Circumcision Status and Risk of Sexually Transmitted Infection in Young Adult Males: An Analysis of a Longitudinal Birth Cohort

Circumcision Status

Circumcision status was assessed though parental report at age 4 months and through medical charts collected at 1 year of age. Parents were questioned at the age 4-month assessment as to whether their child had been circumcised. On the basis of this questioning, 26.1% (N = 130) of the males in the sample were classified as having been circumcised. In addition, medical charts were collected at each assessment that included a record of whether the child had been circumcised in a hospital. On the basis of these records, an additional 24 males were classified as having been circumcised before age 15 years, providing a total of 154 circumcised males (30.2% of the sample).

Self-Reported STIs, Ages 18 to 21 and 21 to 25 Years

At ages 21 and 25 years, cohort members were questioned about a range of sexual activities and practices that they had engaged in since the previous assessment and the consequences of these activities and practices, including whether they had been diagnosed with an STI at any time since the previous assessment. Cohort members who responded “yes” were then asked to provide details of the infection, including the age at which it was contracted, the type of STI contracted (ie, according to a formal medical diagnosis), and the treatment (if any) provided. Fourteen cohort members (2.7%) reported a medically diagnosed STI at the age-21 assessment, and 34 cohort members (6.6%) reported an STI at the age-25 assessment. Six cohort members (1.2%) reported an STI at both the age-21 and -25 assessments.

The details provided of the self-reported STIs were as follows: 22 cases (52.4% of the cases) were reported to have been Chlamydia, 13 (31.0% of the cases) were reported to have been genital warts, 4 (9.5% of the cases) were reported to have been genital herpes, 2 (4.8% of the cases) were reported to have been gonorrhea, 5 (11.9% of the cases) were reported as nonspecific urethral infections, and 1 case (2.4% of the cases) was described as a co-occurrence of genital herpes and genital warts. There were no self-reported cases of HIV infection, syphilis, or genital ulcerative disease among the cohort members.

Covariate Factors

The regression models (see “Statistical Analysis”) used a series of observed covariate factors that were abstracted from the study database and were selected on the basis that they were theoretically relevant predictors of STI risk or known on the basis of previous analysis to be associated with measures of sexual behavior in this cohort. These measures are described below.

Self-Reported Number of Sexual Partners Ages 21 and 25

At ages 21 and 25 years, participants were questioned about aspects of their sexual behavior over the interval since the previous assessment. We asked the following questions: (1) “How many sexual partners have you had during the last year?” (2) “Since you turned (age at previous assessment) how many sexual partners of the opposite gender have you had (including the past year)?” and (3) “How many partners of the same gender have you had since you were (age at last assessment)?” These data were used to obtain an estimate of the total number of sexual partners reported by the participants for each of the intervals, 18 to 21 years and 21 to 25 years. On average, participants reported a mean of 5.4 partners during the interval ages 18 to 21 years and a mean of 7.6 partners during the interval ages 21 to 25 years.

Self-Reported Unprotected Sex Ages 21 and 25 Years

As part of the assessment at ages 21 and 25 years, participants were questioned about their involvement in incidents of unprotected sex in the previous 12 months. To assess sexual activity and unprotected sex, participants were asked how many times they had had sexual intercourse without using a condom, either with an opposite-gender partner or a same-gender partner. For the purposes of the present analysis, participants were classified as having engaged in unprotected sex during the time interval if they reported having engaged in sex with an opposite-gender partner or a same-gender partner without using a condom on ≥1 occasion in the past 12 months. Of the male participants, 77% (n = 390) reported having had sex without a condom with either an opposite-gender or same-gender partner.

Maternal Age

This was recorded at the time of the survey child’s birth.

Maternal Education

This was assessed at the time of the survey child’s birth using a 3-point scale, which reflected the highest level of educational achievement attained. This scale was: “1” where the mother lacked formal educational qualifications (had not graduated from high school); “2” where the mother had secondary level educational qualifications (had graduated from high school and may have attended university or the equivalent); and “3” where the mother had tertiary level qualifications (had obtained a university degree or equivalent qualification).

Family Socioeconomic Status

This was assessed at the time of the survey child’s birth using the Elley and Irving⁴⁴ scale of socioeconomic status (SES) for New Zealand. This scale classifies SES into 6 levels on the basis of paternal occupation, ranging from “1” for professional occupations to “6” for unskilled occupations.

Birth Weight

Birth weight was assessed by the recorded weight of the survey child at birth, measured in grams.

