November 8, 2021

expert reaction to unpublished conference presentation claiming ‘E-cigarette users face 15% higher risk of stroke at a younger age than traditional smokers’

An unpublished conference presentation publicised by the American Heart Association suggested a higher stroke risk at a younger age for e-cigarettes users than for traditional smokers.

Prof Peter Hajek, Director of the Tobacco Dependence Research Unit, Queen Mary University of London (QMUL), said:

“This unpublished conference presentation seems to be based on the same fallacy as the infamous study that claimed that vaping causes myocardial infarction (MI). The MI paper was withdrawn when it transpired that most vapers had their MI before they started to vape. It is likely that e-cigarette users in this cohort were smokers who switched to vaping AFTER they suffered a stroke. Presenting this is if vaping caused these strokes is misleading and could put smokers off switching to vaping.”

Dr Jamie Hartmann-Boyce, Centre for Evidence-Based Medicine, University of Oxford said:

“This study, for which I have only seen a conference abstract, reports that adults who used e-cigarettes had a 15% higher risk of having a stroke at a younger age, compared to adults who smoked traditional cigarettes. Though this may ring alarm bells, there are some important things to bear in mind.

“Stroke was still far more common in people who smoked cigarettes than in people using e-cigarettes – people who smoked cigarettes were over 6 times more likely to have a stroke than people who used e-cigarettes.  Also, we don’t have very many details on this study, as it hasn’t been published in a peer-reviewed journal yet. That means it’s hard to tell how reliable the results are. We don’t know if and how the researchers took into account other important factors-  such as health conditions that may have led people to switch from smoking to using e-cigarettes, and may also have made them more likely to have a stroke earlier. We also don’t know how age has been taken into account, as e-cigarette use is generally more common in younger people.

“Experts continue to agree that, though not risk free, e-cigarettes are considerably safer than smoking, and people who smoke should consider switching. This study does not change that message.”

Prof Paul Aveyard, Professor of Behavioural Medicine at the University of Oxford, said:

“This press release could equally and accurately be headed as ‘E-cigarette users six times less likely to have a stroke than traditional cigarette smokers’. As they point out, ‘Stroke was far more common among traditional cigarette smokers than e-cigarette users or people who used both, 6.75% compared to 1.09% and 3.72%, respectively.’ What they appear to have found is what we call effect modification, whereby while e-cigarette users were at far lower risk overall, there was evidence of a higher risk in younger ages (and correspondingly it must be true that there was a lower risk at older ages). There are several difficulties in interpreting this research.

1. This compares people who use e-cigarettes to people who do not and tries to make inferences that using e-cigarettes leads to these differences in risk. Another equally possible explanation is that people do other things that put them at higher or lower risk.
2. There is no biologically plausible hypothesis that might explain how e-cigarettes could increase the risk of stroke at young ages but decrease it in the older groups. It’s quite possible this is a chance finding.

“It is also important to point out that stroke is mostly a condition that affects older people, where the risk in e-cigarette users was lower than among cigarette smokers. If we were to believe as the authors do that vaping affects risk of stroke, then overall a person who faces a choice between smoking and vaping would be best advised to switch to vaping and this is likely to be true whatever that person’s age. Stopping smoking and vaping is the best choice for health, switching to vaping the second best choice, and continuing smoking the worst – and that maxim remains true.”

Dr Leonie Brose, National Addiction Centre, King’s College London, said:

“It is difficult to say without knowing more about the analysis; however, as far as I know, the survey they used is a cross-sectional survey.  This means respondents had a stroke in the past (maybe years before they completed the survey) and were using e-cigarettes or smoking at the time of the survey. At least some of the strokes would therefore have occurred before e-cigarette use. The strokes then could not have been caused or made more likely by e-cigarette use. It may also be that people switched to e-cigarettes after a stroke to reduce the stroke risk from smoking which would explain the association between a past stroke and current e-cigarette use. In any case, many e-cigarette users will have been smokers before they started using e-cigarettes, so the analysis would have to take people’s past behaviour into account. From the press release, it is unclear if that was done.”

Prof John Britton, Emeritus Professor of Epidemiology, University of Nottingham, said: 

“This finding, if true, is of interest but does not detract from the fact that vapers in this study were overall less likely to have strokes than those who continued to smoke. Without closer scrutiny of the findings, which is not possible given the preliminary nature of this report, it is not possible to determine whether this finding is of any import.”

https://www.eurekalert.org/news-releases/933457  – note ‘This release has been removed upon request of the submitting institution.’

Declared interests

None received.