APPROXIMATELY 19 MILLION CASES of sexually transmitted infection (STI) occur in the United States each year.1 For sexually active persons, male latex condoms are the most commonly used contraceptive method to prevent STIs.2 When properly used, latex condoms prevent contact with semen; genital lesions and infected skin on the penile head and shaft; and penile contact with vaginal, oral, and anal discharges. In vitro studies indicate that latex condoms provide an effective mechanical barrier to passage of infectious agents comparable in size to or smaller than STI pathogens.3–11
Epidemiologic studies show that condoms are highly effective against HIV transmission. Recent meta-analyses of studies of discordant couples indicate consistent condom use reduces HIV risk by at least 80% to 94%.12–14 In contrast, risk reduction associated with condom use has been weaker and less consistent in studies of other STIs.15–22 This inconsistency was highlighted in a 2001 report by the National Institutes of Health17 and has been used by some to question the role of condoms as an STI prevention strategy.23,24
Biologic differences among STIs may contribute to differences in the level of protection that condoms afford. For example, condoms should be more effective against infections that are transmitted primarily from penile fluids (e.g., gonorrhea, chlamydia, trichomoniasis, hepatitis B virus, and HIV) than those also transmitted through contact with skin or mucosal surfaces in areas that may not be covered or protected by the condom (e.g., genital herpes, HPV, syphilis, and chancroid).2,15
Condom effectiveness against different STIs also could vary because of differences in the risk of transmission per sexual contact.25–28 The probability of acquiring infection from a single sexual act depends both on the population prevalence of infection25,29 and infectivity of the STI. Although not well-determined for most STIs, estimates of transmission risk per unprotected sex act with an infected partner are approximately 0.001 for HIV,30,31 0.20 to 0.50 for gonorrhea,32,33 0.45 for chlamydia,34 and 0.70 for chancroid.35 If condoms are less than 100% effective or are not used properly, the cumulative risk of infection for both condom users and nonusers will increase more rapidly for relatively infectious STIs than for those less infectious.26,28,36 Consequently, studies comparing users and nonusers will demonstrate less protection for highly infectious STIs (such as gonorrhea) than for less infectious STIs (such as HIV) even when the number of exposures to an infected partner is identical. Differences in numbers of sex acts occurring with infected partners also could explain variations in condom effectiveness estimates across studies against a specific STI, particularly one with high infectivity (e.g., gonorrhea). Specifically, studies with short observation periods may provide higher effectiveness estimates for condom use (i.e., more suggestive of protection) than studies of longer duration, simply because the number of sex acts occurring with infected partners is lower. This lack of comparability can be minimized by converting the estimate from a proportion to a rate.14
Differences in how STI exposure, condom use, and infection are ascertained may further complicate comparisons of condom effectiveness estimates across studies.15,17–22,29,36–55 Except for human papillomavirus18,19,54 and HIV,12–14 there has been little systematic examination of these factors on condom effectiveness estimates for most STIs.
Although gonorrhea and chlamydia are transmitted almost exclusively through sexual contact and should be easily preventable with proper condom use, previous reviews suggest highly variable risk reduction from condoms against these 2 infections.16,17,21 In this review, we systematically evaluate critical elements of design and measurement in studies of condom use and risk of gonorrhea and chlamydia and assess whether variations in these elements explain differences observed in condom effectiveness estimates.
We searched MEDLINE and article bibliographies to identify English-language, peer-reviewed epidemiologic studies published from January 1966 to April 2005 that assessed the association between male condom use and risk of gonorrhea or chlamydia. Articles were identified using the medical subject heading terms “condoms,” “contraceptive devices, male,” “gonorrhea,” “chlamydia,” and “sexually transmitted diseases, bacterial.” To be eligible, studies had to assess condom use during penile–vaginal intercourse; assess Neisseria gonorrhoeae or Chlamydia trachomatis infection using culture, antigen detection, or polymerase chain reaction (PCR); have a control or comparison group that included condom nonusers; and report a qualitative or quantitative association for the effect of condom use or provide data from which this association could be calculated. Two authors (LW and KMS) reviewed each study to determine eligibility.
