ziyou, My reading suggests that an almost immediate reaction is most likely from anaphylactic shock, in which case having another is like a peanut allergic person saying "Hell that nut didn't kill me, throw some peanuts into my cocktail." It is this possibility that you are asked to wait half an hour before leaving. Such a person should never be bullied into the next.
Anaphylaxis is deadly and swift, fortunately rare in this context.
Another cause of a quick reaction is an accidental IV [not IM] injection. If you get a taste in your mouth during this time RAISE THE ALARM. I'm not sure it isn't too late already.
If you are getting the shot go to a hub staffed by RNs and INSIST that he/she aspirate. I did, and while the greeter had no idea what I was talking about, the RN did and all was well.
For more info search on "Dr John Campbell + Kyle".
