So unfortunately since I specifically and repeatedly referred to asymptomatic cases, which are relatively common in those under 30, it is not good enough.

If I could edit my original post to <B>BOLD</B> asymptomatic even more, I would, since I highly doubt you're going to find many studies on that group.

The sample size is very small and they list other limitations to their findings, but:

"In conclusion, despite concerns of waning immunity, appropriate immunoassays can detect antibodies against SARS-CoV-2 at 8 months after infection in most asymptomatic or mildly symptomatic persons."

However, from my very first post "vaccine immunity is more robust than asymptomatic infections"... and detectable antibodies is a far cry (orders of magnitude) from neutralizing antibody levels, which are not uncommon even 8 months out in healthy vaccinated youth.