Gateway or common liability? A systematic review and meta-analysis of studies of adolescent e-cigarette use and future smoking initiation.

BACKGROUND AND AIMS Studies have consistently found a longitudinal association between e-cigarette use (vaping) and cigarette smoking. Many have interpreted such association as causal. This systematic review and meta-analysis evaluated the plausibility of a causal interpretation by (1) estimating the effect of adolescent vaping on smoking initiation, adjusted for study quality characteristics, (2) evaluating the sufficiency of adjustment for confounding based on the Social Development Model (SDM) and the Social Ecological Model (SEM) and E-value analyses, and (3) investigating sample attrition and publication bias. METHODS Systematic review and meta-analysis of longitudinal studies that examined the association between e-cigarette use at baseline and smoking at follow-up. Participants were non-smokers aged 1000 (OR = 2.49, 95% CI [1.97,3.15]). Overall, the attrition rate was very high (Median: 30%). All but one study reported results from complete sample analysis, despite those dropping out having higher risk profiles. Only two studies comprehensively adjusted for confounding. The median E-value was 2.90, indicating that the estimates were not robust against unmeasured confounding. CONCLUSIONS There is a longitudinal association between adolescent vaping and smoking initiation; however, the evidence is limited by publication bias, high sample attrition and inadequate adjustment for potential confounders.