reaction to 5 way vaccine?

It sounds like your horse has had the most common type of reaction to a vaccine--localized swelling. This commonly goes away in a couple of days but it is possible to have an abcess form. These reactions occur because vaccines are designed specifically to cause a reaction by the immune system and inflammation is a part of that response. So while it is likely nothing to be really concerned about it is a good idea to keep an eye on it and make sure it is going away rather than getting larger and consider hot packing the area for 10-15 minutes twice a day. It's also a good idea to take your horse's temperature daily (preferrably in the morning) and watch for any fever associated with this reaction as well as increase in lethargy, loss of appetite, etc. Call your vet and tell him what has happened and see what he recommends beyond this. Sometimes your vet will have you give oral Bute or Banamine to help reduce the inflammation more quickly.

(Someone else has mentioned that you might have hit a vein--if so you would have had a serious and almost immediate response.)

I would definately call my vet. He can tell you if it is a problem or not. I have heard of people giving vaccines and accidenly hitting veins and things so you'll want to make sure something like this didn't happen.

Before vaccinating, you should have been aware of the risks. Yes people there are risk's, allot more then you think.

Check out these sites, I have found them very helpful.

http://www.shirleys-wellness-cafe.com/petvacc.htm

http://www.buttercuppuppies.com/Immunizations.html

It is possible that this reaction could be severe, but likely common. Call your vet and ask him/her because he/she might have seen reactions like yours before.