copying and pasting my immuno post:
For context, consider that this is true of any adaptive immune system response, because your body is literally throwing whatever works at the wall in the form of antibody gene splicing until by the smallest odds, eventually, one random shuffling will create an antibody that notices that viral antigen, and then you can start replicating the antibody from those b cells that know what the right antibody to code for is. Because this is literally a time consuming process (imagine how long it would take you to guess a 3 digit number if u just had to try all the possibilities), you aren't truly "immune" until this occurs and your body can say "oh snap, covids here.. hey memory b cells! start making those antibodies!". This is why when you get a flu shot, you're counselled to be careful for two weeks until you develop full immunity