Studies were grouped by type of STI and sex of participants. For each, we examined the study design, population or setting, sample size, measure(s) of condom use, duration of recall or follow up, confounders examined in multivariable analyses, and measure(s) of association between condom use and risk of infection. We reported point estimates (odds ratios, risk ratios, or hazard ratios) and 95% confidence intervals from the original source whenever possible; in some situations, we calculated measures of effect and their precision from data presented in the original article. If measures of effect were reported (or calculable) for multiple measures of condom use (e.g., “always use vs. never use” and “sometimes use vs. never use”), we gave preference to comparisons incorporating the category with the highest degree of consistent use. When the highest degree of consistent use was contrasted with multiple levels of less frequent use, we presented multiple comparisons. For comparisons across studies, we presented both unadjusted and adjusted estimates whenever possible; however, only unadjusted estimates were available for many studies. We considered study findings to show statistically significant differences between condom users and nonusers if the 95% confidence interval for the effect measure excluded the null value. Because this review was intended to assess findings and examine methodologic limitations across studies, we followed recommendations of others56,57 to consider studies individually and not aggregate studies for metaanalysis.
We assessed 4 key design or measurement elements by determining whether each study:
1. Distinguished consistent from inconsistent condom use; combining consistent users with inconsistent users introduces one type of nondifferential misclassification that could underestimate condom effectiveness, particularly when examining STIs with high infectivity;
2. Measured correct use and/or condom use problems; combining correct with incorrect users could introduce bias resulting in underestimation of condom effectiveness similar to that produced by combining consistent and inconsistent users. We evaluated whether studies reported on problems with condom use resulting in STI risk, including breakage or slippage during intercourse or withdrawal, application of condoms after onset of genital contact, and removal of condoms before ejaculation;
3. Used a study design that distinguished incident (new) infection from prevalent (pre-existing) infection; unlike cohort studies that assess incident infection, cross-sectional and most case–control studies do not permit determination of whether persons became infected before or after initiating condom use. Misclassifying the temporal sequence between condom use and infection could underestimate the effectiveness of consistent condom use; and
4. Selected a study population with documented exposure to gonorrhea or chlamydia during the time period pertaining to condom use; condoms can prevent transmission of STI only among persons exposed to an infected partner. Differences in STI exposure between condom users and nonusers can confound associations between condom use and infection.47,48,58,59 If condom users are more likely to choose sex partners at relatively low STI risk, the protective effect of condom use could be overestimated.59 Conversely, if condom users are more likely to encounter infected partners, as suggested by studies of sexually transmitted disease (STD) clinic patients, 44,60–62 the protective effect could be underestimated.47,48
We also considered other potentially important design and measurement factors (e.g., validity of self-reported condom use, sensitivity and specificity of diagnostic tests); however, a preliminary review of articles revealed it was not feasible to examine these factors across studies because of inadequate data. We refer readers elsewhere for discussion of these topics.29,38,39,41,43,45,52,55,63,64
Associations Between Condom Use and Infection
Forty-five studies47,48,52,60,65–105 met all eligibility criteria: 10 evaluated gonorrhea only, 19 evaluated chlamydia only, and 16 evaluated both infections individually (12 studies) or as a combined outcome (4 studies). Study populations consisted of attendees at STD clinics (n = 20), family planning clinics (n = 3), teen clinics (n = 3), college health clinics (n = 2), and other medical clinics (n = 1), or multiple clinic types (n = 5); commercial sex workers (n = 6); military recruits (n = 2); vocational school students (n = 1); and community members (n = 2). Studies used cross-sectional (n = 27), cohort (n = 13), and case–control (n = 5) designs. These 45 studies yielded 87 individual point estimates of the association between condom use and risk of gonorrhea or chlamydia after stratification by STI and sex.
Condom use was associated with reduced risk for gonorrhea in most studies; however, point estimates for the magnitude of protective effect varied widely and were not always statistically significant (Table 1 Fig. 1A, B). Eight studies evaluated gonorrhea among males, 6 studies69,79,92,98,100,101 of which reported (or provided information to calculate) risk reduction associated with condom use ranging from approximately 30%100 to 100%.79 Three of the 6 studies found statistically significant protective effects for condoms,69,98,101 whereas 3 reported protective effects that were not statistically significant.79,92,100 The remaining 2 studies52,87 reported no statistically significant association between condom use and gonorrhea, but did not provide sufficient information to quantify the association.
