This is what I have read recently - vaxxed and boosted people (and maybe those with previous infection - not sure) are getting symptoms FIRST, before being infectious - because the increased antibodies immediately pick up that the enemy virus is present so symptoms begin, before the virus has replicated enough to be contagious. So highest chance of correctly testing positive is to test on days 3-5 after onset of symptoms. If you test on just day 1 and/or 2 of symptoms, you may miss the infection.