Earlier quoted context omitted.
Okay, so if the binary thinking is wrong, why stop at three? Why stop at all? Does the science say we just blast our bodies with as much mRNA as we can get away with?
Assuming you are asking earnestly, because of diminishing returns. Pfizer has been found to be 90 something percent effective at 2 months. They've noticed it seems to lose about 3% effectiveness per month, leaving you at 70-something percent after 8 months. That's with the data we have today, it could be much higher or lower as more data is collected. To preemptively answer your question - yes, I could see needing to…
The economics of this are showing increasing returns to the worldwide medical cartels, so this looks like it will continue indefinitely. With broad immunity to product safety laws, why not lay in another billion or two as profit? The C-level company staff will gleefully take home fat bonus checks.