Sixteen studies evaluated condom use and gonorrhea among females. Fourteen67,68,74,76,77,80,84,85,87,93,96,97,100,101 reported (or provided information to calculate) risk reduction associated with condom use. Of these, 11 reported condom use reduced gonorrhea risk by 13%67 to 100%.74 In 9 studies,68,74,76,80,84,85,96,97,100 the protective effect reported was statistically significant. Three other studies77,87,101 reported no effect101 or increased gonorrhea risk among condom users, ranging from 60%87 to 120%.77 The 2 remaining studies52,89 reported no significant association, but available information was insufficient to quantify this association.
One additional study73 that evaluated gonorrhea in both males and females but did not report results by sex found condom use reduced gonorrhea risk by approximately 30% but was not statistically significant.
Condom use was similarly associated with reduced risk for chlamydia in most studies; however, like gonorrhea, point estimates varied widely and were not always statistically significant (Table 1, Fig. 1C, D). Of 8 studies that evaluated condom use and chlamydia among males,52,88,91,94,98,99,101,105 all reported (or provided information to calculate) estimates of risk reduction associated with condom use. Seven studies52,91,94,98,99,101,105 reported a protective effect, ranging from approximately 15%105 to 100%52 risk reduction. Three of these 7 studies52,94,101 reported a statistically significant protective effect. The remaining study88 showed an increased risk of chlamydia among condom users that was not statistically significant.
Of 27 studies that evaluated chlamydia among females, 21 reported (or provided information to calculate) estimates of risk reduction associated with condom use.65,68,70–72,74,76–78,80,81,83,86,89,91,95–97,101,103,104 Eighteen studies65,68,70–72,74,76,78,80,81,86,89,95–97,101,103,104 showed a protective effect, ranging from approximately 10%70,96,97 to 90%65,74 risk reduction. One or more statistically significant protective effects for condom use were reported in 10 studies,65,68,72,74,76,78,81,95,101,104 whereas 8 reported no significant protective effect.70,71,80,82,86,96,97,103 The other 3 reported condom use was associated with nonsignificant increased risk of chlamydia.77,83,91 For the remaining 6 studies,52,75,89,93,94,102 information was insufficient to quantify the association between condom use and chlamydia, although this association was reported as not statistically significant.
One additional study60 that evaluated chlamydia in males and females but did not stratify results by sex found condom use was not associated with risk of infection.
Gonorrhea and Chlamydia (Combined).
Four studies47,48,66,90 reported associations for gonorrhea or chlamydia as a combined outcome. Three reported that condom use was associated with statistically significant reductions in risk,47,48,66 ranging from approximately 50%66 to 58%,47 whereas the other90 reported a nonsignificant increased risk.
Assessment of Key Design and Measurement Factors
Consistency of Use.
Twenty-eight (62%) studies47,48,52,60,65,66,68–70,72–76,78,82,84,89–91,93,94,97–99,101,102,105 clearly distinguished participants who reported using condoms consistently from those who did not (Table 1). These studies categorized the highest frequency of self-reported condom use with terms such as “always use,” “consistent use,” or “100% use.” The remaining 17 studies67,71,77,79–81,83,85–88,92,95,96,100,103,104 categorized the highest frequency of self-reported condom use using terms such as “always or almost always use,” “sometimes or always use,” “ever use,” “any use,” or “use at last sex,” indicating that consistent users were likely grouped with inconsistent users. The proportion of studies that found one or more statistically significant protective effects did not differ between studies that distinguished consistent users from inconsistent users and those that did not (64% vs. 47%, P >0.20).
Correctness of Use and Condom Use Problems.
