Punchline: “The cumulative incidence of SARS-CoV-2 infection remained almost zero among previously infected unvaccinated subjects, previously infected subjects who were vaccinated, and previously uninfected subjects who were vaccinated, compared with a steady increase in cumulative incidence among previously uninfected subjects who remained unvaccinated. Not one of the 1359 previously infected subjects who remained u…
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'.