Teen vaping acts as a "one way bridge" to future smoking among those who have never smoked before, and may not stop those who have smoked before from returning to it, concludes a small US study, published online in the journal Tobacco Control.
The researchers base their findings on a follow up sample of 347 out of 822 originally targeted 12th graders (17-18 year olds), who had been randomly selected from a representative (Monitoring the Future) survey of more than 13,000 12th graders from 122 schools, in 2014.
The 2014 survey and its follow up one year later in 2015 asked the teens about substance use, including vaping and conventional cigarette smoking.
Analysis of the responses showed that e-cigarettes were one of the most popular substances that the teens said they used, and the prevalence of recent vaping (within the past 30 days) was around 50% higher than it was for conventional smoking.
Most of the respondents thought that cigarette smoking was harmful, with 80% in both the 2014 and follow up surveys feeling that one or more packs daily posed a 'great risk.'
Teens who had never smoked a cigarette before reaching 12th grade, but who had used an e-cigarette at least once within the past 30 days, were more than four times as likely to say that they had smoked a cigarette by the follow up survey (31%) as those who hadn't vaped (7%).
But all the new smokers who were also recent vapers said they had smoked only 'once or twice' during the preceding 12 months.
This difference between vapers and non-vapers held true even after accounting for potentially influential factors, such as sex, ethnicity, and their parents' educational attainment.
For those who had ever smoked by the time of the 2014 survey, the prevalence of smoking during the preceding 12 months was more than twice as high among teens who were also vapers in 2014 (80%) than it was among those who weren't (37%).
Vaping also significantly predicted cigarette smoking in the preceding 12 months at the follow up survey among teens who had smoked at some point previously, but not recently (63% vs 27%), even among those who felt that smoking was very harmful.
And among teens who said they had never smoked by the time of the 2014 survey, recent vapers were four times as likely to move away from the belief that cigarette smoking poses a great risk as those who hadn't vaped, possibly because they become desensitised to the harms of smoking, suggest the researchers.
This is an observational study so no firm conclusions can be drawn about cause and effect, added to which the researchers point to several caveats.
The analysis did not take account of the substances in the e-cigarettes the teens vaped, nor the different frequencies of vaping at the time of the 2014 survey. The number of responses to the follow up survey was relatively small, which may have introduced some element of bias, while other factors associated with susceptibility to smoking take-up among teens, such as rebelliousness and the influence of friends, were not included.
But the researchers say the results contribute to the growing body of evidence for vaping as a "one way bridge" to cigarette smoking teens. "These results bolster findings for vaping as a one way bridge to cigarette smoking among adolescents," they write.
"The results support a desensitisation process, whereby youth who vape lower their perceived risk of cigarette smoking," they add.