[f6v]

I think the key is that you get 2 shots. The second one might result in antibodies that bind the antigen better. This might happen because the immune cells undergo another round of mutation and selection after a second round of antigen exposure.

[astrange]

It's typically because of the adjuvants added to vaccines, and some other mutations done for stability.

The major covid vaccines have fewer adjuvants (to allow for boosters) and 2 shots because that's just how the trial was run, and it turned out to work, but later testing shows they're pretty effective just with one and some countries have been delaying the second one until everyone gets their first.