Longitudinal studies show that kids who vape are 4 times more likely to start smoking cigs

Samir Soneji and colleagues just published a meta-analysis of 9 longitudinal studies that examined the effect of e-cigarette use on subsequent cigarette smoking behavior.  Each of these 9 studies started with kids who had never smoked a cigarette and compared the odds that they would go on to smoke cigarettes later (usually a year later) among kids who were and were not using e-cigarettes at baseline (the beginning of the study).  They found that the pooled odds ratio for subsequent cigarette initiation was 3.62 and for current (30 day) smoking was 4.28.
These results are adjusted for a wide range of potential  confounding demographic, psychosocial, and behavioral risk factors.  Soneji and colleagues also do extensive sensitivity analysis to demonstrate that their findings are robust.
In addition, the risks are not that different from earlier cross-sectional studies, including those we did (Dutra and Glantz, Lee et al), but they are stronger than cross-sectional studies.  The reason is that a cross-sectional study is a snapshot in time whereas longitudinal studies follow people forward in time and, so, can make causal statements rather than just the associations that are possible in cross-sectional studies.  On the other hand, cross-sectional studies can be done more quickly than longitudinal studies because one has to wait for time to pass when doing a longitudinal study.
Indeed, once of the excuses that e-cigarette advocates used to deny the evidence from the cross-sectional studies was that they were not longitudinal, effectively delaying the fact that they would have to accept the reality that e-cigarettes are a gateway to cigarette smoking.
Of course, even if they were not, attracting kids to nicotine use is a bad thing.
Here are the Key Points and Abstract from the paper:
Question  Is there an association between e-cigarette use and cigarette smoking among adolescents and young adults?
Finding  A systematic review and meta-analysis showed strong and consistent evidence of an association between initial e-cigarette use and subsequent cigarette smoking initiation, as well as between past 30-day e-cigarette use and subsequent past 30-day cigarette smoking.
Meaning  To minimize the potential public health harm from e-cigarette use, the US Food and Drug Administration, as well as state and local agencies, will need to engage in effective regulatory actions to discourage youths’ use of e-cigarettes and prevent the transition from e-cigarettes to other combustible tobacco products.
Importance  The public health implications of e-cigarettes depend, in part, on whether e-cigarette use affects the risk of cigarette smoking.
Objective  To perform a systematic review and meta-analysis of longitudinal studies that assessed initial use of e-cigarettes and subsequent cigarette smoking.
Data Sources  PubMed, EMBASE, Cochrane Library, Web of Science, the 2016 Society for Research on Nicotine and Tobacco 22nd Annual Meeting abstracts, the 2016 Society of Behavioral Medicine 37th Annual Meeting & Scientific Sessions abstracts, and the 2016 National Institutes of Health Tobacco Regulatory Science Program Conference were searched between February 7 and February 17, 2017. The search included indexed terms and text words to capture concepts associated with e-cigarettes and traditional cigarettes in articles published from database inception to the date of the search.
Study Selection  Longitudinal studies reporting odds ratios for cigarette smoking initiation associated with ever use of e-cigarettes or past 30-day cigarette smoking associated with past 30-day e-cigarette use. Searches yielded 6959 unique studies, of which 9 met inclusion criteria (comprising 17 389 adolescents and young adults).
Data Extraction and Synthesis  Study quality and risk of bias were assessed using the Newcastle-Ottawa Scale and the Risk of Bias in Non-randomized Studies of Interventions tool, respectively. Data and estimates were pooled using random-effects meta-analysis.
Main Outcomes and Measures  Among baseline never cigarette smokers, cigarette smoking initiation between baseline and follow-up. Among baseline non–past 30-day cigarette smokers who were past 30-day e-cigarette users, past 30-day cigarette smoking at follow-up.
Results  Among 17 389 adolescents and young adults, the ages ranged between 14 and 30 years at baseline, and 56.0% were female. The pooled probabilities of cigarette smoking initiation were 30.4% for baseline ever e-cigarette users and 7.9% for baseline never e-cigarette users. The pooled probabilities of past 30-day cigarette smoking at follow-up were 21.5% for baseline past 30-day e-cigarette users and 4.6% for baseline non–past 30-day e-cigarette users. Adjusting for known demographic, psychosocial, and behavioral risk factors for cigarette smoking, the pooled odds ratio for subsequent cigarette smoking initiation was 3.62 (95% CI, 2.42-5.41) for ever vs never e-cigarette users, and the pooled odds ratio for past 30-day cigarette smoking at follow-up was 4.28 (95% CI, 2.52-7.27) for past 30-day e-cigarette vs non–past 30-day e-cigarette users at baseline. A moderate level of heterogeneity was observed among studies (I2 = 60.1%).
Conclusions and Relevance  e-Cigarette use was associated with greater risk for subsequent cigarette smoking initiation and past 30-day cigarette smoking. Strong e-cigarette regulation could potentially curb use among youth and possibly limit the future population-level burden of cigarette smoking.
The full citation for the paper is:  Association Between Initial Use of e-Cigarettes and Subsequent Cigarette Smoking Among Adolescents and Young Adults: A Systematic Review and Meta-analysis.  Soneji S, et al.  JAMA Pediatr. Published online June 26, 2017. doi:10.1001/jamapediatrics.2017.1488
A good Reuters story on the paper is here.