There was no claim that the vaccine could prevent Covid in all cases, only that you may get a milder case and keep you out of the hospital. And yes, some people can get serious side effects, but the worst side effect is death. How quickly we seem to have forgotten the bodies piling up in refrigerated trucks outside of hospitals and the hospital staffers risking their lives and working to the point of exhaustion. The mRNA technology has been in existence for a long time and luckily it wasn't needed in the U.S. until Covid came along. Viruses like Covid and flu mutate constantly and new formulations must be prepared to counter the latest strains, so it is vitally important that the vaccines are are made available quickly.