So what is causing the side effects people in the studies are suffering? There seems to be a lot of concern on people not wanting the second dose. I know flu sh

So what is causing the side effects people in the studies are suffering? There seems to be a lot of concern on people not wanting the second dose. I know flu shots will always make the injected arm hurts for days (never have that reaction with other vaccines for some reason). If this is just creating proteins and not an inactivated virus, what's causing the responses that are being reported?