Repeated vaccine injections, if they do not produce serious adverse reactions, do not have much effect on the body and can lead to a higher titer of antibodies against a certain pathogen in the body and increased protection. Vaccine is a kind of active immunity, the injected vaccine is an antigen that stimulates the body to produce specific antibodies, that is, immunoglobulins against a certain pathogen, and repeated injections are equivalent to booster shots. It can be administered as long as it does not produce serious adverse reactions in the body. Many vaccines do not require a single dose, such as rabies vaccine, which requires five doses for post-exposure prophylaxis. Chickenpox vaccine is usually given twice, HPV vaccine requires three doses, and hepatitis B vaccine also requires three doses, usually the second and third doses are booster immunizations, and immunizations are not lifelong immunizations, but have a certain immunization period. After the immunization period, antibodies can disappear and a booster shot is needed, for example, for the hepatitis B virus vaccination, a booster shot can be given before antibodies disappear or when the titer is low.