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.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.Consistently used condoms provide significant protection against HIV, pregnancy and sexually transmitted infections (STIs).The degree of protection they offer against HIV and STIs is significantly better than any other single prevention method, taken in isolation, other than sexual abstinence or complete mutual monogamy between two people who have tested negative for HIV.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.
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.
Women were much less likely to report inconsistent use of condoms than never using them: over the course of the study, 46% of women said they used condoms ‘always’, 48% ’never’ and only 6% ’sometimes’.
For the reasons described above, there is a convention to use two different words when describing the effect of prevention interventions.
In these circumstances, it is easy to see why condoms sometimes fail, even in consistent users.