It is possible to be re-infected with a certain infectious disease even after vaccination. Vaccination is given for a certain pathogen or even a certain subtype of a certain pathogen, and does not cover all pathogens of diseases, for example, there are many viruses that cause HFMD, and HFMD vaccine is only for HFMD caused by EV71 type, and does not prevent HFMD caused by Coxsackie A group 16. No vaccine can provide 100% protection. Antibodies can be produced after vaccination, but the titer of antibodies gradually decreases over time, and the protection decreases as well. Whether antibodies are produced after vaccination and the titer of antibodies are related to the immunity of a person. People with low immunity, such as AIDS patients, patients with malignant tumors, and people who apply hormones for a long time, have a higher incidence of infectious diseases because they have low immunity and produce low levels of antibodies, and these people are prone to various infections.