Only 2 studies48,69 assessed whether incorrect use or use problems occurred among participants, although neither fully assessed use problems that may result in STI exposure. One cross-sectional study of male STD clinic patients in London69 found gonorrhea prevalence was 4 times lower among those who reported using condoms consistently throughout intercourse with no unprotected genital contact than among those who used condoms incorrectly, inconsistently, or not at all (3.8% vs. 15.9%). Similarly, a large cohort study48 of female STD clinic patients in Alabama found the incidence of gonorrhea and chlamydia was lower among patients who used condoms consistently without breakage or slippage compared with patients who used condoms consistently but experienced these problems or who used condoms inconsistently or not at all. In the remaining 43 studies, correct users were likely grouped with incorrect users during analysis. In one smaller study73 of STD clinic patients in California, researchers did reinterview 2 participants who acquired gonorrhea despite reporting always using condoms, finding that both had applied the condom after starting intercourse.
Choice of Study Design.
Thirteen studies (29%) used prospective48,52,68,72,73,76,79,83,85,87,89,103 or retrospective cohort60 designs to distinguish incident from prevalent infection and could establish whether condom users acquired gonorrhea or chlamydia before or after they reported initiating condom use. Eight of these studies48,52,60,68,72,73,76,89 also assessed whether condoms were used with every sex act to document that infection occurred after onset of consistent condom use. The proportion of studies that found one or more statistically significant protective effects for condom use did not differ statistically between those that assessed incident versus prevalent infection (46% vs. 59%, P >0.20).
Selection of a Population With Gonorrhea or Chlamydia Exposure.
Only one study47 identified a population of participants who had documented exposure to infected sex partners during the same time period for which information on condom use was ascertained. In a cross-sectional analysis of data from STD clinic patients participating in Project RESPECT,106 the protective effect for consistent condom use was stronger among participants with known exposure to gonorrhea or chlamydia (who were referred to the clinic because they had recent sexual contact with an infected person) than among participants with unknown exposure to these infections (58% vs. 18% risk reduction, respectively).
The remaining 44 studies did not identify a population with documented exposure to gonorrhea or chlamydia. Twenty-three studies (51%),48,60,66–68,72,77,78,84,85,87,89,91,94–102,104 did attempt to adjust condom effectiveness estimates for differences in STI risk during analysis by using surrogate markers for STI exposure (including number of sex partners, previous STI diagnosis, number of sex acts, type or perceived risk of partners, and treatment status of partners). The proportion of studies that found one or more statistically significant protective effects was statistically higher between those that adjusted for surrogate markers and those that did not (74% vs. 41%, P = 0.03).
None of the 45 studies fully addressed all 4 factors, one (2%) addressed 3 factors, 9 (20%) addressed 2 factors, 23 (51%) addressed one factor, and 12 (27%) addressed none of these 4 factors (Table 2). The proportion of studies that found one or more statistically significant protective effects for condom use was higher among the 10 studies that examined at least 2 factors than among the 35 studies that examined either zero or one factor (80% vs. 43%, respectively, P = 0.04).
Despite methodologic limitations that tend to underestimate condom effectiveness, most studies in this review found that condom use was associated with reduced risk of gonorrhea and chlamydia in men and women. The magnitude of risk reduction varied widely, however, and was lower than expected given the physical barrier properties of condoms and route of transmission for these infections. All 45 studies had one or more methodologic limitations. Only 28 studies clearly distinguished self-reported consistent from inconsistent use of condoms, 2 assessed whether any type of incorrect condom use or condom use problems occurred, 13 distinguished incident from prevalent infection, and only one included a study population with documented exposure to infection. These inadequacies generally result in underestimation of condom effectiveness.18,20,21,43,45,47,48,52,54,55,60,61
Our findings highlight the general methodologic weakness of studies assessing condom use and risk of gonorrhea and chlamydia compared with studies of HIV, as previously emphasized in a National Institutes of Health report on condom effectiveness17 and elsewhere.15,21,47,48 No studies included in our review assessed the effect of consistent condom use on risk of incident gonorrhea or chlamydia among persons with documented exposure to infection, whereas nearly all studies included in meta-analyses of condom use and risk of HIV12–14 addressed these elements.
Statistically significant protective effects for condom use were more likely to be observed for studies addressing 2 or more design or measurement attributes compared with studies with none or only one of these attributes. This finding is noteworthy because much of the inconsistency in estimates of condom effectiveness has been attributed to inadequacies and variations in study measures, methods, and design. Our review provides empiric support that studies with weak designs to assess condom effectiveness generally underestimate effectiveness, thus contributing to the variation in estimates reported in the literature. This finding is tempered, however, by our inability to directly assess how the magnitude of protective effect related to the presence or absence of specific attributes apart from study sample size or power to detect a statistically significant difference.
