For the reasons described above, there is a convention
to use two different words when describing the effect of prevention
interventions. 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.
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.
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.
The evidence we have is based on three types of trials, and each has potential
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. 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
One disadvantage is that condom use in long-term
relationships, even in serodiscordant couples, is relatively rare. 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.
For these reasons, HIV transmission within long-term serodiscordant
relationships, especially heterosexual ones, may be rarer than it is between
casual sex partners. For all these reasons, large studies may be needed to
establish differences in HIV (and HSV and HPV) incidence between condom users
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. There is opportunity for
qualitative research too, contrasting attitudes and drivers of behaviour
between people who become infected with HIV or other STIs and those who do not.
Condom efficacy against acute STIs can also be measured, if people have
multiple partners, or their partners do.
The weaknesses of this kind of study include the fact
that condom use cannot be corroborated by partners, so self-report is likely to
be even more unreliable. Because behaviours change over time, it can be
challenging to decide whether people really are ‘consistent’ users and the pool
of consistent users will shrink over time; and, like all cohort studies, results
are prone to be confounded by participant characteristics that were not
monitored. A study that measures HIV incidence in condom users and non-users
will be confounded, for instance, if one group has substantially fewer sexual
partners than the other.
For this reason and because HIV seroconversion even in
high-risk populations is a relatively uncommon event, prospective cohort
studies have to be large and can be quite costly.
A third kind of study is to conduct a retrospective
cohort study, asking people about their condom use and contrasting HIV and STI
prevalence in users and non-users. Retrospective cohort studies are subject to
greater limitations that prospective ones. Participants’ recall of behaviour is
often inaccurate; the studies may rely on medical records that may omit
significant demographic and medical information; and it can be difficult to
determine retrospectively people’s degree of STI exposure.
For all these reasons, measuring the efficacy of condoms
(or indeed other established prevention methods and strategies such as
serosorting) can be challenging. Nonetheless, a number of carefully conducted
studies have demonstrated that consistent condom use is a highly effective
means of preventing HIV transmission.
When it comes to STIs other than HIV, most epidemiologic
studies of these are characterised by methodological limitations, and thus, the
results across them vary widely - ranging from demonstrating no protection to
demonstrating substantial protection. However, we now have enough evidence to
demonstrate that condoms offer at least some and in some cases excellent
protection against most STIs.