A game-changing trial has shown that rates of HIV infection can be slashed by treating actively gay men with an anti-viral drug when they are healthy.
The Proud study, conducted in England, provides the first evidence that prophylactic HIV treatment is highly effective in a real-world setting.
It showed that pre-exposure to the HIV drug Truvada can reduce the risk of infection in men-who-have-sex-with-men by as much as 86 per cent.
Previous research had suggested that prophylactic treatment might cut HIV infection rates but it was unclear whether such an approach would work in practice.
The new study of 545 participants divided the men into two groups, one of which was to be given Truvada immediately and the other a year later.
Comparing the two made it possible to assess the effectiveness of pre-exposure prophylaxis (PrEP) in men at high risk of HIV infection.
Of the 276 men treated straight away, just three were infected with HIV over the subsequent year.
During the same period, 19 of the men from the "deferred group" became HIV positive.
Chief investigator Professor Sheena McCormack said the results showed PrEP was highly effective at preventing HIV infection in the real world.
"These results show there is a need for PrEP, and offer hope of reversing the epidemic among men who have sex with men in this country."
Dr Michael Brady, medical director at the HIV/Aids charity Terrence Higgins Trust, said "PrEP is, quite simply, a game-changer".
"We know that most gay men use condoms most of the time, and that this has prevented tens of thousands of HIV infections since the epidemic began in the UK. However, we also know that condomless sex vastly increases the risk of HIV being transmitted.
"This research shows just how effective PrEP can be in preventing transmission of the virus in groups at greatest risk; offering another line of defence alongside condoms and regular testing. It is not a vaccine and it won't be for everyone, but once approved, we expect it to significantly increase the momentum in our fight against the virus."