Correct - I think there's a misunderstanding here. The reason why the OP that you replied to wants to wait is certainly to be more safe of side effects.
In this context it is not as relevant how long the median time is for side-effects to show, but rather how long it takes before the vaccine is no longer used.
When you said "visible" I assumed you mean visible to the OP - but you meant visible in general. So you are not wrong, but as we can see from the case of Pandremix, a median of 42 made it take two years before the usage of the vaccine was stopped.
There are a lot of reasons why it would probably be different in the case of covid (e.g. more media presence from the beginning) but in general, you cannot directly compare thus number (like the 42 days) with the 4 years of the OP.
