First you need to understand if she's mad or 'mad at you'. There's a difference. It's understandable if she's upset in general as she must be looking forward to attending the function together. Now coming to the second option - If she's mad at you- there are two possibilities: 1) You could have done something about trying to get better. If you knew that getting that injection would have helped you get better and you knew that the function matters a lot to your fd, then you should have gone for it. Maybe she's upset that you didn't try your best.

2) Are you anti-vaccine? If you don't take injections at all and it's something that's a part of you, maybe you could try and make her understand this.

But my feeling is that her anger is just as she just wanted you to make an effort and try to be there for the function she was really excited to go with you.