What is a microbicide?

Update December 2009: Microbicides trial results announced.

Microbicides are any substances which protect people against infection by microbes, such as viruses or bacteria, on contact with those microbes. They might do this by directly killing microbes or physically preventing them from entering the body. The term 'microbicides' has replaced 'virucides' to include products that could be active against a wide range of infections, not just viruses.

Microbicides are still at a developmental stage, and no proven safe and effective products are currently available. However, the prospects are good for products with at least some efficacy and there is a growing body of opinion supporting their development. The main focus is on microbicides for vaginal use, which is seen as technically simpler than providing protection during anal sex, but rectal microbicide development is also underway. Microbicides would be used in anal sex, by heterosexuals as well as gay men, and vaginal microbicides need to be tested for rectal use because people may well use them rectally once they are licensed – as indeed happened with condoms. Acceptability and safety studies of rectal microbicides in humans are taking place.

Microbicide research began by analogy with spermicides (contraceptives which kill sperm). Some microbicides may also be spermicidal. The big difference is that while spermicides only act to 'protect' women against sperm, a microbicide may be able to offer protection to either partner. Vaginal microbicides could be used by HIV-positive women to protect uninfected men, as well as by HIV-negative women to protect themselves. Microbicide development advocates have called for sub-studies of microbicide effectiveness on preventing transmission as well as infection in the development of all candidate products. In the case of antiretroviral-based microbicides, this becomes a complex issue because of the possibility that such compounds could lead to drug-resistant HIV.

Microbicides could take the form of a cream, pessary, sponge, foam or jelly. The first products to be tested are gels which closely resemble the lubricants used with condoms.

In reality, some products would have much broader activity than others, raising issues in public education. There is already a great deal of confusion around the meaning of 'safe' or 'safer' sex: safe from what? Pregnancy, HIV, other infections? Such questions must be answered as microbicides are evaluated and, it is hoped, made widely available.

A study commissioned by the Rockefeller Foundation has projected the impact of microbicides in four subpopulations in 73 lower-income countries. In particular: sex workers and their clients; sexually active youth; injecting drug users and their sexual partners; women in regular partnerships. Numbers were estimated for those in each of these groups in contact with services that could distribute microbicides. Using conservative estimates of product efficacy (40-60% vs. HIV, 0-40% vs. STIs), coverage (10% of populations) and usage (50% of sexual acts), it was possible to show that over three years, several million HIV infections could be averted (Watts 2002).