The earlier figure of 5.7 million was deduced from monitoring through surveillance sites where blood samples were taken from groups such as pregnant women, injecting drug users and prostitutes.

The samples were taken over a four-month-period each year and tested.

But a new survey, the population-based National Family Health Survey backed by USAID, indicated the 5.7 million estimate was far too high. The survey tested blood samples of 102,000 people from the general population.

The 2.5 million figure worked out on the basis of the survey puts India behind South Africa, which has around 12 percent of its 47 million people infected, and Nigeria with 3 million cases.


ACTIVISTS' CONCERN

Officials at India's National AIDS Control Organisation, which is due to launch an ambitious $2.8 billion anti-AIDS plan on Friday, refused to comment immediately on the new figure.

But activists said they were concerned about the lower estimate, especially in a country where over 40 percent of the women have not heard of AIDS.

"A lower estimate may reduce the political will to fight the virus and people may start taking the threat of AIDS lightly. This is a danger," said Christy Abraham, the Indian team leader for HIV/AIDS at voluntary group ActionAid International.

The 5.7 million figure gave India an HIV prevalence rate of 0.9 percent, but on the basis of the new survey this would fall to around 0.3 percent, officials say.

Even the higher rate is far behind South Africa's infection rate estimated at 12 percent. In Botswana, more than a third of the population are thought to be HIV-positive.