shows the trial profile. Two schools withdrew after randomisation, one from the control and one from the intervention group, because of changes in decisions by school management. These schools were each replaced by one from the same strata in the list of 113 interested schools, and were then randomly allocated to treatment group as a block of size two. Of the 11 043 potentially eligible students in the 59 participating schools, 313 (3%) were withdrawn by their parents or carers before collection of data at baseline. One school in the control group closed after the follow-up undertaken after the intervention, and one school in the intervention group closed after the 1-year follow-up. However, of the 123 students registered at these two schools, 117 transferred to other schools within the trial and were therefore not lost to follow-up. At every data collection point, more than 90% of eligible students provided self-reported data for smoking ().
We recorded few differences between the characteristics of schools at baseline in both groups, but more students in control schools reported smoking every week than did those in intervention schools (). However, when we restricted this comparison to students who reported smoking at baseline and who also provided data at 1-year follow-up, this difference was smaller (229/4436 [5%] vs
195/4711 [4%]). We noted no differences between intervention and control schools in the proportion of students who were occasional, experimental, or ex-smokers and therefore at high risk of regular smoking uptake (). We recorded very slight differences in the family affluence scores33,34
and family ownership of vehicles, suggesting that a slightly larger proportion of students in control schools came from less affluent backgrounds and did not have a family car than did those in intervention schools ().
Baseline characteristics of schools and students according to experimental group
835 (16%) of 5358 students completed the training and agreed to work as peer supporters, achieving the prespecified target of 15% of the year group. Furthermore, peer supporters were generally representative of the year group in terms of sex, ethnic origin, smoking status, and whether or not they intended to remain in full-time education after 16 years of age (data not shown). Very high retention rates were achieved: 99% (835 of 848) of students who trained agreed to continue to work as peer supporters, and 84% (687 of 816) handed in a completed diary at the end of the intervention period.
Overall smoking rate in the whole year group increased from 5·7% (570 of 10 047) at baseline when the students were aged 12–13 years to 13·8% (1366 of 9909) at 1-year follow-up and 20·3% (1963 of 9666) at 2-year follow-up when the students were aged 14–15 years (). Smoking prevalence was lower in intervention than in control schools at all three follow-up points, even after adjustment for baseline differences (). At 1-year follow-up, the odds ratio of being a smoker in intervention compared with control group was 0·77 (95% CI 0·59–0·99). At 2-year follow-up, the corresponding odds ratio of 0·85 (0·72–1·01) was not significant (p=0·067; ), which suggests an attenuation of this intervention effect over time. For the high-risk group, the odds ratios at 1-year follow-up of 0·75 (0·56–0·99) and at 2-year follow-up of 0·85 (0·70–1·02) suggest that, contrary to the findings of the feasibility study, there is no evidence that the intervention had a more beneficial effect on students who were identified as occasional, experimental, or ex-smokers at baseline.
Rate of smoking in the past week and intracluster correlation coefficients at every follow-up point according to experimental group
Odds ratios of smoking in the past week at every follow-up point according to experimental group
Results from the multilevel modelling () show a 22% reduction (odds ratio 0·78 [95% CI 0·64–0·96]) in the odds of being a regular smoker in an intervention school compared with a control school, with the 95% CIs not including a null effect. Further, results of the planned subgroup analyses provided no evidence of the intervention having a differential effect according to sex (ratio of odds ratios 0·90 [0·72–1·13]), peer supporter status (0·92 [0·70–1·21]), or deprivation measured by free school meal entitlement (0·99 [0·65–1·51]). However, the intervention does seem to have had a more pronounced effect in schools located in south Wales valleys (0·58 [0·36–0·93]; ).
Odds ratios from multilevel model for overall intervention effect and according to time of follow-up, baseline smoking status, sex, peer supporter status, free school meal entitlement, and school location
Comparison of the self-reported smoking data and concentrations of salivary cotinine shows that only 125 (1%) of 9282 students who reported not smoking had a salivary cotinine concentration greater than 15 ng/mL at 1-year follow-up (). At 2-year follow-up the corresponding proportion was 3% (101 of 3755). We recorded almost no difference in proportions between intervention and control schools ().
Concordance and discordance between self-reported smoking behaviour and salivary cotinine findings at 1-year and 2-year follow-up
The average cost of the intervention was GB£27 (95% CI 19–48) per student and £4700 (2408–6786) per school. The trial design involved trainers travelling between Wales and England; however, such distances would be unlikely if the intervention were to be implemented in local areas. The average cost excluding travel was £23 (16–43) per student and £3937 (2221–5511) per school.