As far as needed multiple booster to maintain immunity, do you take the flu shot yearly? We have to do that because there are different strains (also read as variants) of the flu virus that are assumed to be prevalent in the coming year. Maybe in another 20 years (hopefully sooner), if corona hasn’t died out by then, doctors, scientists, and researchers will understand enough about the virus and its variants to provide a yearly booster aimed at the most prevalent variant at that time. For now we’re behind the 8 ball, trying to play catch up to a disease that seems to always be 2 steps ahead of us.
Also, do you know how many doses of vaccine for DTaP are given to infants in order to provide some level of immunity? Check it out. And even after that there are further boosters given in childhood/adolescence, and still a person needs a tetanus booster shot every 10 years to maintain immunity. Does that mean it is not effective?