1 Department of Family and Preventive Medicine, University of South Carolina School of Medicine, Columbia, SC 29208
2 Department of Family Medicine and Rural Health, Florida State University College of Medicine, Tallahassee, FL 32306
Warner et al. (1) are to be congratulated for their excellent study examining the impact of partner infection status on condom effectiveness for reducing transmission of gonorrheal and chlamydial infections. Their results demonstrate significant protection in consistent condom users known to have been exposed to an infected partner, while no significant impact of condom use was detected among those who did not know their exposure status. The authors point out that limiting the analyses to persons with infected partners avoids potential bias from increased condom use with partners considered to be "high risk."
This study also incorporates a methodological advancementmeasuring the number of "protected" and "unprotected" sexual encounters, rather than simply estimating the proportion of encounters during which condoms were used. As we have previously proposed, this distinction is important because the number of exposures to an infected partner may have a substantial impact on infection risk, particularly for highly contagious infections (2).
It is perplexing, however, that the risk of infection in condom-using subjects did not increase even when the frequency of protected exposures increased. Although increases in the proportion of exposures in which condoms are used would be expected to result in fewer new infections, the cumulative risk of experiencing condom failure would be expected to increase with greater numbers of exposures. This assumes that even consistent condom users remain at some risk of infection from incorrect condom use or method failure. The adjusted odds ratio of 0.42 in this study indicates that consistent condom users still face some risk of infection.
A lack of statistical power resulting from a small number of total exposures may help to explain the absence of a relation between increasing numbers of condom-protected exposures and an increase in infection risk. Larger sample sizes and greater total numbers of protected exposures may be necessary to detect the effect of increasing cumulative risk of condom failure. A second possibility is that the increased risk from repeated exposures is offset by more effective condom use in those who use condoms more frequentlya practice effect (3).
A third possibilitythe one we believe is most likelyis that, despite improvements in measurement, the measurement of condom use in this study was still not precise enough. Although the authors provide information regarding the number of protected and unprotected exposures in persons with known infectious partners, no information is provided regarding the actual number of protected and unprotected exposures with the infected partner, as opposed to other, likely noninfectious partners. Thus, a direct measurement of the number of protected and unprotected exposures with a risk of disease transmission is lacking. In the absence of this information, there remains an element of conjecture in any estimate of condom effectiveness.
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