Regarding the question about how the vaccine could protect against severe but not mild infection, I think they answer that here:
> Antibodies can prevent infection, your T cell-responses ensure that those antibodies keep doing their job and they kick in after you're infected OR oh my goodness stop the presses- VACCINATED (how about them apples). In other words, while robust T-cells responses cannot protect you from a mild or moderate infection sometimes (think cough, sniffles, etc.) they can however proliferate rapidly and prevent the build up of viral load. Psst- you want this to be LOW. VL drives disease severity.
> Antibodies can prevent infection, your T cell-responses ensure that those antibodies keep doing their job and they kick in after you're infected OR oh my goodness stop the presses- VACCINATED (how about them apples). In other words, while robust T-cells responses cannot protect you from a mild or moderate infection sometimes (think cough, sniffles, etc.) they can however proliferate rapidly and prevent the build up of viral load. Psst- you want this to be LOW. VL drives disease severity.