Individually, none of the attributes we examined was associated with the likelihood of detecting statistically significant protective effects for condom use across studies. This finding likely resulted from poor measurement of individual attributes, simultaneous confounding from other methodologic limitations, and inadequate statistical power to detect this difference. For example, although the majority of studies assessed consistent use, reports that condoms were used “consistently” may not indicate use during every act of intercourse58,101,107,108 Self-reports of condom use also may be inaccurate,17,21,29,38,43,45,51,52,55,109,110 with overreporting being of greater concern than underreporting because of social desirability, which could lead to underestimation of effectiveness.
Only 2 studies in our review provided any information on incorrect use and condom use problems. In these studies, failure to use condoms throughout genital contact69 and breakage and slippage48 were predictive of infection. Similar findings regarding condom problems and the occurrence of STI have been reported in other studies.111,115,118,119 The general lack of information on condom problems was disconcerting in this review, given the high infectivity of these infections and evidence that problems occur commonly. For example, a cross-sectional study49 of 98 university men found that condoms broke, slipped off, or were not used throughout intercourse during 13% of uses in the previous month. Subsequent studies also have suggested a high frequency of these condom use problems111–119.
Notably only one study47 assessed directly whether participants had documented exposure to gonorrhea or chlamydia, and thus whether condom users and nonusers had comparable risk of infection. That study demonstrated that failure to adjust for differences in STI risk between condom users and nonusers led to substantial underestimation of condom effectiveness. Another study48 that used design techniques to approximate STI exposure revealed similar findings. These studies demonstrate that, although rarely done, populations of persons with known exposure to gonorrhea or chlamydia can be identified. As important, they illustrate how the protective effect for consistent and correct use can be masked when the critical variable, the infection status of the sex partner, is unknown.
The majority of studies did attempt to assess exposure indirectly by using surrogate markers for STI exposure. We also found studies that adjusted for surrogate markers were more likely to detect statistically significant protective effects for condom use than those that did not. These surrogates, although frequently associated with the infection status of study participants, may still be unreliable markers of the partner’s infection status.37 For example, there is no risk associated with having multiple partners if none are infected yet significant risk with having a single partner who is. In one analysis of condom effectiveness estimates,47 adjusting for surrogate markers of infection reduced confounding negligibly compared with restricting analyses to persons with known STI exposure. Failure to assure that condom users and nonusers were similarly exposed to infected partners could lessen the general validity of estimates of condom effectiveness.17,45,47,48,58
The possibility of differential exposure to infected partners between condom users and nonusers remains a somewhat major limitation in studies of curable STI.8,17,37,42,47,48,114,117 Recent studies have documented that people use condoms more often with partners who are likely to be infected with STI and less often with partners at low risk.44,60–62 A retrospective cohort study60 of 3568 STD clinic patients in Denver found, for example, that consistent and inconsistent condom users were both significantly more likely than nonusers to report having multiple partners (30% and 39%, respectively, vs. 18%) or a new partner (48% and 48%, respectively, vs. 26%). A prospective cohort study62 of 869 female STD clinic patients in Alabama also found that condom use was 60% higher among those with new or casual partners than among those with regular partners. If condom users as a group are at higher risk for STI than nonusers, condom effectiveness could be underestimated.
Theoretical solutions to the methodologic problems of differential exposure to STI are straightforward but typically unethical in practice. A randomized study in which participants were assigned to use or not use condoms with their prospective sex partners would generally be considered unethical17 except under very limited conditions.120,121 Prospective cohort studies of persons in relationships with infected partners (i.e., discordant couples) also would reduce differences in exposure between condom users and nonusers. Although this design has been used to evaluate condom effectiveness against chronic, incurable STIs such as HIV12–14 and herpes simplex virus,122 it would be unethical for curable STIs such as gonorrhea and chlamydia because infected persons must be treated promptly.15,17,45,47
This review updates previous reviews of gonorrhea and chlamydia16 that were less systematic17 or less focused on these 2 infections.21 By examining 2 STIs with high biologic plausibility for condom effectiveness, we could better assess the effect of design and measurement factors on observed estimates. Our findings extend those reported from a workshop on condom effectiveness led by the National Institutes of Health17 by examining a substantially larger number of studies for gonorrhea and chlamydia and by systematically reviewing key design and measurement factors for each. Our findings generally support arguments that better designed studies may be more likely to document protective effects for condom use.
