This is not, however, the answer to the nearly 30 year epidemic of HIV and AIDS. Since it is just one trial, many more studies will need to follow. But according to the Terrence Higgins Trust (THT) it is "potentially very significant and could change the HIV landscape in the future".
Some brief facts about the trial: it involved about 2,500 men at high risk of HIV infection in Peru, Ecuador, Brazil South Africa, Thailand and the United States (San Francisco and Boston). At the start of the study they had on average 18 sexual partners, which dropped to 6 by the end.
Half were given a daily pill - Truvada, a combination antiretroviral drug - which interferes with the virus's ability to replicate in cells. The other half were given a placebo - or dummy pill - each day. All were encouraged to use condoms. After a year 36 men taking Truvada had got infected with HIV compared to 64 on the dummy pills.
Many of the men failed to take the pill every day. Among those who took the medication more than 90% of the time, there were 73% fewer infections.
The treatment is known as pre-exposure prophylaxis. Dr Kevin Fenton, AIDS prevention chief at the US Centers for Disease Control and Prevention called it "a major advance". But he said it should never be seen as a first line of defence against HIV: "It's not time for gay and bisexual men to throw out their condoms".
There are many issues to resolve. The trial involved gay men and no studies have been done to show whether it would reduce heterosexual transmission. The medication was not without side-effects, including nausea. It is also expensive - costing between $5,000-$14,000 in the United States. There are also concerns that it might allow HIV-resistant strains to develop.
For the sexually active the long-standing advice to help prevent HIV infection is:
- use condoms consistently and correctly
- get tested to know HIV status and get treatment for other sexually transmitted infections
- reduce drug use and risky sex
- reduce the number of sexual partners
Given that the drug treatment is not a guarantee against infection and that it involves taking a pill every day, who might benefit?
Sir Nick Partridge, Chief Executive of THT, said it may help prevent HIV transmission among those most at risk: "We are nearly 30 years into this epidemic and yet there are still far too many people getting infected. The majority of new infections in the UK are among gay men. This might help men who seem incapable of using condoms and regularly put themselves at risk."
He also pointed out that the average annual cost of treating one person with HIV was between £7-10,000 a year, so it made good economic sense to try to prevent infection.
The news of the trial comes as figures from the United Nations show that new HIV infections and deaths from AIDS-related illnesses are declining.
Last year there were 2.6m new HIV infections worldwide - the main route of transmission being through heterosexual sex - 20% fewer than a decade ago. In 2009, 1.8m died from AIDS-related illnesses, down from 2.1m in 2004.
There are an estimated 33m people living with HIV.
There have been other encouraging developments in the field of HIV in the past 14 months. In September 2009, scientists announced that a vaccine against HIV had protected nearly one in three people from getting infected in a trial Thailand. This is clearly not a high enough success rate, but it is the best news on vaccination to emerge. In July this year research in South Africa showed that a vaginal gel containing an antretroviral nearly cut in half a women's chance of getting HIV from an infected partner.