According to the instructions in the Technical Guide to Vaccination against New Coronavirus (First Edition) issued by the Health Care Commission, the recommended interval between two doses of inactivated New Coronavirus vaccine is not less than 3 weeks. Therefore, the 21-day algorithm for neo-coronavirus vaccination generally means that the day of vaccination is counted as the first day, and the next dose is administered at the earliest 21 days. Inactivated New Coronavirus vaccination does not strictly require a 21-day interval, but usually no earlier than 21 days and no later than 56 days. Therefore, if symptoms such as fever, diarrhea, colds or acute attacks of chronic diseases occur 21 days after the first vaccination, the second vaccination can be postponed and the vaccination can be given in time after the disease has been relieved, but it is important to note that it should not be later than 56 days. In addition, patients need to be clear about the type of vaccine they are receiving. The recombinant subunit neocoronavirus vaccine is currently available clinically and requires three doses, with an interval of at least 4 weeks or 28 days between the two doses, so it is important to check which vaccine you have received and to receive it within the prescribed time. However, if you have already completed two doses of the vaccine and the interval is less than 21 days, there is no need to worry too much. According to the guidelines, people who have completed two doses of inactivated neocoronavirus vaccine between 14 and 21 days can also be immunized without the need for a catch-up vaccination.