From what I understand, it's peer-reviewed scientific research done in 2016 by the Royal College of Physicians and Surgeons of Canada, and they state that vaping is likely at least 95% less harmful or toxic than smoking a cigarette. Recently, on February 6, Public Health England did a review on the success and progress of their liberal, open-minded approach to vaping and harm reduction in their country.
On February 14th, 2018. See this statement in context.