One possibility is that better air circulation in closed rooms due to open windows, higher vitamin D levels, and people meeting others outdoors rather than indo

One possibility is that better air circulation in closed rooms due to open windows, higher vitamin D levels, and people meeting others outdoors rather than indoors more often have been contributing factors. These ameliorating factors would play less of a role in Southern countries like Spain or Italy, since they were already present in spring in those regions and so all infections took place in spite of them. Covid could be "lightly seasonal", so-to-say, though by far too infectious to be as seasonal as influenza. This is speculation, of course, unless it's backed up with studies. By the way, maybe I'm mistaken about this, but it seems to me that any increase of the number of immunized people will statistically decrease the R-value even long before herd-immunity is reached. Together with the measures and heightened awareness this might also explain the data.