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You should be very worried. Any time that a cat won't eat for more than 48 hours, you are in serious trouble. Their liver can fail and then the cat can die. It is called hepatic lipidosis where the liver thinks it is being starved and starts "grabbing" fat cells...and actually ends up killing the liver.
I suggest that you see the vet first thing tomorrow for a full exam and possibly bloodwork to find out what's going on.
This could be a vaccine reaction or it could be something more serious. I would be more worried about the cat not eating than anything.
Let me know if you still have questions.