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The meta-analysis by Khouja et al. confirms the strong association in young people between e-cigarette use and subsequent smoking. The critical issue is whether the relationship is causal. If there is a causal relationship, there are several factors which diminish its impact.
Firstly, most of the studies used ‘ever smoking’ as the outcome. Ever smoking is a poor marker for smoking-related harm as most smoking by vapers who later smoke is experimental and infrequent and few progress to established smoking (100+ lifetime cigarettes). Shahab et al. found that only 2.7% of youth who tried e-cigarettes first progressed to established smoking. Only established smoking is linked to significant smoking-related death and disease.
Secondly, the absolute number of non-smokers who progress from vaping to smoking is small as smoking precedes vaping in the vast majority of cases (70-85%). If there is a gateway from vaping to smoking, this only affects a minority of young vapers.
Thirdly, the authors use Bradford Hill’s dose-response and specificity criteria to assess whether the association between vaping and subsequent smoking is likely to be causal.
They acknowledge that the dose-response criterion is mostly based on nicotine dependence, indicating that that nicotine dependent vapers are more likely to progress to smoking. However, nicotine dependence in non-smoking vapers is rare, less than 4% in the 2018 National Youth T...
They acknowledge that the dose-response criterion is mostly based on nicotine dependence, indicating that that nicotine dependent vapers are more likely to progress to smoking. However, nicotine dependence in non-smoking vapers is rare, less than 4% in the 2018 National Youth Tobacco Survey (NYTS).
They point out that studies with ‘negative control outcomes’ would reduce specificity but do not cite any studies to demonstrate this. A number of studies have found that vaping also predicts other risky behaviors such as alcohol, marijuana and other substance use.[5,6] There is no biologically plausible mechanism for e-cigarette use being a causal factor for these other behaviours. We think that, like smoking, these associations are best explained by a common liability.
Finally, the recent study by Shahab et al. using NYTS data found that nicotine vaping appears to be protective against future smoking. Teens who vaped first were significantly less likely to subsequently become established smokers than 1) those who smoked first and 2) a matched group of non-vapers.
Their findings suggest that, if there is a gateway from vaping to smoking it is very small and is outweighed by a much larger effect of diverting youth away from cigarette smoking.
1. Khouja JN, Suddell SF, Peters SE, et al. Is e-cigarette use in non-smoking young adults associated with later smoking? A systematic review and meta-analysis. Tobacco control 2020 doi: 10.1136/tobaccocontrol-2019-055433 [published Online First: 2020/03/12]
2. Shahab L, Beard E, Brown J. Association of initial e-cigarette and other tobacco product use with subsequent cigarette smoking in adolescents: a cross-sectional, matched control study. Tobacco control 2020 doi: 10.1136/tobaccocontrol-2019-055283
3. Berry KM, Reynolds LM, Collins JM, et al. E-cigarette initiation and associated changes in smoking cessation and reduction: the Population Assessment of Tobacco and Health Study, 2013-2015. Tobacco control 2018;28(1):42-49. doi: 10.1136/tobaccocontrol-2017-054108
4. West R, Brown J, Jarvis M. Epidemic of youth nicotine addiction? What does the National Youth Tobacco Survey reveal about high school ecigarette use in the USA? 2019 [Available from: https://www.qeios.com/read/article/391 accessed 24 February 2020.
5. Park E, Livingston JA, Wang W, et al. Adolescent E-cigarette use trajectories and subsequent alcohol and marijuana use. Addictive behaviors 2020;103:106213. doi: 10.1016/j.addbeh.2019.106213 [published Online First: 2019/12/22]
6. Rigsby DC, Keim SA, Adesman A. Electronic Vapor Product Usage and Substance Use Risk Behaviors Among U.S. High School Students. J Child Adolesc Psychopharmacol 2019;29(7):545-53. doi: 10.1089/cap.2019.0047 [published Online First: 2019/07/26]
7. Vanyukov MM, Tarter RE, Kirillova GP, et al. Common liability to addiction and "gateway hypothesis": theoretical, empirical and evolutionary perspective. Drug and alcohol dependence 2012;123 Suppl 1:S3-17. doi: 10.1016/j.drugalcdep.2011.12.018 [published Online First: 2012/01/21]