Scientists this week announced the first anti-HIV pill to provide effective protection against the disease that affects 33 million people globally.
Gay men at extremely high risk of HIV who took the oral pill daily cut their risk of contracting the infection by almost 44 per cent. Aids organisations and researchers said it heralded a new era of Aids prevention. After the failure of almost 30 large-scale trials of protective therapies, recent positive results for an Aids vaccine and for a microbicidal gel suggest progress.
"This discovery alters the HIV prevention landscape for ever," said Jim Pickett of the Aids Foundation of Chicago. Anthony Fauci, head of the National Institute for Allergy and Infectious Diseases - which provided two thirds of the £27.6m ($57.3m) cost of the study - said: "The results are extremely important." Margaret Chan, director general of the World Health Organisation, said the trial opened "exciting new prospects".
Almost 2,500 men from the US, South Africa, Thailand, Brazil, Ecuador and Peru took part in the study. They were highly sexually-active, reporting an average of 18 partners over 12 weeks, with 60 per cent saying they had receptive, unprotected anal intercourse during that time, the riskiest sexual activity for HIV.
Half the men took the pill, called Truvada, containing an antiretroviral medicine, amongst whom there were 36 infections after 14 months. There were 64 infections among those who took a placebo, amounting to a 43.8 per cent reduction with Truvada.
All the men received regular advice about how to reduce their risks, a supply of condoms and treatment for other sexually transmitted infections.
Kevin O'Reilly, an HIV prevention specialist at WHO, said the results were "somewhat weaker than had been hoped". Many of the men did not take the drug regularly and protection was highest in those who had the most consistent use.
He said side-effects were minor and compared the treatment to the use of oral contraceptives by women.
"It is only one ARV and the most easily tolerated compared with three taken by people who are HIV-positive and on treatment. Its safety has been very well studied."
The preventive strategy is also being trialled in heterosexuals in Africa and injecting drug users in Thailand and the results are expected next year.
Experts accept that every extra preventive measure may encourage riskier behaviour but they say the more weapons against the disease the better. Problems with compliance may mean the drug has to be delivered in a different form, for example as a microbicidal gel to be used during intercourse.
Sir Nick Partridge of the Terence Higgins Trust said: "It's not ready for widespread use yet. Three major hurdles are still going to be: its cost, the risks of drug-resistant strains of HIV developing and taking a drug treatment every day.
"For now - and for the foreseeable future - condoms remain the most effective, easily available and cheapest way of preventing HIV transmission."
- INDEPENDENTBy Jeremy Laurance