Table 4

Association of baseline ever used e-cigarettes with ever used cigarettes at follow-up (among never users of cigarettes at baseline; n=1726; left-hand column) or increased use of cigarettes at follow-up (among baseline once or used to use cigarettes; n=318; right-hand column) controlling for clustering by school

 Predictors Baseline never used cigarettes Baseline once or used to use cigarettes OR (95% CI) p Value OR (95% CI) p Value Model one without covariates Never used e-cigarettes 1.00 1.00 Ever used e-cigarettes 5.38 (4.02 to 7.22) <0.001 2.16 (1.01 to 4.62) 0.046 Model two with covariates Never used e-cigarettes 1.00 1.00 Ever used e-cigarettes 4.06 (2.94 to 5.60) <0.001 1.89 (0.82 to 4.33) 0.13 Friend smokers = none 1.00 1.00 Friend smokers = a few 1.87 (1.35 to 2.58) <0.001 1.15 (0.50 to 2.66) 0.75 Friend smokers = most 2.99 (1.52 to 5.87) 0.001 3.23 (1.19 to 8.77) 0.022 Male 1.00 1.00 Female 1.32 (0.97 to 1.79) 0.08 0.83 (0.45 to 1.52) 0.55 Family smokers = none 1.00 1.00 Family smokers = one 0.76 (0.51 to 1.13) 0.18 1.69 (0.61 to 4.68) 0.31 Family smokers = two 2.05 (1.37 to 3.06) <0.001 1.41 (0.48 to 4.12) 0.53 Family smokers = three or more 1.90 (1.23 to 2.94) 0.004 1.23 (0.45 to 3.41) 0.69 Intentions 0.70 (0.52 to 0.96) 0.03 1.50 (0.87 to 2.57) 0.14 Attitudes 0.68 (0.44 to 1.04) 0.08 0.51 (0.28 to 0.90) 0.020 Norms 0.89 (0.57 to 1.39) 0.61 1.12 (0.56 to 2.23) 0.75 Perceived behavioural control 1.00 (0.73 to 1.37) 0.99 0.99 (0.58 to 1.69) 0.96 Self-efficacy 1.09 (0.75 to 1.57) 0.66 0.57 (0.35 to 0.94) 0.027 Free school meals 0.99 (0.97 to 1.02) 0.60 1.01 (0.96 to 1.07) 0.62 Model three with covariates and interactions Never used e-cigarettes and Friend smokers = none 1.00 Ever used e-cigarettes and friend smokers = none 7.74 (4.68—12.79) <0.001 Never used e-cigarettes and Friend smokers = a few 2.57 (1.72 to 3.84) <0.001 Ever used e-cigarettes and friend smokers = a few 7.84 (5.08–12.09) <0.001 Never used e-cigarettes and friend smokers = most 6.32 (2.68 to 14.91) <0.001 Ever used e-cigarettes and friend smokers = most 8.75 (3.68–20.83) <0.001 Male 1.00 Female 1.37 (1.01 to 1.86) 0.04 Family smokers = none 1.00 Family smokers = one 0.76 (0.51 to 1.14) 0.19 Family smokers = two 2.02 (1.35 to 3.03) <0.001 Family smokers = three or more 1.87 (1.21 to 2.90) 0.005 Intentions 0.70 (0.52 to 0.96) 0.03 Attitudes 0.67 (0.44 to 1.01) 0.06 Norms 0.91 (0.59 to 1.41) 0.69 Perceived behavioural control 1.00 (0.73 to 1.37) 0.99 Self-efficacy 1.09 (0.75 to 1.59) 0.65 Free school meals 0.99 (0.96 to 1.02) 0.47
• Follow-up ever used cigarettes: model without covariates, AIC=1281.3; model with covariates, AIC=1226.5; model with covariates and interactions, AIC=1218.7; follow-up escalation of cigarette use: model without covariates, AIC=334.1; model with covariates, AIC=327.5.