Currently, it mostly refers to the new crown vaccination with only two doses, which are completed if it is the adenovirus carrier vaccine, or the third dose in time if it is another vaccine. The adenovirus carrier vaccine currently requires a total of two vaccinations, i.e. the first and booster shots. Since the primary immunization is one dose, followed by a booster, the total number of vaccinations is two. However, for inactivated and recombinant protein vaccines, a total of three doses are required. Administering only the first two doses may affect the effectiveness of the vaccine and prevent it from achieving its intended preventive effect. The recombinant protein vaccine is a three-dose series, and the third dose should be completed as close as possible to six months after the first dose. Therefore, if only two doses have been given it is recommended to get the vaccination in time.