HIV patients who take antiretroviral therapy (ART) early enough not only protect themselves but also their partners, new research has shown.

The finding is the first from a major randomized clinical trial to indicate that treating an HIV-infected person can make them less contagious, as well as keep them healthy.

The study, which spanned nine countries, involved more than 1,700 couples, in which one partner was HIV-positive and the other was not. Each couple was randomly assigned to one of two study groups by researchers at the University of North Carolina, US.

In the first group, the partner with HIV began receiving antiretroviral drugs as soon as they enrolled in the study. In the second group, the infected partner started antiretroviral treatment once their CD4+ count – a key measure of immune system health – fell to between 200 and 250 cells/mm3, or when they developed an AIDS-related illness.

According to the results, 27 previously uninfected partners in the second group contracted HIV from their partner. But in the first group, only one such case of new HIV infection occurred. This means that starting ART earlier led to a 96% reduction in HIV transmission within relationships.

UK HIV charities have given a cautious welcome to the news. Sarah Radcliffe, a policy officer at the National Aids Trust (NAT) said the findings backed up anecdotal evidence. She added: “These results are very exciting. Back in 2008, the Swiss Federal Commission for HIV/AIDS published a statement saying that patients on effective ART who have no other sexually transmitted infections are sexually non-infectious. This is the first time we have an experiment which backs this up. The fact that it finished early is very significant and it is a very robust piece of research.”

AVERT spokesperson Rachel King described the research as “compelling” but continued: “Scaling up testing to improve knowledge of HIV status and increasing treatment coverage will be crucial to gaining the public health benefits of treatment as prevention.” Universal access to treatment was still the main goal, she added.

The Terrence Higgins Trust, however, suggested that the research could have more impact in the developing world than in Britain. Blake Smith, information officer at the trust, said: “With up to one in four people with HIV being undiagnosed in the UK, the people who are likely to be most infectious are not under care and so would not receive treatment.”