another vaccine, another reaction

My older son (now 5) received his Quadracel vaccine this morning.
He got a few rather large hives on his back and sides, so he was given a liquid antihistamine right away.  He was tired, grumpy and complained of being sore all day (hard to tell if that is because of the vaccine, the antihistamine, or other factors).
This evening, we noticed his shoulder (where the vaccine was injected) was all swollen and had white patches of hives approx 4 inches diameter.  It is pretty clear to us that it is an allergic reaction to the vaccine.
I checked the ingredients on a Canadian manufacturers website (can't be sure it is same one that produced the vaccine he was given). 
Since there were none of the usual suspects, it could be an as-of-yet undiscovered allergy.  Most of the ingredients of the vaccine seem reasonably innocuous.  The only one I suspect is Neomycin, an antibiotic (remember he is allergic to penicillin).  Turns out neomycin is also used in the varivax and MMR shots he was given when younger, shots to which he also had allergic reactions (which we had attributed to egg and gelatin, but if indeed allergic to neomycin, we could add that to the list).
So, we'll keep an eye out for the signs of a neomycin allergy (1-2 week papule within 24-48 hours) and add it to the list of allergies at the pharmacies and report it to our allergist next time we see him.
I've also submitted an adverse reaction report just in case this is part of something else.

On another note, my other son (almost 4 months) got two shots today too.  He was mighty pissed (still is) and cried a lot all day (normally he beams whenever anyone even looks at him and only cries out of impatience for food; but today he sounded in pain).  For sure it's too early to tell whether he'll develop any allergies or not (genetics put the odds against him), but so far so good.