"This is a big moment we need to celebrate because this is the first time in the history of fighting HIV/AIDs that we can talk about a prevention revolution that will help us have a tool that can help women initiate and control their prevention methods," Michel Sidibe, the executive director of the United Nations' AIDS-fighting agency, said.
"The only things we've known until now has only been for men," he told Al Jazeera.
"I am so proud that this discovery is coming from Africa and also from researchers who are themselves African."
Anthony Fauci, an expert from the US National Institutes of Health, said: "This is the first time we've ever seen any microbicide give a positive result.
"Even partial protection is a huge victory that could be a boon not just in poor countries"
Salim Abdool Karim,
South African researcher
"Given that women make up the majority of new HIV infections throughout the world, this finding is an important step toward empowering an at-risk population with a safe and effective HIV prevention tool," Fauci said.
The gel, spiked with the AIDS drug tenofovir, cut the risk of HIV infection by 50 per cent after one year of use and 39 per cent after 2 1/2 years, compared to a gel that contained no medicine.
Research is under way to try to improve the gel and if further studies show the gel to be safe and effective.
The level of protection is probably currently not enough to win approval of the microbicide gel in countries like the United States, but researchers say they are optimistic it can be improved.
To be licensed in the US, a gel or cream to prevent HIV infection may need to be at least 80 per cent effective, Fauci said.
Researchers suggested that better results could be achieved by adding more tenofovir or getting women to use it more consistently.
In the study, women used the gel only 60 per cent 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 would be important because other sexually spread diseases raise the risk of catching HIV.
Dr. Salim Abdool Karim, the associate director of the programme that carried out the study, told Al Jazeera that "the traditional four tools, which is abstinence, being faithful, condoms and circumcision, have very limited capability" to fight HIV transmissions.
"So essentially we have very little to offer a woman to protect herself from HIV, but this finding offers new hope," he said, speaking from Austria's capital Vienna.
"We did some mathematical modeling that showed what would happen if we took this gel and made it available to the whole of South Africa.
"Our model shows that over the next 20 years if the gel is available, we could prevent 1.3 million HIV infections and could avert over 800,000 deaths in South Africa.
"39 per cent (efficiency) is not the last word. What we have done is showed that the concept works. This is the first time an anti-retroviral has been shown to prevent sexual transmission.
"It is also the first time that we have a microbicide gel that has been shown to prevent HIV infection. This is a starting point.
"The most important thing at this point is for us to is to put all of our sources together to find the fastest ways to get it approved by regulator authorities.
"Until a regulator says that this product is safe and effective, we will not be able to make it available. "
Karim will present results of the study in Vienna at the International AIDS Conference on Tuesday.