In that case it seems like the headline reads wrong based on a colloquial and imprecise but often correct heuristic. Should read more like "non-necessity of C19 vaccination for previously infected..."
[EDIT: I read further down this page, dang commented that he'd edited in the 'evaluating' just 1m before parent's comment: https://news.ycombinator.com/item?id=27453952 So, yes, fair enough, 'necessity of' is ambiguous, as I went on to say in my original comment continuing below.]
Necessity is the concept as well as describing one possible state of it.
Another example: when 'my hunger is sated', I do not 'have a hunger', I am 'not hungry'.