The limitations of our review generally reflect limitations among the available studies. For example, the time period for observation varied widely across studies (from 4 days to 1 year) and was unspecified in more than one third of studies, thereby complicating our ability to evaluate specific design and measurement factors. We also could not assess the relative importance of each of the 4 factors examined because so few studies examined more than one. We suspect, however, that knowledge of STI exposure likely is the single critical factor through which the importance of other factors is demonstrated. Although we limited our review to 4 factors that influence effectiveness, other limitations not examined (e.g., potential inaccuracy of self-reported condom use, imperfect performance of diagnostic tests)38,52,55,58,61 also may contribute to underestimates of condom effectiveness. Lastly, by focusing on only these 4 factors, our review likely missed other strengths of individual studies. For example, one highly recognized cohort study79 of gonorrhea transmission among sailors who had had intercourse with sex workers while on shore leave demonstrated strong protection for condoms yet did not report on consistent use, correct use, or exposure to infection. Those limitations may have minimally impacted observed effectiveness because of other strengths, notably the study’s short duration and recall for condom use and the fact that all sailors were exposed to the same group of sex workers (and presumably same risk of infection).
Conclusions and Recommendations for Future Research
Most studies showed that condom use was associated with reduced risk for gonorrhea and chlamydia among men and women. However, the exact magnitude of risk reduction is difficult to quantify because of limitations and variations in the methods and design of these studies. We focused on 4 design and measurement elements minimally necessary for obtaining meaningful estimates of condom effectiveness against gonorrhea and chlamydia: measurement of consistent use and correct use, and assessment of incident infection and exposure to infection. Few studies addressed 2 or more factors simultaneously and none addressed all 4.
Finally, this review highlights recent advancements in design and measurement as well as areas to be addressed by future evaluations of condom effectiveness, especially for STIs with high infectivity. The 2 largest issues remain how best to measure condom use and how best to identify study populations with documented exposure to infection. Investigators can assess consistency and correctness of use with a brief module of questions, provided that other critical issues (e.g., exposure to infection) are adequately addressed. Consistency of use should be assessed during a time period appropriate for the population and STI under study (e.g., last 3 or 6 months)29,38,109 that also coincides with the period in which STI status is examined.51 Consistent use should be defined as use during every act of intercourse.38 Correctness of use and use problems should be limited to condom behaviors that result in direct STI exposure risk.49 Additional research is needed to determine whether the most appropriate referent category for consistent and correct use is nonuse as opposed to inconsistent/nonuse or the number of unprotected sex acts. Investigators should continue efforts to document the validity of self-reported use, given direct observation of condom use is not feasible. Recent advances in data collection may facilitate accurate reporting of condom use by increasing participant willingness to report less socially desirable behaviors and by facilitating recall of condom use and sexual behavior.29,38,41 New biologic markers that detect the presence of semen or other male genital fluids in the vagina42,45,123,124 also hold promise for documenting use or nonuse of condoms.
Documentation of exposure to infection, absent from all but one study in this review, remains critical for improving the accuracy of condom effectiveness estimates. Condom use and STIs should be assessed during the same time period when STI exposure occurred. Previously suggested prospective cohort designs for assessing condom effectiveness against curable STI,38 which can detect incident infection and somewhat minimize differences in STI exposure, still cannot ensure adequate measurement of exposure during the relevant time period; even in this situation, participants may choose to use condoms based on their partner’s risk. Limiting the study population to persons exposed to infection ensures that participants are at risk for STI during the actual period of observation, regardless of condom use. Recent evaluations of condom use and risk of gonorrhea and chlamydia47,48 suggest that populations likely exposed to these infections during the period of condom use assessment can be identified efficiently and inexpensively without requiring prohibitively large sample sizes.
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