E.T.
This of course makes sense, the more people are infected, the more chance of a mutation. You could argue that other factors may also come to play as the US for example has a higher infection and death rates per population but no mutations yet to this scale. It could of course also be a function of population density and infection and death rate, but I am not sure whether those would not also be a function of the total population.
The other extremely worrying development which seems to have been contained for the moment, is the outbreak in mink in Denmark as an escape to another host indicates also mutations that allow this to happen, and that such adaptations can also occur to pets and other farmed animals and wildlife.
The original ‘herd immunity’ model that was briefly flirted with and criticized has I think remained the essential policy of this government and at no stage was there a policy of total suppression, merely flattening the curve, and even that was done incompletely and not for a sufficiently long period.