Women are physically more vulnerable to sexual transmission of HIV than men, probably due to the nature of the cervix. Unlike the vaginal epithelium (surface), which consists of several layers of flat, sturdy squamous cells, parts of the surface of the cervix are made up of a single layer of fragile columnar cells, which are more easily damaged. This is the cellular equivalent of a stack of sumo wrestlers versus a single line of figure skaters.
In younger women, these cervical columnar cells are even more exposed than in adult women, probably a major factor in adolescent girls' higher risk. In addition, several target cells for HIV, including CD-4 cells, are found more frequently on the cervix than throughout the rest of the vagina. The passage of infectious fluids into the upper genital tract (also highly susceptible) via the cervix may be another factor in women's HIV acquisition. Other STIs besides HIV also target the cervix. The bacteria that cause gonorrhea and chlamydia, for example, can only replicate in the cervical epithelium, not in the squamous epithelium of the vagina.
Protecting the cervix may be key.
Though the cervix is not the only factor in vaginal transmission of HIV or STIs, it is highly possible that a woman could reduce her risk by protecting her cervix. Cervical barrier methods include the diaphragm, cervical cap and, to a lesser extent, the contraceptive sponge. Developers are working on variations of these traditional methods that would make them easier to use.
- SILCS Diaphragm PATH (Program for Appropriate Technology in Health) is designing a silicone alternative to the diaphragm. The device would be easier to insert and remove and would not require a health care provider to fit.
- Lea's Shield , sold in Germany, is a silicone cup barrier with a loop for removal.
- FemCap and the Oves Cap are silicone variations of the current latex cervical cap.
- Contraceptive sponges , though not completely impermeable barriers, deliver and release spermicide at the cervix and provide some physical coverage. Two sponges, Protectaid and Pharmatex, are available in Canada and Europe. The Today sponge is expected back on the market in the US shortly.
The Contraceptive Research and Development Program (CONRAD) is supporting studies to demonstrate that some of these devices are equivalent to those currently approved by the FDA for contraceptive efficacy. However, so far no studies have been published that specifically tested whether cervical barrier methods can reduce the risk of HIV transmission. On July 13, 2007, the MIRA (Methods for Improving Reproductive Health in Africa) diaphragm study released their preliminary findings on the use of a cervical barrier to prevent acquisition of HIV. The trial results show that in the context of a comprehensive HIV prevention package (including condoms, counselling and STI screening and treatment) there is no added protective benefit from the use of a diaphragm and lubricant.
Given these results, the study authors conclude that a diaphragm should not be used or promoted as an effective means of HIV prevention at this time. These data, however, do not conclusively rule out that cervical barriers may have a role in HIV prevention. The diaphragm findings are still preliminary and only speak to whether this particular cervical barrier, when used with a non-microbicidal lubricant, can reduce the risk of HIV infection compared to a comprehensive prevention package of STI screening and treatment, risk reduction counselling, and male condom promotion and use. The MIRA study was not designed to determine whether the use of a cervical barrier was better than nothing at all. Other cervical barrier methods, particularly when combined with future microbicide candidates, may prove effective and many feel the HIV prevention field should continue to develop and pursue such combination approaches.