A new gel can block the Aids virus, halving a woman's chances of getting HIV from an infected partner, US scientists say.
The breakthrough in the long quest for a tool to help women whose partners will not use condoms was reported in a study in South Africa.
The results need to be confirmed in another study, and that level of protection is probably not enough to win approval of the microbicide gel in countries like the US, researchers say. But they are optimistic it can be improved.
"We are giving hope to women" who account for most new HIV infections, said Michel Sidibe, executive director of the World Health Organisation's UNAids programme.
A gel could "help us break the trajectory of the Aids epidemic", he said.
And Dr Anthony Fauci of the US National Institutes of Health said: "It's the first time we've ever seen any microbicide give a positive result" that scientists agree is true evidence of protection.
The vaginal gel, spiked with the Aids drug tenofovir, cut the risk of HIV infection by 50% after one year of use and 39% after two and a half years, compared with a gel that contained no medicine.
To be licensed in the US, a gel or cream to prevent HIV infection may need to be at least 80% effective, Dr Fauci said. That might be achieved by adding more tenofovir or getting women to use it more consistently.
In the study, women used the gel only 60% of the time; those who used it more often had higher rates of protection.
The gel also cut in half the chances of getting HSV-2, the virus that causes genital herpes. That is important because other sexually-transmitted diseases raise the risk of catching HIV.
Even partial protection is a huge victory that could be a boon not just in poor countries but for couples anywhere when one partner has HIV and the other does not, said Dr Salim Abdool Karim, the South African researcher who led the study.
In the US, nearly a third of new infections each year were among heterosexuals, he noted.
Countries may come to different decisions about whether a gel that offers this amount of protection should be licensed. In South Africa, where one in three girls is infected with HIV by the age of 20, this gel could prevent 1.3 million infections and 826,000 deaths over the next two decades, he said.
Dr Karim will present results of the study today at the International Aids Conference in Vienna, Austria. The research was published online yesterday by the US journal Science.
"We now have a product that potentially can alter the epidemic trends ... and save millions of lives," said Dr Quarraisha Abdool Karim, the lead researcher's wife and associate director of the South African programme that led the testing.
It is the second big advance in less than a year on the prevention front. Last year scientists reported that an experimental vaccine cut the risk of HIV infection by about 30%. Research is under way to try to improve it.
If further study shows the gel to be safe and effective, WHO would work to speed up access to it, its director-general Dr Margaret Chan said.
But the gel is in limited supply - it is not a commercial product and was made for this and another continuing study from drug donated by California-based Gilead Sciences, which sells tenofovir in pill form as Viread.
If further study proves the gel effective, a full-scale production system would need to be geared up to make it.
The study tested the gel in 889 heterosexual women in and near Durban, South Africa. Researchers had no information on the women's partners, but the women were heterosexual and, in general, not in a high-risk group, such as prostitutes.
Half of the women were given the microbicide and the others, a dummy gel. Women were told to use it 12 hours before sex and as soon as possible within 12 hours afterwards.
At the study's end, there were 38 HIV infections among the microbicide group versus 60 in the others.