I found it. Actually, they’re not talking about intelligence or mental stability over time.They’re talking about relief from stress and worry after vaccination.

I agree with that wholeheartedly; I felt much less worried after my first vaccination, even more after the second. That’s why delta was such a disappointment; it took away much of that sense of relief. For a month or so I didn’t wear a mask; now I wear one.

There’s a link to a summary of the study. If I can find it again I’ll come back and post it.

And here it is:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0256406