ONE of the most contentious issues in the field of AIDS is the use of condoms. Some people—including many on America's religious right—object to them because they think that contraceptives of any sort encourage promiscuity. Others—the pope and couples who want to have children—object simply because they are contraceptives. Others again object because condoms detract from the pleasure of sexual intercourse. The last two objections, though, would be rendered irrelevant if a different barrier to the entry of HIV could be found.

The next few years could see the discovery of just such a barrier, for one of the most promising ideas being discussed at the AIDS conference in Bangkok was that of vaginal microbicides.

The idea of developing an antiviral agent that a woman could put in her vagina prior to intercourse is not new. It fell out of favour four years ago when trials of what was then thought the most promising candidate, nonoxynol-9, failed to deliver. Now it is back. Two microbicides are in phase-III clinical trials (to determine efficacy) in Africa, and four others are expected to start in the next few months.

These first-generation drugs operate in one of two ways. Some work like detergents, disrupting the membrane that surrounds an individual virus in the way that washing-up liquid disrupts an oily film. The others are large, negatively charged molecules that attach themselves to viral particles (which are positively charged on the outside) and thus render them harmless. Further down the track, two more types await large-scale trials. One works by inhibiting the activity of a viral enzyme called reverse transcriptase. That stops the viruses reproducing. The other stops viruses being taken into the cells of the vaginal wall, from which they can spread to the rest of the body.

Microbicides have many advantages. They would be controlled by women, and controlled in such a way that a man would not necessarily have to know that one was being used. And, depending on which particular type of microbicide was being used, they need not have a contraceptive effect. That means that if only one individual in a partnership was infected, the couple would be able to have children without risking an infection in the other.

Microbicides, of course, will never provide the long-term protection afforded by a vaccine—should one ever be developed. But they are likely to be available for use far sooner. Zeda Rosenberg, the head of the International Partnership for Microbicides, a charity based near Washington, DC, says that if any of the six phase-III trials were to come up trumps, a product could be on the market within five years. Unfortunately, a vaccine is nothing like that close to delivery.

Microbicides also offer another benefit. Like a vaccine they offer a public good as well as a private one in that they curb the rate of viral transmission. Mathematical modelling suggests that if microbicides were used by only 20% of women at risk, and were used only half the time, this would still prevent more than 2.5m infections over three years.

The first microbicides may prove a little awkward to use, since they would come in the form of gels or foaming pessaries that would need to be applied not more than a few hours before a sexual encounter. But Dr Rosenberg believes that at least some of the substances being considered as microbicides could be packaged into slow-release rings that a woman could wear in her vagina for a month or more. If she is right, what was once seen as an orphan technology could emerge as one of the most hopeful developments in the field of AIDS for almost a decade.