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CDC Researchers Publish Estimate of Effectiveness of Condom Use for Gay Men


A paper published this month in the Journal of Acquired Immune Deficiency Syndromes finally makes publicly available a study-- originally reported nearly 2 years ago from the 2013 Conference on Retroviruses and Opportunistic Infections but until now unobtainable even as an abstract -- which gives an estimate for the effectiveness of 100% condom use as the strategy of choice for the prevention of HIV infection among gay men. The CDC researchers estimate that condoms used consistently stop 7 out of 10 HIV infections acquired through anal sex between men.

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This analysis, by Dawn Smith and other researchers from the Centers for Disease Control and Prevention (CDC), analyzes 2 historic studies of condom use and HIV incidence among gay men and other men who have sex with men (MSM) in the U.S. – one, VAX004, from 1998-1999, and the other, EXPLORE, from 1999-2001.

The analysis finds an overall effectiveness of 70.5% in both studies for all anal sex with a partner who has HIV. The effectiveness when the HIV negative partner is the receptive partner is slightly higher at 72.3%. This contrasts with an approximately 80% effectiveness of consistent condom use as a prevention strategy in several meta-analyses of studies of heterosexuals.

The difference between the MSM and heterosexual studies is not actually statistically significant, but Smith and colleagues argue that "it is more appropriate to use the MSM-specific point estimate of 70% effectiveness for discussions and models involving anal sex among MSM than to continue use of the heterosexual 80% effectiveness point estimate for MSM."

The Studies Analyzed

The VAX004 study was the first large-scale Phase 3 HIV vaccine study to be conducted in the U.S. and the EXPLORE study was the largest study ever conducted of a behavioral HIV prevention intervention for gay men. Taken together, 7725 men from both studies were eligible for this analysis. The average age of men in the 2 studies was 31 years, 82% were non-Hispanic white and 4.5% African American, and 76% had had some tertiary education.

These studies were chosen first because they are among the very few studies that report both self-reported condom use and HIV incidence in a large group of gay men longitudinally, or over time. Second, they were large enough, and had enough seroconversions, for the figures computed for the risk of HIV infection to be, in the main, statistically significant.

According to U.S. national surveillance data, the proportion of gay men who report using condoms consistently over the last year has fallen from 52% in 2005 to 43% in 2011. By comparison, in these 2 pre-2005 studies, the proportion of gay men who reported "always" using condoms in the 6 months before each study visit was 51%, and the proportion who "never" did was 22%. The remaining 27% reported "sometimes" using condoms. (As reported below, 100% condom use over the whole length of the studies was much less common.)

There was an element of "seropositioning" in the behavior of study participants. In terms of number of sex acts reported (rather than number of partners), study participants reported taking the insertive role 62% of the time. Whereas 30% of insertive sex acts involved participants who reported never using condoms during the previous 6 months, only 20% of acts of receptive anal sex did. The figures for "always" using condoms in the previous 6 months was 36% for insertive sex and 46% for receptive sex. The figures for "sometimes" using condoms were, however, the same regardless of sex role: 34%.

Study Differences

Condom effectiveness differed between the 2 studies. The effectiveness seen in the EXPLORE study was 86.2% for all sex and 86.8% for receptive sex. In contrast, the respective effectiveness figures seen in the VAX 004 study were only 61.3% and 64.1%.

Although participants in EXPLORE were on average younger, less likely to be white, better educated, and more likely to be either west coast or east coast residents than in VAX 004, these factors are controlled for in this analysis and cannot explain the difference seen in condom effectiveness between the 2 studies. The difference may possibly be due to the fact that the earlier study was a trial that assessed the behavior of gay men who were participating in a vaccine study while the second study was a trial of a behavior-change intervention. Although condom use only increased by 24.5% in this study and HIV infections decreased by 15.7%, it is possible that participants, especially in the intervention arm, acquired better condom-use skills during the 4 years of study follow-up.

The increase in condom use in EXPLORE was with partners of known HIV status, whether HIV positive (from about 60% to about 70% condom use) or HIV negative (from about 33% to 41%); condom use with unknown-status partners remained flat at about 67%. This could suggest that EXPLORE participants may have developed a bit more ease with disclosure and negotiation as they went along. In contrast, condom use declined in VAX004 for all partner HIV status categories (from about 67% to 53% with HIV positive partners and from 47% to 32% with negative partners.)

Consistent Condom Use and HIV Infections

The main comparison made by the researchers is between study participants who reported "never" using condoms during discrete 6-month periods of follow-up, and ones who reported "always" using them during those periods. The researchers also determined how many study participants had partners in each 6-month period that they knew to be HIV positive and looked specifically at condom usage rates with these partners.

Clearly the known HIV positive partners would not be all the partners with HIV that study participants had contact with. However, there is no way of knowing how many of their other partners whose HIV status was unknown or assumed to be negative in fact had HIV, and estimating this would not add clarity because it cannot take into account differences in individual participant behavior.

Combining figures from both trials, and excluding some participants such as those who reported sex with women or injection drug use, the researchers included 7725 men, of whom 3490 (45%) reported sex with a known HIV positive partner at any time during the studies. In total, there were 42,451 twice-yearly appointments attended, and sex with a known HIV positive partner was reported by participants at 22% of those visits. Among these men, 225 (6.4%) acquired HIV infection, with no significant difference between VAX004 (7%) and EXPLORE (6%); 2.5% of men per 6-monthly visit turned out to have acquired HIV since their last visit.

