We knew perfectly well back then that bad cases of Covid were rare in teenagers.
https://www.thelancet.com/journals/lanwpc/article/PIIS2666-6...
> Full vaccination of household contacts reduced the odds to acquire infection with the SARS-CoV-2 Delta variant in household settings by two thirds for mRNA vaccines and by one third for vector vaccines. For index cases, being fully vaccinated with an mRNA vaccine reduced the odds of onwards transmission by four-fifths compared to unvaccinated index cases.