A new gel can block the Aids virus, halving a woman's chances of getting HIV from an infected partner, US scientists have said.
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 said. 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 herpes virus that causes genital warts. 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.