The evidence we have is based on three types of trials, and each has potential weaknesses.
For efficacy against HIV and other chronic STIs, studies of the incidence of HIV (or HSV or HPV) in monogamous serodiscordant couples provides the best evidence.
Another kind of study is to conduct a prospective cohort study, looking at differences in HIV incidence between two groups of people according to their usage of condoms.
This cannot be a randomised controlled study, but participants’ HIV and STI incidence can be related to their condom use either at baseline or preferably (because behaviours change over time) by means of regular questionnaires and monitoring.
Laboratory studies and product testing have shown that reputable condoms tested in the laboratory are completely impermeable to micro-organisms as small as viruses.
However, the same studies show that condoms come off the penis altogether 3 to 5% of the time but may slip down (but not off) up to 13% of the time.
These margins of uncertainty...should represent an obligation on the part of the health ministries and all these campaigns to act in the same way as they do with regard to cigarettes, which they state to be a danger." Finding out the degree to which condoms protect against HIV is important both for HIV-negative people who want to protect themselves against HIV, and HIV-positive people who want to avoid transmitting it.
Knowing how well they protect against other STIs is important for sexual health in general and may be particularly important for people with HIV, who may be more vulnerable to the effects of certain STIs.The next problem is deciding what kind of study provides truly reliable evidence.It would be unethical to mount a randomised trial of condom use because the control group would have to stop using them altogether.Another is that the HIV-positive partner will be chronically infected and so will not have the very high viral load characteristic of acute HIV infection.Thirdly, in long-term serodiscordant relationships, studies have shown that the HIV-negative partner can acquire a degree of immunity to their partner’s HIV.In these circumstances, it is easy to see why condoms sometimes fail, even in consistent users.