HIV infections in southern India have fallen by a third over a four-year period, raising hopes that the tide of the disease could be reversed in one of the country's worst-hit regions.

A study published in the Lancet looked at data from the four south Indian states that account for 75% of India's 5.1 million people infected with the virus.

The research, by a joint Indian-Canadian team, tracked HIV prevalence among 204,050 young women and nearly 60,000 men between 2000 and 2004 in the north and south of the country. Doctors reported HIV prevalence among women aged 15 to 24 in the states of Tamil Nadu, Maharashtra, Karnataka and Andhra Pradesh had fallen from 1.7% to 1.1% in four years. New infections fell by almost 35% between 2000 and 2004.

"We are seeing a decline and it's real," Prabhat Jha of the Centre for Global Health Research at the University of Toronto said.

The team found new infections among men visiting STI clinics in the four southern states - which are home to a third of India's billion-plus population - fell by more than 30%. Two large states, Maharashtra and Tamil Nadu, have been promoting condom use and partner loyalty as key to their anti-Aids strategies. They have been at the forefront of India's Aids prevention campaign.

But many campaigners, while welcoming the results of the study, said the data masked the disparity between regions with the declines in Maharashtra and Tamil Nadu hiding the size of the problem elsewhere in the country.

"The data shows that there continues to be a challenge in the south, especially in Karnataka and Andhra Pradesh," said Ashok Alexander of Avahan, the Bill and Melinda Gates Foundation's Aids initiative in India. "The study is weighted in favour [of Maharashtra and Tamil Nadu] as they have many more measuring sites than anywhere else in the country."

The authors of the study admitted that surveillance of HIV in north India, which includes states like Uttar Pradesh and Bihar with a combined population of around 250 million people, made precise measurements difficult.

India has the world's highest number of HIV infections after South Africa.