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.Research early on in the epidemic showed that 40 to 70% of men who claimed they use condoms 100% of the time in fact did not use them for every act of intercourse.Twenty years later, 51 to 66% of women taking part in a microbicide study reported 100% condom usage at different time points during the trial, but the researchers calculated from inconsistency in their answers that the actual proportion who maintained 100% condom use was 25%.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.Moral questions about condom use are not within the remit of this resource, but questions of fact are, and condoms’ ability to stop HIV is periodically questioned by people opposed to their use on religious or moral grounds.Therefore questions of condom efficacy have to be addressed and misapprehensions corrected.
These are based upon observations of their use in contraception: studies have shown that 98% of women relying on condoms as their sole form of contraception remain pregnancy free if condoms are used perfectly, meaning that they are used consistently and correctly at every act of sexual intercourse.
Studies of condom efficacy have therefore largely contrasted HIV and STI incidence or prevalence in people who claim 100% consistent use against people who use them inconsistently or not at all.
Because these studies involve private behaviours that investigators cannot observe directly, it is difficult to determine accurately whether an individual is a condom user and whether condoms are used consistently and correctly.
These can be done in individuals whose characteristics are known and can be controlled for, and if the relationship truly is monogamous then infections by acute STIs and from outsiders can be ruled out.
One disadvantage is that condom use in long-term relationships, even in serodiscordant couples, is relatively rare.