Injection after 20 days after a cat scratches is ineffective, and the risk period of rabies has passed after 20 days, and the wound has basically recovered, so there is no need to vaccinate again. The best time to inject rabies vaccine is within 24 hours after being bitten, so if you are bitten by a cat, you should clean and disinfect the wound immediately, and then go to a pet hospital immediately to inject rabies vaccine to avoid being infected with rabies virus.