He probably realizes that almost every vaccine was an attenuated (weakened) virus vaccine. Those are more effective than inactivated (killed) virus vaccines. Inactivated vaccines are frequently poorly effective because they do not stimulate much immune response and don’t go through the same processing that live viruses do. They typically have only been used for immunocompromised patients in the past in order to mitigate the potential that an attenuated vaccine has to still overcome the immune response if a patient is weak and can’t fend off much of anything. Most previous vaccines were attenuated, which poses very little threat to someone with a natural and normal strength immune system.