In a decisive breakthrough against the spread of HIV, an international study has demonstrated conclusively that antiretroviral therapy blocks the spread of HIV from an infected person to their uninfected partner.

The study, which was halted early because the results were so compelling, covered 13 countries and involved 1763 “discordant” couples, in which one partner carried the infection at the outset.

In 886 of the couples, the infected partner received antiretroviral drug therapy (ART) straight away, while treatment was delayed in the other 877 couples until the infected partner showed pre-defined signs of sickness.

Preliminary results, announced yesterday by the US National Institute of Allergy and Infectious Diseases (NIAID), showed that only one of the 886 individuals who received immediate treatment passed the virus to their partner. There were 27 cases of cross-infection in the 877 couples where treatment was delayed.

Faced with this 96 per cent reduction in risk, the study was halted between three and four years early so all of the couples could be treated.