The second dose of the vaccine does not have to be given 21 days apart. In the case of the new crown inactivated vaccine, the second dose is required to be injected ≥ 21 days and controlled within 56 days. It can be injected either at 21 days or after 21 days, but earlier injection is not advocated. If symptoms such as fever, diarrhea, cold, etc. occur 21 days after the first dose of vaccine or in the acute attack period of chronic diseases, the second dose can be postponed and the vaccination can be given in time after the condition is relieved. However, it should be noted that it should not be later than 56 days. In addition, patients need to be clear about the type of vaccine they are receiving. Currently, there are three doses of recombinant neocoronavirus vaccine available in the clinic, and the interval between the two doses should be at least 4 weeks, so it is important to check the vaccine you have received and try to ensure that the vaccination is completed within the window. In addition, there are many different vaccines for different pathogens, such as hepatitis B vaccine, human papillomavirus vaccine, rabies vaccine, etc. The interval between the second dose of the vaccine varies for different vaccines, and you generally need to follow the prescribed interval to get the vaccination. If you are not able to get the injection on time for various reasons, do not postpone the injection for too long if possible.