I haven't heard about cortisone for a preventative for that kind of a reaction but it can be used for allergies, vomiting, and it helped my cat Pepper recover from his illness. It sounds like you have found a very good and caring vet. Good luck. My cat Sunny had a sneezing reaction to the FIP vaccine and he developed a fever too. Now I only get the FVRCP shots for my furbabies. They are also indoor only cats but they do go to a salon to have their nails clipped. Good luck with Bassett and please keep us posted.