A scheme in six Indian states that concentrated safe-sex campaigns on a few niche groups prevented 100,000 HIV infections over five years, according to estimates published in The Lancet on Tuesday.

The so-called Avahan project was launched in 2003 in Andhra Pradesh, Karnataka, Maharashtra and Tamil Nadu, along with the northeastern states of Manipur and Nagaland, using a massive grant from the Bill and Melinda Gates Foundation.

These states, with a total population of 300 million, had the highest prevalences of the human immunodeficiency virus (HIV) in India at the time.

Avahan's goal was to boost prevention among prostitutes and their customers, gays, injecting drug users and truck drivers to stop HIV from leaping from high-risk groups to the wider population.

Tactics included one-on-one safe-sex counselling, free condoms, exchanging used needles for sterilised ones, clinics to treat sexually-transmitted disease and advocacy work within the community.

"Overall, we estimated that 100,178 HIV infections were averted at the population level from 2003 up to 2008 as a result of Avahan," says the study.

Its estimate derives from HIV prevalence in key districts in the six states.

The campaign was most effective in districts that received the most resources but also worked better in the heavily-populated southern states rather than in the remote northeastern ones, say the authors.

Overall, a targeted strategy -- as opposed to a generalised effort spread across the population -- was a big success and a useful lesson for other countries, they say.

Prevention has been in the doldrums in recent years, given the success of antiretroviral drugs that treat HIV but do not cure it.

But experts caution that drugs alone are not enough to roll back the global pandemic. As the infection tally rises higher, so does the drugs bill, as the medication has to be taken daily for the rest of one's life.

Avahan was launched at a time when India was gripped by fears that as many as 25 million of its people could be infected by HIV by 2010.

These projections were eventually cut back sharply. In 2009, India had an estimated 2.4 million people with HIV in 2009.

Avahan gained Gates funding of 258 million dollars for 2003-2008, a sum that sparked criticism in some quarters that this was too lavish and poor value for money.

But the paper disputes this, saying that the cost per HIV infection averted, "is in a similar range" to the government scheme, run by the National AIDS Control Programme of India.

The national programme had funding of 460 million dollars form 1999-2006, which rose to 2.5 billion for 2007-2012, of which two-thirds were allocated for HIV prevention.

The study's authors were funded by the Gates Foundation. However, The Lancet is a peer-reviewed journal, which means that data proposed for publication is vetted by outside experts.

In 2009, the Gates Foundation promised an additional 80 million dollars into Avahan to help it integrate with the government's AIS programme by 2013.