Examining differences in real-world effectiveness of e-cigarettes for smoking cessation by source of purchase in England: an observational study of different contexts before and after the COVID-19 pandemic

The effectiveness of e-cigarettes for smoking cessation may differ by source of purchase. The changing influence of self-selection on purchase location caused by COVID-19 pandemic-related vape shop closures means we can examine the association between smoking abstinence e-cigarette use by purchase source and test for moderation of this association by the timing of the pandemic.