A Cochrane review identified 134 randomized controlled trials involving a total of 428 293 participants. Pure Prevention cohorts (Group 1) included 49 studies (n=142 447). Pooled results at follow-up at one year or less found no overall effect of intervention curricula versus control (odds ratio (OR) 0.94, 95% CI 0.85 to 1.05). In a subgroup analysis, the combined social competence and social influences curricula (6 RCTs) showed a statistically significant effect in preventing the onset of smoking (OR 0.49, 95% CI 0.28 to 0.87; 7 arms); whereas significant effects were not detected in programmes involving information only (OR 0.12, 95% CI 0.00 to 14.87; 1 study), social influences only (OR 1.00, 95% CI 0.88 to 1.13; 25 studies), or multimodal interventions (OR 0.89, 95% CI 0.73 to 1.08; 5 studies). In contrast, pooled results at longest follow-up showed an overall significant effect favouring the intervention (OR 0.88, 95% CI 0.82 to 0.96). This represents a risk reduction of 12%. Subgroup analyses detected significant effects in programmes with social competence curricula (OR 0.52, 95% CI 0.30 to 0.88), and the combined social competence and social influences curricula (OR 0.50, 95% CI 0.28 to 0.87), but not in those programmes with information only, social influence only, and multimodal programmes.
Change in Smoking Behaviour over time (Group 2) included 15 studies (n=45 555). At one year or less there was a small but statistically significant effect favouring controls (standardised mean difference (SMD) 0.04, 95% CI 0.02 to 0.06). For follow-up longer than one year there was a statistically nonsignificant effect (SMD 0.02, 95% CI -0.00 to 0.02).
Of the thirteen studies of social influences interventions, nine found some positive effect of intervention on smoking prevalence, and four failed to detect an effect. The largest and most rigorous study, the Hutchinson Smoking Prevention Project, found no long-term effect of an intensive eight-year programme on smoking behaviour. Of the three RCTs of combined social influences and social competence interventions, one provided significant results and one only for instruction by health educators compared to self-instruction. The four valid studies of multi-modal approaches provided limited evidence about the effectiveness of these approaches including community initiatives.Comment: The quality of evidence is downgraded by inconsistency (heterogeneity in interventions and outcomes and variability in results across studies).