Earlier this spring, we provided a review of studies into the relationship between young people using e-cigarettes and their subsequent use of tobacco. This post reports on four more large studies which support the general conclusion that young e-cigarette users are three to four times more likely to become tobacco cigarette smokers
All four studies provide systematic reviews of previous evidence. The most recently published of these studies provides a a new analysis of longitudinal data from the Population Assessment of Tobacco and Health (PATH) survey in the United States. Two others are meta-analyses commissioned by government health agencies – one from Ireland and Australia. One other, from the European Union, reviewed previous studies, meta-analyses, and e-cigarette components particularly attractive to young people.
Evidence from an analysis of longitudinal data from the PATH survey
Smoking Intention and Progression From E-Cigarette Use to Cigarette Smoking was published in the journal Paediatrics in November 2020. Olusegun Owotomo and his colleagues at the University of Texas used data from the Population Assessment of Tobacco and Health (PATH) study to look at changes in adolescent behaviour from 2014-15 to 2015-16. They used a US nationally representative population of adolescents who were 12 to 17 years old at the beginning of the study period to find out whether the teenager's stated intention to smoke cigarettes or their use of e-cigarettes predicted actual cigarette smoking one year later. There were four previous analyses of PATH data that examined the relationship between e-cigarette use and subsequent smoking, but this was the first to also examine the effect of intention to smoke. References to these previous studies can be found in the Australian study by Olivia Baenziger and her colleagues.
It turned out that both intention to smoke and e-cigarette use predicted later smoking.. "Among adolescents who had expressed intention to smoke conventional cigarettes at baseline, e-cigarette use did not predict cigarette smoking at follow-up. However, among adolescents without previous intention to smoke conventional cigarettes, e-cigarette use predicted cigarette smoking."
If teens said they intended to smoke, they were three times more likely to actually become cigarette smokers. Children who smoked e-cigarettes in 2014-15 were 4.6 times more likely to become cigarette smokers by 2015-16, even if they earlier had no intention of becoming cigarette smokers. This latter finding is very much in line with most other studies of same phenomenon, as we shall see in the following sections.
Evidence from meta-analyses performed by the Irish Health Research Board
Smoking Intention and Progression From E-Cigarette Use to Cigarette Smoking was published in the journal Paediatrics in November 2020. Olusegun Owotomo and his colleagues at the University of Texas used data from the Population Assessment of Tobacco and Health (PATH) study to look at changes in adolescent behaviour from 2014-15 to 2015-16. They used a US nationally representative population of adolescents who were 12 to 17 years old at the beginning of the study period to find out whether the teenager's stated intention to smoke cigarettes or their use of e-cigarettes predicted actual cigarette smoking one year later. There were four previous analyses of PATH data that examined the relationship between e-cigarette use and subsequent smoking, but this was the first to also examine the effect of intention to smoke. References to these previous studies can be found in the Australian study by Olivia Baenziger and her colleagues.
It turned out that both intention to smoke and e-cigarette use predicted later smoking.. "Among adolescents who had expressed intention to smoke conventional cigarettes at baseline, e-cigarette use did not predict cigarette smoking at follow-up. However, among adolescents without previous intention to smoke conventional cigarettes, e-cigarette use predicted cigarette smoking."
If teens said they intended to smoke, they were three times more likely to actually become cigarette smokers. Children who smoked e-cigarettes in 2014-15 were 4.6 times more likely to become cigarette smokers by 2015-16, even if they earlier had no intention of becoming cigarette smokers. This latter finding is very much in line with most other studies of same phenomenon, as we shall see in the following sections.
Evidence from meta-analyses performed by the Irish Health Research Board
Electronic cigarette use and tobacco cigarette smoking initiation in adolescents: An evidence review was made public by the Irish Health Research Board in October. It was prepared to respond to Ireland's Department of Health's request for an answer to the question "Does e-cigarette use by adolescents who are cigarette naïve at baseline lead to subsequent cigarette smoking?"
This review predated the Owotomo study discussed above, so the latter was not included among the papers analyzed.
