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 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.
The efficacy of an intervention is how well it works in a scientific trial or when people use it as indicated, i.e.
consistently; its effectiveness is how well it actually works to prevent disease or infection in a given population, given actual levels of use.
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.
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%.
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.