These analyses show that smokers who are regularly exposed to other people using e-cigarettes were more motivated to stop smoking and were more likely to have tried to quit, but this was not independent of their own use of e-cigarettes; in fact, there was moderate evidence for no independent association between exposure to e-cigarette use by others and motivation to stop smoking or quit attempts.

This is the first study, to our knowledge, that has assessed the impact of exposure to other people’s e-cigarette use on smokers’ motivation and behaviour. One in four smokers reported that someone else regularly used e-cigarettes in their presence. In contrast to experimental studies that have reported increased desire and urges to smoke in smokers exposed to e-cigarette use by others [18, 19], our unadjusted models were suggestive of beneficial effects of e-cigarette exposure. Smokers who were regularly exposed to e-cigarette use by others were around 20% more likely than those not regularly exposed to report being highly motivated to stop smoking in the next 3 months or to have made a serious quit attempt in the last year. One possible explanation for these discrepant findings is that the outcomes of interest in the experimental studies (desire, urges) are momentary and vary in response to features of the current environment, whereas outcomes in our study were likely to be more stable. It could also be that the impact of single exposure to an unknown person using an e-cigarette differs to the impact of regular exposure to e-cigarette use by a close social connection such as a friend, relative or colleague, particularly if the latter is known to have used e-cigarettes to successfully reduce or stop smoking [28,29,30]. Thus, while seeing someone use an e-cigarette may momentarily increase desire to smoke, repeated exposure to other people’s e-cigarette use may draw non-users’ attention to the potential utility of these devices in helping them to quit smoking, increasing their motivation to make a quit attempt.

When we adjusted for potential confounding by age, sex, social grade, survey year, reported regular exposure to cigarette smoking by others and participants’ own e-cigarette use, there was no association between exposure to e-cigarette use by others and motivation or quit attempts, although exposure to cigarette smoking by others was independently associated with reduced high motivation to quit. The differences we observed in the unadjusted models were driven primarily by participants’ own e-cigarette use, which was strongly associated with motivation and quit attempts and more commonly reported by smokers who were regularly exposed to e-cigarette use by others. Among smokers who did not use e-cigarettes at all, there was no evidence that exposure to other people using e-cigarettes affected motivation or quit attempts. It is possible that excluding e-cigarette users from these analyses might have overrepresented smokers who have little or no interest in e-cigarettes and who therefore might be less influenced by other people using e-cigarettes, thereby underestimating the association with e-cigarette exposure. The same pattern of results was seen when we retained e-cigarette users in the sample and adjusted for participants’ own use of e-cigarettes. These findings should ease concerns about potential adverse effects of e-cigarettes on non-users’ motivation and efforts to stop smoking [15, 16, 31].

While we found no evidence that exposure to other people using e-cigarettes adversely affected smokers’ motivation to stop or attempts to quit, there was a strong association between exposure to e-cigarette use by others and own e-cigarette use. Smokers who were regularly exposed to other people using e-cigarettes were more than twice as likely as those who were not to report using e-cigarettes themselves. While we cannot disentangle the direction of this association because it was not investigated longitudinally, previous research into network phenomena in smoking behaviour suggests that exposure to e-cigarette use by others might encourage non-users to take up using the devices. In a landmark paper that studied a densely interconnected social network of around 12,000 people, Christakis et al. demonstrated that smoking behaviour spreads through social connections, such that people are substantially more likely to smoke if they have a family member, friend or colleague who smokes and more likely to quit if a close social connection quits [32]. The effective international mass media campaign ‘Stoptober’ was predicated on this social contagion [33]. It is possible that the same might be true for e-cigarettes, such that having a close social connection who uses an e-cigarette may increase a person’s likelihood of using the devices themselves. In a large sample of US adults who reported ever trying e-cigarettes, friends or family members using or offering e-cigarettes was cited as one of the most common reasons for trying e-cigarettes [34], and knowing people who use e-cigarettes has been shown to predict e-cigarette use in adolescent samples [35, 36]. Qualitative studies of e-cigarette users have found that the majority were introduced to e-cigarettes by friends or family who already used them [37, 38].

This study had a number of strengths, including a large sample that was representative of smokers in the English population. We used data from monthly surveys spanning 3.5 years, which limited potential bias from seasonal differences in the rate of quit attempts. Models were adjusted for a range of potential confounders that are often either not assessed in other surveys or omitted from analyses. However, there were also limitations. The assessment of quit attempts relied on recall of the last 12 months for past quit attempts and the last 6 months for prospective quit attempts, introducing scope for bias. Due to differential drop-out, the follow-up sample was not representative of the baseline sample (typically being older and from a higher social grade), which may have biased the results. In addition, the findings may not generalise to populations outside of England where tobacco control policies and regulations (e.g. the legal status of e-cigarettes) differ. The wording of the item used to assess exposure to e-cigarette use by others did not define ‘regular’ so this measure relies on each respondent’s interpretation of regular exposure. Experimental research has indicated that smokers have an attentional bias for e-cigarette cues [20]. We did not adjust for tobacco dependence, and it is possible that highly dependent smokers might react more strongly to e-cigarette cues than smokers who are less dependent. However, previous research has found that while dependence is predictive of whether a quit attempt is successful, it is usually not associated with whether a quit attempt is made [26]. A priori we therefore decided there was not sufficient justification to warrant its inclusion as a potential confounder. Participants’ own e-cigarette use was assessed with an item that asked about use ‘to help you stop smoking, to help you cut down or for any other reason at all’; it is possible that the emphasis on quitting or reducing smoking may have accounted in part for the strong association we observed between own use of e-cigarettes and motivation to quit and quit attempts. However, estimates of current e-cigarette use in the Smoking Toolkit Study are extremely close to those produced by the Office for National Statistics using the Opinions and Lifestyle Survey, which uses a less prescriptive assessment [39]. Exposure to e-cigarette cues other than vaping itself (e.g. e-cigarette advertisements) was not assessed; this means that e-cigarette exposure may have been underestimated, and this study cannot inform any discussions about e-cigarette advertisements which might have an effect on smokers’ motivation to stop or make a quit attempt. E-cigarette advertising in England has been tightly regulated by the EU Tobacco Product Directive since 2016. Sensitivity analyses using Bayes factors indicated that the data favoured the null hypothesis but were insensitive to detect small effects, so it is possible that regular exposure to e-cigarettes has small effects on smokers’ motivation and attempts to stop smoking that were not detected in this study. It is also possible that there are effects beyond motivation and attempts. Further, qualitative research may be able to identify other important outcomes.