Even more evidence that ecigs increase risk of starting to smoke cigs, this time from Scotland and Canada

While the evidence that kids who initiate tobacco product use is already pretty bulletproof, all the studies so far have been from the US.  Now two new studies, one from Scotland and one from Canada, are taking the case global.
The first study, "Relationship between trying an electronic cigarette and subsequent cigarette experimentation in Scottish adolescents: A cohort study," by Catherine Best and colleagues, followed 3807 Scottish adolescents (11-18 years old) who had never smoked a cigarette forward in time for 1 year.  The youth who smoked e-cigarettes at baseline were twice as likely to have smoked a conventional cigarette by follow-up 1 year later.  The authors controlled for a wide range of variables, including susceptibility to smoking, a well-established predictor of future smoking.  The fact that the authors followed the youth forward in time supports concluding that e-cigarette use was increasing the liklihood of subsequent cigarette smoking.
Another important finding in this paper, which is consistent with the US literature, is that the effects of e-cigarettes on subsequent youth smoking were larger in youth at low risk of initiating tobacco use with convnetional cigarettes.  Thus, in the UK, as in the US, e-cigarettes are expanding the tobacco epidemic.
Being the first paper from the UK, one would think it is giving the e-cigarette enthusiasts there a bad case of indigestion, since they have been saying that the links between e-cigarette use and subsequent smoking cigarettes was a weird American phenomenon.  It's not.  
Here is the abstract:

This study examines whether young never smokers in Scotland, UK, who have tried an e-cigarette are more likely than those who have not, to try a cigarette during the following year.
Prospective cohort survey conducted in four high schools in Scotland, UK during February/March 2015 (n=3807) with follow-up 1 year later. All pupils (age 11-18) were surveyed. Response rates were high in both years (87% in 2015) and 2680/3807 (70.4%) of the original cohort completed the follow-up survey. Analysis was restricted to baseline 'never smokers' (n=3001/3807), 2125 of whom were available to follow-up (70.8%).
At baseline, 183 of 2125 (8.6%) never smokers had tried an e-cigarette and 1942 had not. Of the young people who had not tried an e-cigarette at baseline, 249 (12.8%) went on to try smoking a cigarette by follow-up. This compares with 74 (40.4%) of those who had tried an e-cigarette at baseline. This effect remained significant in a logistic regression model adjusted for smoking susceptibility, having friends who smoke, family members' smoking status, age, sex, family affluence score, ethnic group and school (adjusted OR 2.42 (95% CI 1.63 to 3.60)). There was a significant interaction between e-cigarette use and smoking susceptibility and between e-cigarette use and smoking within the friendship group.
Young never smokers are more likely to experiment with cigarettes if they have tried an e-cigarette. Causality cannot be inferred, but continued close monitoring of e-cigarette use in young people is warranted.

I don't know why the authors added the statement that causality cannot be inferred.  One of the strengths of longitudinal studies like this is that cauality can be inferred.
Sunday Azagba and his colleagues paper, “Susceptibility to cigarette smoking among middle and high school e-cigarette users in Canada”  is a cross-sectional study that showd that nonsmoking Canadian kids who use e-cigarettes have about twice the susceptibility to cigarette smoking as kids who do not use ecigs.   Because it is cross-sectional (i.e., a snapshot in time), they could not draw causal conclusions, but it is still a well-done study that controlled for a wide variety of potential confounders.  The effect was even bigger for younger adolescents than older teens. 
Here is the abstract:

There is a growing concern that the historic reductions in tobacco consumption witnessed in the past decades may be undermined by the rapid increase in e-cigarette use. This study examined the association between e-cigarette use and future intention to smoke cigarettes among middle and high school students who had never smoked cigarettes. Data were drawn from the 2014-2015 Canadian Student Tobacco, Alcohol and Drugs Survey (n=25,637). A multivariable logistic regression model was used to examine the association between e-cigarette use and susceptibility to cigarette smoking. In addition, an inverse probability of treatment weighted regression adjustment method (doubly robust estimator), which models both the susceptibility to smoking and the probability of e-cigarette use, was conducted. About 10% of the students had ever tried an e-cigarette. There were higher rates of ever e-cigarette use among students in grades 10-12 (12.5%) than those in grades 7-9 (7.3%). Students who had ever tried an e-cigarette had higher odds of susceptibility to cigarette smoking (adjusted odds ratio=2.16, 95% confidence interval=1.80-2.58) compared to those that had never tried an e-cigarette. Current use of an e-cigarette was associated with higher odds of smoking susceptibility (adjusted odds ratio=2.02, 95% confidence interval=1.43-2.84). Similar results were obtained from the doubly robust estimation. Among students who had never smoked cigarettes, e-cigarette use was associated with a higher susceptibility to cigarette smoking.

The fact that e-cigarette use was associated with higher susceptability to smoking suggests that, by including susceptabilty to smoking as a predictor in their model, Best and colleagues may have underestimated the effects of e-cigarettes on subsequent smoking. 
The full citations are:  
Best C, Haseen F, Currie D, Ozakinci G, MacKintosh AM, Stead M, Eadie D, MacGregor A, Pearce J, Amos A, Frank J, Haw S.  Relationship between trying an electronic cigarette and subsequent cigarette experimentation in Scottish adolescents: a cohort study.  Tob Control. 2017 Jul 22. pii: tobaccocontrol-2017-053691. doi: 10.1136/tobaccocontrol-2017-053691. [Epub ahead of print]
Azagba S, Baskerville NB, Foley K.  Susceptibility to cigarette smoking among middle and high school e-cigarette users in Canada.  Prev Med. 2017 Jul 20. pii: S0091-7435(17)30264-5. doi: 10.1016/j.ypmed.2017.07.017. [Epub ahead of print]


Please stop with the gateway theory hyperbole

As with all the 'bullet proof' data you speak about there is no way to show causality in these types of studies. Unlike many of the other studies on this subject, these researchers admit this very important limitation to their work: "we cannot infer if the association between e-cigarettes use and cigarette smoking initiation is causal or not". Further there is still no study that shows a trajectory from being a nonsmoker to regular smoker that passes through vaping. There simply is no evidence of regular smokers (of any age, youth, young adult or otherwise) that started with vaping. Further still, every survey to date that has looked at e-cigarette use with an eye to smoking status and frequency of use shows that the only people that use e-cigarettes regularly are current and former smokers. It is high time that the hyperbolous Gateway Theory gets put to rest.

The "bulletproof" link goes to longitudinal studies

There are 9 of them and they do allow causal conclusions.
The new paper is cross-sectional and so they did not make causal statements, which is appropriate.  The new Canadian study is, however, consistent with what would expect from the US longitudinal studies.