A combination of drugs used to treat HIV reduces the risk of people becoming newly infected by 44 per cent, raising the prospect of a powerful new prevention technique against Aids, according to a new study.

Published in Tuesday’s New England Journal of Medicine, the study of 2,499 uninfected gay and transgender men in the Americas, Asia and South Africa found that nearly twice as many who took a placebo became infected compared with those who took the drugs tenofovir and emtricitabine.

The details came on the day that UNAids released its latest annual report estimating that last year 2.6m people were newly infected with HIV, increasing to 33.3m the numbers living with the virus. Last year, an estimated 1.8m died from complications of the virus, even though more than 5m are receiving treatment.

However, the UN report said the global incidence of HIV infection fell by 19 per cent between 1999 and 2009. It said the adoption of safer sexual practices by the young and greater availability of condoms in countries most at risk were important factors in the fall.

The results of the new drug study represent a significant boost to researchers struggling to find ways to prevent the spread of HIV.

However, a number of experts gave warning that concerns over funding, resistance, use and logistical challenges could limit widespread use of the drugs for prevention.

In the trial – called iPrEx (pre-exposure prophylaxis initiative) – 36 men who took tenofovir and emtricitabine over an average of more than a year developed HIV, compared with 64 who took the placebo.

The development is the latest breakthrough in the use of drugs or vaccines to cut infection. In clinical trials last year an experimental HIV vaccine reduced infections by 31 per cent, and a vaginal microbicide gel cut transmission by 39 per cent.

The conclusions are a fresh boost to Gilead, the US drug company that developed tenofovir, which has become an increasingly widely used “first line” treatment for HIV and was also used in the microbicide trial.

Mitchell Warren, head of Avac, an HIV prevention advocacy group, said: “This builds on an incredible year in prevention. Each of these results is a bit better and the hat-trick in combination is leading us in new directions.”

The latest study, which recruited men at high risk of infection across the Americas, South Africa and Asia, was co-ordinated by Robert Grant from the Gladstone Institute of Virology and Immunology at the University of California San Francisco.

A separate editorial in the New England Journal cautioned over cases of drug resistance in participants who had already become infected but whose HIV was not detected before the trial began, and poor rates of compliance with the daily dose, partly because of side-effects.

Mr Warren suggested that this meant any future prophylaxis use of drugs would require counselling and testing, regular monitoring to boost compliance, and the need to continue with other forms of prevention, including greater use of condoms and circumcision.

Kevin Frost, head of amfAR, the Foundation for Aids Research, welcomed the findings but warned of “the incredible costs and other logistical hurdles that will be faced in trying to operationalise this approach”.