Sexual health in adolescents: methodology and evidence
30 January 2007
Stammers’ editorial on sex education raises a number of interesting public health issues. However, in the light of our recent investigation into abstinence-based programmes for HIV prevention in high- income countries, several of his assertions appear flawed on key methodological principles.
-Conclusions drawn from Henderson et al.- Stammers’ conclusions do not follow logically from the trial design and results reported by Henderson et al., because his editorial has not drawn appropriate attention to the actual comparison made. Stammers concludes, “The trial found no significant difference between the intervention and control groups ... The results should stimulate urgently needed change from previous ineffective approaches to school sex education.” This assertion overlooks that Henderson et al. compared the SHARE programme to existing sex education programmes, which already included 7 to 12 lessons of information provision and discussion. In fact, the evaluation did not show that SHARE (or sex education in general) is ineffective; it simply showed that SHARE is no more or less effective than existing sex education programmes. This is an important distinction. The trial results do not necessarily motivate changes to existing methods; instead, they suggest only that SHARE does not improve on the standard sex education already delivered in east Scotland.
-The relevance of behavioural and biological outcome measures- We agree with Stammers’ assessment of the clinical relevance of outcomes frequently measured in sexual health programme trials. As we have previously suggested, the field of HIV prevention could benefit from consensus on a set of operationalised and clinically relevant outcome measures, both biological and behavioural, for the evaluation of risk. This recommendation may apply to the fields of pregnancy prevention and sexual health as well. The medical evaluation of HIV, other sexually transmitted infections (STIs), and pregnancy will yield the most clinically relevant assessments of sexual health promotion programmes, especially as there are limits to the reliability of self-reported data[4- 7] and to the predictive validity of sexual behaviour outcomes[8 9] as indicators of actual risk.
However, while medical evaluations of biological outcomes indicate risk most directly, self-reported sexual behaviour data remain relevant. Stammers suggests that evaluations showing protective effects for “intermediate outcomes” such as condom use might lead to “false claims of success, whereas more robust outcome measures such as rates of terminations, unplanned conceptions, and sexually transmitted infections show no benefit.” Here, Stammers does not account for the substantial problem of floor effects. Often, rates of terminations, conceptions, and STIs are so low that sample sizes of many hundreds or thousands of adolescents would be necessary to detect a significant programme effect. In the US, for example, the prevalence of HIV among young adults is estimated at approximately 1.0 per 1,000 in the general population. To detect a difference of even one HIV infection per trial arm, a two-arm trial among a general young adult population would require data for at least 2,000 individuals at follow-up. It would not require as many participants to detect a significant difference in pregnancy rates, but the problem of floor effects remains relevant. Most trials lack such expansive sample sizes, but they are often adequately powered to detect significant behavioural differences.
As a result, while medically evaluated biological outcomes are crucial, sexual behaviour data such as condom use are a necessary proxy for most trials. The trials that find significant results for behavioural, but not biological outcomes are not necessarily making “false claims” of programme success; few are adequately powered to reliably detect corresponding differences in biological outcomes. Notably, even small trials should collect data on biological outcomes, because the aggregation of many underpowered studies in a meta-analysis can produce adequate power to detect significant effects.
-Causes of recent declines in adolescent pregnancy rates- Recent research also contests Stammers’ suggestion that declines in adolescent pregnancy rates (specifically, US adolescent pregnancy rates) are primarily attributable to delayed first sex. A study published this month by Santelli et al. analyses data that correspond to decreases in the US teen birth rate during the years 1995-2002. Analysing data for youth aged 15-19, the study found that “the overall pregnancy risk index declined 38%, with 86% of the decline attributable to improved contraceptive use.” This contrasts with earlier research cited by Stammers.
-The evaluation of “Managing the Pressures Before Marriage” by Blake et al.- Referring to a randomised controlled trial by Blake et al., Stammers reports that adding parent-child homework “greatly enhanced” the abstinence-only programme’s effectiveness as measured by “usual intermediate outcomes only, such as self efficacy and intention to have sex.” Although Stammers then calls for more evaluations that assess sexual debut, he does not mention that Blake et al. did assess sexual debut at short-term follow-up: the trial found no significant difference between trial arms in whether students had ever had sexual intercourse. The evaluation also found no significant effects on whether students reported having sexual intercourse in the previous 3 months. Attitudes can be important mediators of sexual activity, but the results of Blake et al. actually indicate that the addition of parent-child homework did not enhance short-term programme effects on sexual behaviour. (This finding was also limited by floor effects, as only 6% of participants in the trial reported ever having had sex.) It is interesting to note the contradiction between Stammers’ suggestion that sex education studies assessing condom use make “false claims of success,” and his acceptance of attitudinal outcomes of an abstinence-only programme trial as indications of “greatly enhanced” effectiveness.
-The effectiveness of abstinence-based programmes- Safe sex and “saving sex” need not be mutually exclusive focuses of sexual health interventions, as Stammers has also acknowledged in the past. No one disputes that consistent abstinence from oral, anal, and vaginal sex can be an effective way to avoid pregnancy, STIs, and the sexual acquisition of HIV. However, given limited resources, a major issue is whether it is more effective for programmes to promote only abstinence, or to promote abstinence along with condom use and other safe-sex strategies.
The most methodologically rigorous systematic reviews to date have documented no behavioural or biological evidence that abstinence-only programmes can reduce sexual risk with respect to HIV infection or pregnancy, as compared to a range of control groups. One review has even indicated that abstinence-only programmes may cause harm. However, rigorous reviews[17-20] suggest that hierarchical interventions that promote both abstinence and condom use (i.e., “abstinence-plus” programmes) have had long-term protective effects on self-reported sexual behaviour and/or pregnancy. (To date it appears that only three trials of abstinence-plus HIV prevention programmes[21-23] have assessed self- reported STI incidence or treatment, without finding significant effects among approximately 1,734 participants. The control groups for these studies were usual care[22 23] and a time-matched HIV prevention program without skills training. All may have been underpowered to detect significant differences, and more evaluations are necessary to assess this outcome.)
Direct comparisons between abstinence-only and abstinence-plus programmes are lacking. In our research, we have found only one randomised controlled trial that explicitly compared an abstinence- only programme against an equal-format programme that prioritised abstinence but also recommended condom use. This evaluation found no significant differences in condom use, incidence or frequency of sex, or number of partners (with analyses representing approximately 194 students aged 18-21 in the two-arm comparison), but it was limited to six-week follow-up and faced serious methodological limitations. Many cite Jemmott et al. as another example, but the abstinence-focused arm in this trial acknowledged the protective effects of condoms and was therefore not an abstinence-only intervention. The lack of abstinence-only vs. abstinence-plus programme trials is problematic, and more evaluations directly comparing the two programme types will provide key evidence in this ongoing investigation.
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Competing interests: None declared
Competing interests: None declared
Centre for Evidence-Based Intervention, University of Oxford, 32 Wellington Square, Oxford, OX1 2ER
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