We used separate logistic regression models to assess the relationships between condom use and 1) prevalent STI at enrollment or 2) incident STI during follow-up. Outcome measures consisted of composite STI variables for prevalent infection at enrollment or incident infection at follow-up based on having a positive diagnosis for at least 1 study STI (chlamydia, gonorrhea, and/or trichomoniasis) versus no STI. We assessed the following potential confounders: randomization group; age (>median age of 26 yr vs. ≤26 yr); union status (married or cohabiting vs. not); education (less than high school vs. more); weekly income (<2000 Jamaican dollars vs. more); coital frequency (continuous variable); recent condom malfunction (≥1 breakage or slippage in past week vs. none); having a nonmain sex partner in past week; and having a prevalent STI at enrollment (assessed for condom use and incident STI model only).
We fitted full logistic regression models for each exposure classification with all potential confounders and then performed backward elimination of covariates that did not result in more than a 10% absolute change from the crude exposure estimate. In the models where exposures were measured during follow-up, we used generalized estimating equations with the exchangeable correlation structure to account for multiple visits from individual participants. Based on these methods, the only confounders in any of the models were having a nonmain partner and coital frequency during the past 7 days. To improve the comparability of estimates, we included these confounders in all final models. We did not assess effect modifiers because insufficient power precluded the inclusion of these terms in all models. We used SAS 9.1 software for the analysis.
The study enrolled 414 participants, of whom 355 (86%) provided data on incident STI and self-reported sex and condom use for at least 1 follow-up visit. Most participants (56%) had data from 4 study intervals while the remainder contributed data on 3 (16%), 2 (13%), or 1 (15%) interval. Thus, the analyses on condom use and prevalent STI included data from 414 men, while those on incident STI were based on 1111 intervals from 355 men. Table 1 presents the study population’s enrollment characteristics.
We diagnosed prevalent STI (chlamydia, gonorrhea, and/or trichomoniasis) in 54.6% (n = 226) of the participants at enrollment. Because participants were treated for STI presumptively at enrollment and following diagnosis during follow-up, each individual could have up to 4 incident STI diagnoses. Three men had 2 infections at a single visit. Incident chlamydia was diagnosed the most frequently during follow-up (n = 34), followed by gonorrhea (n = 23) and trichomoniasis (n = 11). Overall, 51 participants (14.4%) were diagnosed with 65 cases of incident STI (i.e., at least 1 of the study STIs) during follow-up.
The adjusted odds of prevalent infection for participants who reported no unprotected acts in the prior 7 days compared with those reporting at least 1 unprotected act was 0.7 (95% CI, 0.4–1.0; Table 2). However, the remaining 6 exposure classifications did not result in statistically significant associations with prevalent STI. Restricting the data set to the 355 participants who had at least 1 follow-up visit had a negligible impact on the estimates and did not change any of the conclusions (data not shown).
Self-reported condom use was more strongly associated with incident than prevalent STI (Table 3). Also, classifications based on the number of unprotected acts resulted in findings similar to those based on the proportion of acts protected. For example, participants reporting no unprotected acts in the interval preceding the follow-up visit were 60% less likely (adjusted OR, 0.4; 95% CI, 0.2–0.9) to have an incident STI than those reporting at least 1 unprotected act. Similarly, men reporting that all acts were protected were less likely (adjusted OR, 0.4; 95% CI, 0.2–0.9) to have an incident STI than those reporting that none of their acts were protected. Effect estimates from the 3 models based on trichotomous measures for the number or proportion of unprotected acts revealed a dose–response relationship between self-reported exposure during follow-up and incident STI (Table 3). The odds ratios for the groups with the middle level of exposure, though, were not statistically significant.
Two of the exposure classifications involved recalling condom use over a longer time period (during the prior 6 months or during study follow-up). Self-reported condom use within the 6 months preceding the enrollment visit was not significantly related to prevalent STI diagnosis (Table 2). We did not find a difference in prevalent infection between men who reported at enrollment that they “always” used condoms during the past 6 months and those reporting inconsistent or no condom use during this time frame (adjusted OR, 0.5; 95% CI, 0.2–1.3). In contrast, participants who reported at their last visit that they had “always” used condoms for coitus during study follow-up had significantly less incident STI than those reporting inconsistent or no condom use (adjusted OR, 0.4; 95% CI, 0.2–0.8).
