I don't know about you, but I've had to go to more than one class to learn stuff for lots of complicated things. Think of each vaccine dose as a class in a larger course and not the entire course all at once.
How many articles do people need to read to learn the "best" way to get on the most rides at Magic Kingdom? I'll bet most people need to read more than one. Most probably even need to read the same articles more than once to get all the meaning out of them.
Teaching your body how to fend off a virus sounds at least as hard as learning how to get the most out of
Fast Pass Lightning Lane. Look at that, it mutated to a different variant and needed new directions.