E-cigarettes and equity: a systematic review of differences in awareness and use between sociodemographic groups.

Objective: To assess whether electronic cigarette (e-cigarette) awareness, 'ever use' and current use vary significantly between different sociodemographic groups.

Design: Systematic review.

Data Sources: Published and unpublished reports identified by searching seven electronic databases (PubMed, MEDLINE, Web of Science, EMBASE, Global Health, PsycINFO, CINAHL Plus) and grey literature sources.

Study Selection: Systematic search for and appraisal of cross-sectional or longitudinal studies that assessed e-cigarette awareness, 'ever use' or current use, and included subgroup analysis of 1 or more PROGRESS Plus sociodemographic groups. No geographical or time restrictions imposed. Assessment by multiple reviewers, with 17% of full articles screened meeting the selection criteria.

Data Extraction: Data extracted and checked by multiple reviewers, with quality assessed using an adapted tool developed by the Joanna Briggs Institute.

Data Synthesis: Results of narrative synthesis suggest broadly that awareness, 'ever use' and current use of e-cigarettes may be particularly prevalent among older adolescents and younger adults, males, people of white ethnicity and-particularly in the case of awareness and 'ever use'-those of intermediate or high levels of education. In some cases, results also varied within and between countries.

Conclusions: E-cigarette awareness, 'ever use' and current use appear to be patterned by a number of sociodemographic factors which vary between different countries and subnational localities. Care will therefore be required to ensure neither the potential benefits nor the potential risks of e-cigarettes exacerbate existing health inequalities.