Condom effectiveness in reducing heterosexual HIV transmission

It is logistically and ethically impossible to conduct randomized controlled trials to find out whether condom use reduces the risk of HIV transmission. Hence, we have to rely on observational studies, which inherently carry a risk of bias. In such studies, consistent use of condoms has been found to result in an 80% reduction in HIV incidence.

RHL Commentary by Wilkinson D


This Cochrane review is an excellent attempt to estimate the effectiveness of condom use in reducing heterosexual transmission of HIV. While several studies have looked at this issue, many of these have been small, and the reviews done to date have had a number of methodological problems associated with them.

The authors of the Cochrane review identified and selected 14 studies involving discordant couples (i.e. couples in which one of the partners is HIV-positive and the other free from HIV). A total of 587 people who reported "always" using condoms during sexual intercourse, and 276 people who reported "never" using condoms were included in these studies.

The incidence of HIV infection among those who reported always using condoms was 1.14 per 100 person-years (95% confidence interval 0.56-2.04), while it was 5.75 per 100 person-years (95% CI 3.16-9.66) among those who never used them. This gave an 80% reduction in the incidence of infection with condom use.

It is important to note, however, two important limitations: (a) the meta-analysis was done using data from observational studies; and (b) the authors did not provide confidence intervals for their estimated effect of 80%.

This is a well done review. As the authors correctly point out, it is logistically and ethically impossible to do randomized controlled trials to find out whether condom use reduces the risk of HIV transmission. As such, we have to rely on observational studies. The studies used in this review typically compare rates of HIV infection in groups of people who report always or never using condoms. Observational studies inherently carry a risk of bias as people make choices for reasons, and if the choice about using condoms or not is related to other risk factors for HIV, then the estimate of effect that we get will be inaccurate.

So, in this study, in spite of the best efforts of the authors, we can’t be sure that condoms really reduce HIV incidence by 80%. It may be, for example, that people who use condoms always also practice safe sex and have fewer partners, thereby reducing their HIV risk irrespective of condom use. It might also be that some people who report always using condoms do not actually do so but tell the researchers that they do in order to “look good”. Similar points could be made about people reporting not using condoms. Self-reported data always have the risk of being unreliable; and in this case there is no other means of confirming the findings.

If we take the extreme values from the 95% confidence intervals of the HIV incidence rates reported in the two cohorts, we find that condom use may be associated with reduced HIV incidence of between 94% and 35%. The wide range of values is explained by the small numbers of people (863) and HIV infections (51) in the studies. Concurrent use of other measures could also contribute to this variation. As a result, the estimate of the incidence of HIV in the two groups is fairly imprecise (between 0.56 and 2.04 per 100 person-years for users, and between 3.16 and 9.66 per 100 person-years for non-users).

This lack of precision is not the fault of the authors and their review. It is an inherent feature of the available studies. As such, it is not really appropriate to estimate condom effectiveness at 80%. While 80% is the best single estimate of effectiveness, it is also fair to say that the true measure of the effect could be as low as 35% or as high as 94%, as the authors state in the review.


2.1. Magnitude of the problem

This review is of high relevance to under-resourced settings. The very high rates of HIV infection, and sexually transmitted infections (STIs) that contribute to HIV transmission, have been well documented in much of sub-Saharan Africa (levels of HIV around 30% in some parts), Asia and the rest of the developing world. Another related problem is the low and inconsistent use of condoms in most of the affected areas.

2.2. Feasibility of the intervention

The widespread, correct and consistent use of male condoms is probably the most feasible HIV prevention strategy available. Compared with other health strategies that require doctors, nurses, health workers, clinics, fridges, needles and so on, condoms can be safely and effectively promoted, distributed and used. Yes, there are barriers—including cultural and social ones—but there can be no denying the ready availability of an intervention that provides substantial protection.

2.3. Applicability of the results of the Cochrane Review

The cost of condoms is low, and various social marketing projects and programmes have shown widespread distribution to be feasible. Without doubt different difficulties will be faced in different locations, but local expertise and interest will overcome these.

2.4. Implementation of the intervention

Condom distribution already occurs in many different ways through most of the developing world, using a wide range of networks ranging from formal health sectors, through social marketing efforts, and also the private sector. These efforts need to be redoubled.

2.5. Research

Perhaps the key focus for future research needs to be on how best to increase distribution and uptake of condoms in as many different locations as possible, rather than on further refinement of knowledge of the actual effectiveness of condoms.

This document should be cited as: Wilkinson D. Condom effectiveness in reducing heterosexual HIV transmission: RHL commentary (last revised: 11 November 2002). The WHO Reproductive Health Library; Geneva: World Health Organization.

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