My daughte got her mmr today and broke iout in hives. is this a common reaction? if so how bad is it? should i be concerned? should i halt future vax

Don't stop vaccines: About 5% of chldren develop a rash after their first mmr. This doesn't necessarily indicate an allergy, and benefits of getting vaccinated usually outweigh the risk except with severe allergic responses (breathing problems, etc). Typically these go away in a few days but if your child shows any more worrisome signs (wheezing, fever), let your dr know immediately.

1st or 2nd MMR?: Hives after MMR booster occurs rarely and have been reported to be due to traces of gelatin in the vaccine. Mmr vaccine is grown on gelatin. Hives in children with egg allergy receiving MMR vaccine is exceedingly rare. In either case bring this reaction to the attention of your pediatrician. An antihistamine may be prescribed. A referral to any allergist for evaluation may also be helpful.