Among seroconverters, 14% in EXPLORE (19% for receptive sex) and 29% in VAX004 (38% for receptive sex) maintained that they had always used condoms with HIV positive partners in the 6-month period during which they acquired HIV; 42% and 30%, respectively, said they never had; and 46% and 33%, respectively, said they sometimes had. This, of course, says nothing about condom usage in the previous 6 months with partners assumed to be negative or of unknown status.

In terms of the absolute likelihood of catching HIV, the researchers calculated that the 100% consistent use of a condom transformed the per-act likelihood of HIV infection in EXPLORE from 1 infection per 286 sex acts for men who never used condoms to 1 in 1429 for men who always did so, and in VAX004, from 1 in 556 acts to 1 in 1000 acts. When only sex as the receptive partner was considered, the observed per-act transmission risk in EXPLORE was 1 in 90 acts without any condom use vs 1 in 667 acts with 100% use, and in VAX004 the figures were 1 per 244 vs 1 per 454 acts, respectively.

Intermittent Condom Use

One of the other headline findings of this study, however, was that "sometimes" using condoms was at best only marginally effective as a prevention strategy. While, as reported above, the effectiveness of "always" versus "never" condom use in the 2 studies overall was 70.5%, there was no significant difference in HIV acquisition rates between men reporting they "never" used condoms in the previous six months and men reporting they "sometimes" used them. There was a slight reduction in HIV incidence among men who reported receptive anal sex, but this approximately 25% reduction in HIV infections in men who "sometimes" versus "never" used condoms was not statistically significant, in either study.

It is important to note here that intermittent condom use was not stratified by frequency of use, so men reporting they "sometimes" used condoms could have been using them 90% of the time, or 10% of the time. Therefore, the above effectiveness figures for intermittent use do not mean that intermittently using condoms is never effective: it means that there is some point, as yet undetermined, between always and never using condoms, where they become ineffective as a strategy.

Long-term Consistency of Condom Use

The other headline figure in the study was that always using condoms proved to be very difficult for trial participants over the long term. Over a 3-year period (the length of the VAX004 study; the extra year’s data in EXPLORE was cut off at the 3-year limit) just 13% of participants maintained 100% condom use throughout for any anal sex, and 20% maintained condom use throughout as the receptive partner.

In fact, inconsistency of condom use was the most consistent behavior: 95.6% of participants used a condom at least once during the 3 years, so consistently never using condoms was rare. It is, however, notable that when only taking the receptive role is counted, 9% of participants never used a condom once throughout the study period when they were the receptive partner in anal sex.

Limitations and Conclusions


This study has a number of limitations. Its findings will no doubt continue to be debated and it is notable that the study includes a disclaimer saying that, "The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the CDC."

First, all reports of condom use, sexual activity and partner serostatus are self-reported. We know that participants in research trials are likely to under-report behavior that they think will be disapproved of (so-called "social desirability bias"). This would imply that condom use may have been lower than stated, which in turn would imply that condoms, when they were used, were more efficacious on an individual level than this study would imply.

Condoms used absolutely perfectly -- i.e., with only mechanical failure, not human error, contributing to failure -- are up to 99.5% effective, according to laboratory tests. What this study shows, though, is that even consistent use is hard to maintain over the long term, let alone perfect use. The CDC, in their own condom fact sheet, states that it is almost impossible to provide an estimate for the real-life effectiveness of consistent and correct use, "because of numerous methodological challenges inherent in studying private behaviors that cannot be directly observed or measured."

This study is also based on findings from only 2, very different, HIV prevention studies. However, its findings do agree with the only previous study done of the effectiveness of condoms in preventing HIV in gay men (Detels), which found the same figure for effectiveness -- 70%.

Partner serostatus was also reported by participants, and clearly some participants may have been infected by partners believed or assumed to be HIV negative. But since the comparison in this study is between always and never using condoms with partners known to have HIV, this should not make a strong difference to the condom efficacy seen.

The publication of this study in a journal may give additional impetus to the need to research and roll out methods of HIV prevention for gay men in addition to condoms.

"These data," say the researchers, "suggest the need to further intensify efforts to educate HIV-uninfected MSM and improve their ability to accurately assess both their risk of HIV acquisition and the effectiveness of their current use of condoms (consistently or inconsistently) during anal sex."

In addition, they add, their data also suggest "a need to provide access to additional highly-effective HIV prevention tools and strategies, including more frequent HIV testing (including testing as couples) and daily oral antiretroviral pre-exposure prophylaxis (PrEP) when indicated."



DK Smith, JH Herbst, XJ Zhang, and CE Rose. Condom Effectiveness for HIV Prevention by Consistency of Use among Men Who Have Sex with Men (MSM) in the U.S. Journal of Acquired Immune Deficiency Syndromes. December 2, 2014 (Epub ahead of print).

R Detels, P English, BR Visscher, et al. Seroconversion, sexual activity, and condom use among 2915 HIV seronegative men followed for up to 2 years. Journal of Acquired Immune Deficiency Syndromes 2(1):77-83, 1989.