Three and a half years after a child has had the rabies vaccine it has lost its effectiveness in preventing rabies. If you want to prevent it, you need to have a second vaccination. Rabies vaccination mainly refers to the injection of attenuated or inactivated pathogens into the organism, thereby stimulating the immune system to produce the appropriate antibodies, which can effectively prevent rabies virus infection and attacks. The amount of antibodies produced by rabies virus vaccine stimulation has a certain metabolic loss and will gradually decrease with the extension of the vaccination period. Rabies virus antibodies below the effective level are not effective in preventing rabies virus. Rabies vaccine is effective in preventing rabies virus infection within one year after vaccination. However, the amount of rabies virus antibodies present in the body is not stable for 1 to 3 years after vaccination and may lose its preventive effect. Therefore, one year after vaccination, if there is a chance of rabies virus infection due to an animal bite, it is recommended to receive a second vaccination to effectively increase the concentration of antibodies in the body. If a child is bitten by an animal after receiving rabies vaccination, he/she should consult his/her doctor to determine whether revaccination is necessary.