This is very anecdotal, but I know a number of young people who had COVID and a number of young people who got the vaccine. Of the ones who had COVID, about half report continuing brain fog and thinking issues—one person went from a 99th percentile score on the PSAT to a 80th percentile score on the SAT. Of the ones who got the vaccine (including one who got the vaccine and booster shot after previously having COVID) there have been no noticed issues with thinking/​intelligence.
This is very anecdotal, but I know a number of young people who had COVID and a number of young people who got the vaccine. Of the ones who had COVID, about half report continuing brain fog and thinking issues—one person went from a 99th percentile score on the PSAT to a 80th percentile score on the SAT. Of the ones who got the vaccine (including one who got the vaccine and booster shot after previously having COVID) there have been no noticed issues with thinking/​intelligence.