I try to avoid repeat posts but this,
from another thread, is probably significant...
There is a growing body of research that points to T-cells playing an important role in the immune response to this virus, particularly in the degree of severity of outcome.
Depletion of certain T-cells (CD3+) may be key in severe cases.
T cell senescence is well known to be a feature of ageing.
Clinical evidence reveals how host factors affect coronavirus infection.
www.nature.com
Whereas a robust T cell response (here, CD4+ and CD8+) now appears to be a feature of mild and asymptomatic COVID-19 cases.
SARS-CoV-2-specific memory T cells will likely prove critical for long-term immune protection against COVID-19. We systematically mapped the functional and phenotypic landscape of SARS-CoV-2-specific T cell responses in a large cohort of unexposed individuals as well as exposed family members...
www.biorxiv.org
Which would go a long way to explaining the apparent (false) negative test results in cohorts who have experienced COVID-19 symptoms. (The bulk of antibody testing is looking for IgG and, some, IgM antibody responses - certainly that currently available to the public at home or via testing sites/labs).