[nailer]

> This states that a Covid vaccine passing the placebo-controlled study requirement 4 years ago will not suffice to accept updated versions of the same Covid vaccine

Yes. That is what the comment you were replying to states.

> More concisely: it's an upper bound, but not a lower bound.

You can’t say it’s any bound at all. Maybe more than two milliseconds old (less than four years) is unacceptable, maybe 16 millennia (more than four years) is unacceptable. They’re just thinking about four years as being unacceptable. Which sounds reasonable.