Most COVID vaccine information is focused on how effective they are at preventing serious disease, hospitalization, and death. How is it possible that the vaccine is more effective at preventing serious illness and death than it is preventing a mild infection?
It’s actually very common for vaccines to be much better at preventing serious illness and death than preventing infection or mild infection. For example, with the flu vaccine, people can still often get the flu, but they are much less likely to get seriously ill or die if they get the flu vaccine.
The question is why. It partly depends on how the immune system responds to vaccines. Any infection whatsoever is a certain type of immune response, and very few vaccines give what people call a “sterilizing immune response.”
What vaccines do cause is an immune response that is strong and multifaceted inside your body. So, even if you knew that the virus can replicate a bit for a mild infection, it can’t cause that huge overwhelming infection that really puts people at risk.