The interval between injections differs with different types of New Crown vaccine: In the case of inactivated New Crown vaccine, a second dose is not recommended if it is less than 21 days old. When the interval between two doses of inactivated New Crown vaccine is less than 21 days, the vaccine effect cannot be optimal. Therefore, for the second and subsequent doses of inactivated Newcastle vaccine to produce optimal immunization, an interval of more than 21 days is required. The recommended interval between 2 doses of basic immunization with inactivated vaccine is 3-8 weeks (21 days-56 days). After the first dose of the vaccine, antibodies are produced in the human body against the new coronavirus. However, the amount of antibodies produced is not enough and the duration of antibodies is short. Only through re-vaccination, the immune system is stimulated to produce immunity again, so that the memory cells can rapidly produce antibodies against the new coronavirus, so that the number of antibodies produced is large, more efficient, and of longer duration, and the level of resistance to the virus is greatly increased. It takes time to generate memory cells, and it takes 21 days for memory cells against neo-coronavirus to develop a certain size to achieve the expected effect at the second injection. This 21-day period is the result of a large number of clinical trials, so only after 21 days can the second injection be given to achieve effective immunity, and early administration of the second injection will compromise the effectiveness of the vaccine.