Statistical Analysis

The unadjusted association between circumcision and the rate of STI was tested for significance by fitting a random effects logistic regression model to the repeated-measures data linking circumcision status to STI risk in the intervals 18 to 21 and 21 to 25 years. The fitted model was of the form: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathrm{logit}(\mathrm{Y}_{\mathrm{it}}){=}\mathrm{B}_{\mathrm{0t}}{+}\mathrm{B}_{1}\mathrm{X}_{\mathrm{i}}{+}{\nu}_{\mathrm{i}}\] \end{document} where logit(Y_it) was the log odds of STI for the ith participant in the tth time period; Xi was a dichotomous variable representing the individual’s circumcision status (circumcised or not circumcised); and ν_i was an individual’s specific random effect. The intercept term B_0t was permitted to vary with time to allow for changes in the base rate of STI between assessment periods. In this model, the coefficient B₁ represents the effect of circumcision on STI risk pooled over the assessment periods 18 to 21 and 21 to 25 years. A test of the significance of the association between circumcision and STI risk was obtained from a Wald χ² test of the hypothesis H₀: B₁ = 0. An estimate of the strength of association was provided by the pooled odds ratio (OR) and corresponding 95% confidence interval (CI) estimated using the fitted model parameter and SE. The pooled OR was given by exp(B₁).

The associations between STI risk and covariate factors were tested for significance using the χ² test of independence for comparison of percentages and the t test for independent samples for comparison of means. To adjust the association between circumcision and STI risk for covariates, the random effects model above was extended to include covariate factors. The adjusted parameter estimate for circumcision was used to calculate the adjusted OR for risk of STI. The adjusted effect size estimate was used to assess the contribution of circumcision status to the overall rate of STI in the cohort, through calculation of the population attributable risk (PAR) estimate for circumcision.⁴⁵ Finally, the above analyses were repeated using only those self-reported cases of Chlamydia as the STI outcome measure (n = 22) to determine whether there was any specific association between circumcision status and Chlamydia infection.

Associations Between Circumcision Status and STIs

Table 1 shows rates of self-reported STI at ages 18 to 21 and 21 to 25 years related to circumcision status. Table 1 shows that, at both times, uncircumcised males were at increased risk of STI. To estimate the association between circumcision status and rates of STI, a random effects model was fitted to the repeated-measures data (see “Methods”). The estimates from the random effects model showed that uncircumcised males had odds of STI that were 2.66 times higher (95% CI: 1.17–6.11; P < .05) than circumcised males.

Associations Between Covariate Factors and STI Risk (Ages 18 to 25 Years)

Table 2 shows the associations between STI risk from ages 18 to 25 years and a number of covariate factors. These factors included measures of social, family, and related background factors measured at birth and measures of sexual activity in adolescence and young adulthood. Table 2 shows the following: (1) risks of STI were unrelated to all measures of childhood social background and birth weight (P > .10); and (2) risks of STI were related to both the number of sexual partners (P < .001) and engaging in unprotected sex (P < .05).

Associations After Adjustment for Covariate Factors

The results in Table 2 raise the possibility that the associations between circumcision status and STI risk were because of the covariate factors associated with STI. To address this issue, the results were adjusted for the covariate factors (number of sexual partners and unprotected sex) using logistic regression. The results of this analysis are summarized in Table 3, which shows the ORs between circumcision status and STI risk before and after adjustment for covariate factors. These comparisons make it clear that adjustment for covariates did not decrease, but rather increased the magnitude of the association between circumcision status and STI (β = 1.16; SE = .45; P < .01). After adjustment, the OR was 3.19 (95% CI: 1.32–7.75). The increase in the OR was because of the fact that those who were circumcised reported a greater number of sexual partners and higher rates of unprotected sex than those who were uncircumcised.

Estimates of the PAR of Circumcision After Controlling for Covariates

The results in Table 3 show that circumcision status was related to overall risk of STI. To assess the contribution of circumcision status to overall rates of STI, the PAR was calculated (see “Methods”). The PAR estimates the percentage decrease in the overall rate of STI that would be observed if all males in the population were circumcised.⁴⁵ The PAR for circumcision was 48.2% (95% CI: 17.7%–60.9%), suggesting that if all of the cohort members were circumcised, the overall rate of STI would reduce by 48.2%.

Supplementary Analyses

The analyses conducted above were repeated using a subset of self-reported STIs, namely those respondents who reported contracting Chlamydia. The unadjusted associations between circumcision status and reports of Chlamydia were similar to those for overall STI (β = .92; SE = .63; P > .10; OR: 2.50; 95% CI: 0.73–8.53) but were not statistically significant because of the small number of cases of Chlamydia (n = 22). Adjustment for the potentially confounding factors listed above followed the same pattern as overall STI (β = 1.11; SE = .63; P > .05; OR: 3.03; 95% CI: 0.88–10.45).