I actually think it was probably people with egg allergies and for whatever reason that batch may have had more allergen left over in it. I have always had egg allergies that fortunately are not life threatening, but I didn't know until I was in my 20s and got an allergy test for something totally unrelated (hayfever). I would have never known otherwise...I really thought I might have irritable bowel/got a lot of food poisoning but it was so random I never bothered going to my doctor...so if somebody was allergic, didn't know, and had it injected into their blood stream with a product meant to ramp up their immune response I can see how that could be a real problem.
Well now it's been posted (with a link) that the vaccines in the US contain sulfa drugs and various forms of the penicillin family.. That TOTALLY knocks me right out of the box in terms of getting this shot if they all have it..

And - as has already been mentioned - the mom (in the allergic reation thread) could have been non-allergic to eggs a month ago and then got knocked for a loop when she got a second vaccine containing eggs.. I've been allergic to drugs in the penicillin family since I was a child, but the sulfa drugs came right out of the blue.. Had been on one recently - off - needed to take it again, and WHAM!! Major allergic reaction..
