Challenges in determining condom efficacy

These degrees of protection may be lower than some readers expect, and rates of 98% reliability are still sometimes quoted for condoms. 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 do not become pregnant if condoms are used perfectly, meaning that they are used consistently and correctly at every act of sexual intercourse.¹

However because they are not always used correctly even if they are used consistently, studies have found efficacy rates of 85 to 87% when young women use condoms as their sole form of contraception.² This contraceptive efficacy compares with an estimated:

• 92.5% for contraceptive pills
• 96.5% for implants and intrauterine devices
• 99% for the pill and condoms used together
• 99.8% for vasectomy,

but only

• 77% for the rhythm method (calendar-based method)
• 75% for withdrawal, and
• 72% for spermicides.

Condoms are, however, the only method on that list that has been shown to protect against STIs as well as pregnancy.

Laboratory studies and product testing have shown that reputable condoms tested in the laboratory are completely impermeable to micro-organisms as small as viruses. Research has also found that during vaginal intercourse condoms break less than 2% of the time³ and during anal intercourse less than 4% of the time.⁴

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. In these circumstances it is easy to see why condoms sometimes fail, even in consistent users.

In addition, however, people are not consistent in their use of condoms, and may not even be consistent when they claim to be, or think they are. Research has shown that 40 to 70% of men who claim they use condoms 100% of the time in fact do not use them for every act of intercourse.

The first question we have to answer, then, in assessing condom efficacy, is whether we are talking about their efficacy in perfect use, consistent use, or typical use.

Perfect use sets too high a standard for individual behaviour, and measuring typical use is more about studying what motivates people to use condoms than whether they work. Indeed there is a convention to use two different words when describing the effect of prevention interventions for these reasons. Condom efficacy  is how well they work when people use them as indicated, i.e. consistently; condom effectiveness is how well they actually work to curb the spread of HIV in a given population, given actual levels of use.

The studies that have been done of condom efficacy have therefore largely contrasted HIV and STI incidence or prevalence in people who claim 100% consistent use and 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 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. 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. 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 sexual partners. For all these reasons, large studies may be needed to establish differences in HIV (and HSV and HPV) incidence between condom users and non-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 catch 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-reporting 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.