Not all dog bite cases require serum, but only dog bites that reach level III exposure. Tertiary exposure refers to a bite that is bleeding or has a deep bite wound, in which case the rabies virus may enter the nervous system directly, resulting in a significantly shorter incubation period for rabies. Therefore, the rabies vaccine alone may not be able to prevent rabies attacks, and a serum injection is necessary. After the rabies vaccine is injected into the body, it takes a period of time (usually about 10 days) for the amount of antibodies to reach a more effective level. In the time between the start of the injection and the time when the antibody level reaches an effective level, there is a lack of effective protection, and this is where the rabies serum can make up for this deficiency. Because it is a passive immunization, it is a direct injection of antibodies to the rabies virus that gives a person immediate resistance to rabies.