Usually, vaccines can have a preventive or immunizing effect, while the third dose of vaccine is usually a booster immunization, which has both positive and negative effects on the body. Therefore, it is recommended that vaccinators follow a regular vaccination program under the guidance of a doctor to better reflect the positive effects while avoiding other adverse effects of the vaccine on the body as much as possible. The positive effects of the third dose of vaccine on the body mainly lie in the enhanced prevention of diseases. Common vaccines that require a third dose include the New Crown vaccine and HPV vaccine. It is only after receiving three doses that the full process of immunization can be completed, which can produce sufficient antibodies in the body to prevent the corresponding diseases and have certain health benefits. Especially for the new crown vaccine, the third injection also plays a greater role in the prevention of variant strains and can reduce the probability of serious cases. Therefore, it is recommended that those who are suitable for vaccination actively receive the third dose of the vaccine within an appropriate time frame. However, since most of the vaccines are protein-based and foreign to the body, some people may experience temporary negative effects on the body, i.e. adverse reactions, when the third vaccination is administered to the immune system, depending on their individual constitution. Common adverse reactions include localized redness, swelling, nodules, pain, and generalized transient fever (usually low fever), dizziness, nausea, weakness, drowsiness, and muscle aches, etc., which may gradually resolve a few days after vaccination. Some people may also have allergic reactions to the vaccine, such as skin rash and itching, and in severe cases, even chest tightness, throat edema, respiratory difficulty or even shock, but such cases are relatively rare. Therefore, if symptoms such as hives and itchy skin occur during the first two doses, it is not recommended to receive a third dose of the vaccine again.