The Irish HRB review included 21 studies, up from the 17 included in the meta-analysis reported by Jasmine Kouja and her colleagues a few months earlier. Of these, 15 longitudinal studies reported adjusted odds ratios. The HRB concluded: "We found a four-fold association between ever using e-cigarettes and initiating smoking tobacco cigarettes in adolescents in a combined analysis of nine cohort studies conducted with follow-up periods between 4 and 24 months"
The analysis considered this relationship using different theoretical constructs. "We identified three theories that attempt to explain the move from using e-cigarette use to smoking tobacco cigarettes, and these are: the gateway theory, the common liability theory, and the catalyst model." The HRB recommends that further studies be designed to test these theories.
Evidence from umbrella review and meta analyses of gateway effect and relapse undertaken researchers in Australia
With support from the Australian Government Department of Health, Olivia Baenziger and her colleagues prepared a report on E-cigarette use and combustible tobacco cigarette smoking uptake among non-smokers, including relapse in former smokers: umbrella review, systematic review and meta-analysis. It was made available in September ahead of peer-review.
This study included several components. First, they reviewed three previous meta-analyses, "an umbrella review" and noted that all three had similar findings, that young e-cigarette users were 3-4 times more likely to later become cigarette smokers. Secondly, they identified 25 studies as methodologically suitable for inclusion in their own meta-analysis of the the relationship between e-cigarette use and subsequent cigarette smoking. When all were combined in the meta-analysis, it was estimated that e-cigarette users were 3.25 times more likely to become cigarette smokers.
Thirdly, the researchers found three studies that studied whether former smokers who took up e-cigarettes would relapse to cigarettes smoking. This is the first systematic review to consider the relationship of e-cigarette use to smoking relapse. It found former smokers were 2.4 times more likely to relapse to cigarette smoking if they took up e-cigarette use.
"Our umbrella and systematic review, along with an updated meta-analysis using data from primary studies, shows strong and consistent evidence that never smokers who have "used e-cigarettes are more likely than those who have not used e-cigarettes to try smoking conventional cigarettes and to transition to become regular tobacco smokers."
Evidence reviewed by the European Commission
Another scientific assessment of the role of e-cigarettes in the initiation of smoking was conducted by the European Commission's Scientific Committee on Health, Environmental and Emerging Risks (SCHEER). In February 2019, the Commission requested SCHEER to review the health effects, cessation impact and also "their role as a gateway to smoking." The SCHEER released its Preliminary opinion on electronic cigarettes in late September and will make public its final version after reviewing comments that were submitted in response.
In this paper, the SCHEER reported that "Regarding the role of electronic cigarettes as a gateway to smoking/the initiation of smoking, particularly for young people, the SCHEER concludes that there is strong evidence that electronic cigarettes are a gateway to smoking for young people. There is also strong evidence that nicotine in e-liquids is implicated in the development of addiction and that flavours have a relevant contribution for attractiveness of use of electronic cigarette and initiation." (emphasis in original)
The question of causality and the gateway effect
The reviews discussed above took different approaches and applied different models when considering whether smoking is causally related to e-cigarette use.
* The Health Research Board reported that its study design "does not allow us to say there is a definitive causal relationship, but it does allow us to say that the findings builds a case towards a causal relationship as the findings are consistent across all studies included in the meta-analysis." It recommended further studies to establish "what drives the relationship between e-cigarette and tobacco cigarette use."
* The EU SCHEER committee considered both the gateway hypothesis and the renormalisation hypothesis to explain the relationship between e-cigarette use and later smoking by adolescents. They concluded "that there is strong evidence that electronic cigarettes are a gateway to smoking for young people."
* The Australian team found that the similarity of results in many places suggested a causal link ("... the consistency of findings across multiple studies and settings supports the likelihood of a causal relationship"), but that the cross-sectional observational design of most studies meant they could not yet rule out "the possibility of residual confounding."
* The longitudinal design of the U.S. study (which was not included in the other reviews) adds further evidence to support a causal relationship. The authors' conclusion implies that the results should be treated as support for a causal connection. "Abstinence from e-cigarette use should be framed as an adolescent smoking prevention strategy."
Take-away
- Multiple studies and multiple meta-analyses from many countries show remarkably consistent results. Young people who take up using e-cigarettes are 3-4 times more likely to progress to becoming cigarettes smokers than their peers who do not use e-cigarettes.
- This risk of becoming a smoker also applies to those young e-cigarette users who declare they have have no intention of becoming smokers. Despite their intentions, they are over four times more likely to become cigarette smokers.
- Former smokers who take up e-cigarettes are over twice as likely to relapse to cigarette smoking.
