Introduction
The objective of this systematic review was to identify, report and critically appraise studies that have reported health outcomes from use of ENDS.
Methods
We conducted a systematic review of all published literature on the health impact of ENDS products from 1st January 2015 until February 1, 2020, following the PRISMA protocol, including across the databases, PubMed, Embase, Scopus and Google Scholar. Medical subject headings were used in the execution of PubMed searches.
A category for the level of evidence was assigned blindly using the Centres for Evidence Based Medicine framework. A similar approach was adopted to evaluate methodological quality of each study utilizing the National Institutes for Health (NIH) Quality Assessment Tools.
Results
The database search identified 755 studies and a further 265 were identified from other sources and reference reviews of which 37 studies met the eligibility criteria.
The majority of studies were of low strength for levels of evidence including 24 (65%) cross-sectional, 1(2.7%) case-control and six (16%) case studies. There were four (11%) cohort studies and only one (2.7%) RCT. There was only one (2.7%) meta-analysis or pooled study of observational study designs; there were no pooled results of randomized controlled trials. Of 37 studies, eight (22%) studies reported on benefits, two (2%) studies were neutral, reporting on both harm and benefits, the remaining 27 (73%) reported only on harms. The quality ratings were poor (20, 54%), fair (9, 24%) and good (8, 22%).
In our review ENDS use has not been shown to be causative for any CVD outcomes and has been shown to be beneficial for hypertensive patients. Switching from cigarettes to e-cigarettes resulted in reduced exacerbations of COPD, with no evidence of long-term deterioration in lung function. There was a suggestion of short-term reductions in respiratory function in asthmatics, but no increased risk of asthma in ENDS users who were never smokers. Mental Health, cancer and mortality have not been adequately studied to form any consensus with regards to health outcomes from ENDS use.
Conclusion
This review found numerous gaps in the evidence base for the harms from ENDS and benefits of switching from cigarettes to ENDS. Studies are skewed towards those reporting on negative health impacts from ENDS.
The evidence of significant harms to health outcomes from ENDS is lacking. There is a failure of study designs to be able to rigorously establish causation. There is some evidence of positive health outcomes in those switching from cigarettes to e-cigarettes but further studies would need to replicate the findings.
We lack the information required to accurately and correctly weigh the health impact of increased uptake of ENDS against the vast majority of users whose main reason for ENDS use is to quit smoking. Only with this full picture can we ensure that we are totally informed about the net impact on health outcomes from ENDS.