With AIDS vaccine efforts at an impasse, microbicides — virus-blocking gels inserted into the vagina before sexual intercourse — have risen from their own string of setbacks to once again offer hope of preventing HIV infections, at least in women.

Microbicides blocked the primate form of the virus in monkeys in studies reported online Wednesday in the journal Nature and last month at an AIDS meeting. A third microbicide tested in 3,100 women showed potential for protection in findings presented at the meeting, in Montreal.

Identifying a microbicide that works even partially could have a huge effect. Almost half of the more than 33 million people living with HIV worldwide are women; in sub-Saharan Africa, the figure is 60 percent.

“It would empower women to protect themselves in a sexual situation in which they may not have complete control,” said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.

Abstinence and condoms have been the only proven ways to prevent or reduce the spread of HIV among adults. Circumcision lowers the risk for men, and already-existing AIDS drugs are used prophylactically to reduce mother-to-child transmission during birth or breast-feeding.

In many parts of the world, it is difficult or impossible for women to refuse sex or persuade a partner to use a condom. An effective microbicide would be something that women could initiate on their own.

Microbicides might also be effective in preventing infection during anal intercourse, although most studies have focused on vaginal transmission.

That scientists would be so encouraged by two animal studies and a minor success in a human trial is a sign, in part, of how stymied they have been in two decades of trying to find a microbicide or a vaccine to stem the spread of HIV.

“There’s no vaccine on the immediate horizon,” said Dr. Lynn Paxton of the U.S. Centers for Disease Control and Prevention. “After a number of microbicide trials failed to show any effect or in some cases showed a negative effect, this is good news.”

Until now, all microbicides tested in human trials have failed — two spectacularly, in that they increased the risk of infection.

In 2000, human trials of nonoxynol-9, a spermicide gel already used for birth control, were halted because the gel was found to irritate the vaginal lining, making it easier for the virus to gain entry. In 2007, human trials of a cellulose sulfate gel were halted after it was also found to increase vulnerability to the virus.

Experimental vaccines have proven equally disappointing. For 25 years, U.S. health officials have vowed to get a vaccine to the market, saying it was the best hope for containing the AIDS pandemic.

But two years ago, a much-anticipated human trial of a Merck & Co. vaccine was halted not only because the vaccine wasn’t working but also out of fear that it — like the failed microbicides — might have increased the risk of infection.

And last year, U.S. health officials called off plans for a large human trial of what had been considered a promising government-developed vaccine and urged scientists to go back to basic research on the immune system and animal tests.

Antiretroviral drugs have been the one concrete success in the AIDS battle, almost overnight commuting what had been a certain death sentence into, for most people, a manageable chronic disease.