This is true but then we still don't know how long the antibodies last for immunity. Some people who've been symptom positive have been PCR/antibody negative.Yes, PCR tests are very sensitive. The whole question of what influences how contegious someone is is still in research. It's a very complex question.
We don't know the viral load to be contagious vs that to be Swan/PCR positive. There's still loads needed to be researched and understood before we can make definite conclusions on pathophysiology.