We found a strong reduction in the risk of incident STI associated with self-reported consistent condom use measured during follow-up. Men who reported no unprotected sex acts during the week preceding the follow-up visit had less than half the odds of incident STI compared with those who reported at least 1 unprotected act. Given that condoms should reduce the risk of urethral infection,9 prior studies that have failed to demonstrate a strong association between condom use and incident urethral infection could have been subject to several biases. First, social desirability bias or imperfect recall could have influenced the measure of condom use.10,11 For example, 2 studies comparing self-reported recent unprotected sex with prostate-specific antigen, a biologic marker of semen exposure, found that substantial proportions of female sex workers appeared to underreport exposure.12,13 Moreover, evidence suggests that people are less likely to use condoms with regular partners than with occasional partners or partners with perceived high STI risk.14–16 This differential use of condoms could obscure the reduction in STI risk afforded by condom use. Recent research has demonstrated the importance of controlling for partner infection status or related unmeasured confounding.6,17–20 While our study potentially was subject to these same measurement problems, our findings indicate that—even in the presence of these possible biases—the choice of STI outcome (prevalent vs. incident infection) affects the magnitude of measured condom effectiveness.
Undoubtedly, consistent and correct condom use should reduce the risk of urethral infection, as evidenced by findings from laboratory studies as well as biologic plausibility.21–23 However, determining a single numerical estimate for the “true” protective effect of consistent condom use for specific STI may be unrealistic.24 Many factors clearly influence this estimate, including number of partners and their individual infection status6,17–20, the validity of the exposure (i.e., condom use) data10; infectivity of the STI25,26; individual’s susceptibility to the STI; direction of transmission (e.g., male-to-female or male-to-male); study follow-up length; test properties for diagnosing STI27; coital frequency; correct use of condoms28; and brand, type (e.g., latex or “natural membrane”),29 and quality of condoms used. Here, we found that the type of STI outcome (prevalent vs. incident infection) influenced the measurement of this relationship.
Many have proposed that studies should use the number of unprotected acts instead of the proportion of acts that were protected because the former may provide a more direct measure of infection risk.2–4,19,30 Surprisingly, though, we did not find a stronger exposure–outcome relationship when exposure was based on the number of unprotected acts. Our study was limited, because the longitudinal assessment was based on the frequency of unprotected acts on the preceding 7-day period, which might not have been representative of the entire interval between visits. However, other methods for measuring condom use over lengthy recall periods might not have yielded more accurate self-reports. For example, the use of resource-intensive coital diaries to attempt to measure all exposures during the period of risk for incident infection could result in biased measures if participants fail to follow instruction for its completion.31,32
Previous studies have shown that recall periods 1 or 2 weeks in duration are more reliable for self-reported sexual behaviors than longer periods.33,34 Participants in the current study were asked at their last study visit to report overall condom use during study participation. We found a statistically significant inverse relationship between “always” condom use during study follow-up and incident STI. As a result, the appropriateness of lengthy recall periods for research on coitus and condom use remains unclear.
Additionally, although we assessed the role of self-reported breakage and slippage, the study failed to account for condom misuse that could have caused exposure to pathogens. For example, retrospective surveys among university students in the United States have found that 38% to 43% of respondents admitted to donning condoms after initial penetration at least once in the prior 3- or 6-month period.35–38 Consequently, bias from exposure misclassification could have attenuated the relationship between condom use and risk of infection. The analysis also was limited in that it was based on a study of males presenting with urethral discharge to an STI clinic in Jamaica. Participants who were condom users might have been more likely to use condoms incorrectly or experience condom failures at baseline than during follow-up, which possibly could explain the weak link between self-reported condom use and prevalent infection. Study results might not apply to more general populations.
In conclusion, we found that self-reported consistent condom use was associated with reduced risk of urethral STI. The magnitude of the protective effect observed for self-reported condom use against incident infection was similar when different measures (e.g., proportions vs. number of acts) for condom use were used. The more attenuated association observed between self-reported condom use and prevalent infection compared with condom use and incident infection suggests that future studies of condom effectiveness should avoid relying on prevalent STI outcomes.
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