https://www.nhs.uk/smokefree/help-and-advice/e-cigarettes https://www.nhs.uk/smokefree/help-and-advice/e-cigarettes Click on "how safe are e-cigarettes" which

https://www.nhs.uk/smokefree/help-and-advice/e-cigarettes https://www.nhs.uk/smokefree/help-and-advice/e-cigarettes Click on "how safe are e-cigarettes" which link to two studies, one by the royal physicians and one by public health England. The one by the PHE is 118 pages and is a summary of all the available evidence as of 2015. And the RCP does something similar in 189 pages. They do not exclusively focus on comparing the relative harm, but that is a large section in each.