File this under ‘something I should have seen but didn’t.’
So, why did so many vaccinated people test for COVID, but still not get sick?
Intramuscular vaccinations stimulate strong IgG antibodies and cellular immunity which protect you from serious illness, but they don’t result in high IgA antibodies which are found in nasal and other mucous.
Nasal spray vaccines and natural infections do stimulate IgA more effectively.
So, COVID, especially Delta, take hold in the nasal passages and throat, so you test positive, but the disease doesn’t get much beyond that.
That explains a lot. but I’ll still get my booster when I can.