NAC = N-acetyl cysteine
There is a time release form from Jarrow I would suggest.
I do believe these supplements are fine to hold off taking until you notice any symptoms. However, we should have zinc every day and a cup or two of green tea or a capsule of quercetin is harmless every day, regardless of COVID.
Good info— GNC, now that it is in bankruptcy and being taken over by... some other company, in certain stores is not well stocked in the NAC (weightlifters/sports peeps use it commonly completely aside from this immunity support utility).
Source it when you see it.