Here is a pretty good (and recently updated) article from the Crime Prevention Research Center (CPRC) regarding the prevalence of mass public shootings in place where guns are prohibited for the unbadged versus localities where concealed carry is not meaningfully restricted. The author also explain how the propaganda from groups like "Everytown for Gun Safety" and "Mayors Against Illegal Guns" distorts the numbers to better serve their agenda.
I did not realize the the difference was so marked. In the 55 years since 1950, 98.4% of mass public shootings happened in places where carry for ordinary persons was prohibited either by local law or the venue owner's requirements. If only the years from 1998 to 2016 are considered that number drops slightly to 96.2%. While no solid conclusions can be drawn from just those two numbers, they do make it reasonable to consider the proposition that: If guns are a "Pubic Heath" problem, the best vaccine is not prohibition but empowering peaceable citizens to carry firearms for defense of self and, where appropriate, others.