Backing up the post above, a very good explanation of t-cell immunity here from CEBM in Oxford
Annette Plüddemann, Jeffrey K. Aronson On behalf of the Oxford COVID-19 Evidence Service Team Centre for Evidence-Based Medicine, Nuffield Department
www.cebm.net
This part is important
Potential for long-term immunity
Early research suggests that the antibodies in people infected with SARS-CoV-2 dropped significantly within 2 to 3 months [
21,
22], causing concern that humoral immunity against the virus may decline rapidly. However, it is a normal part of the immune response that antibody levels fall after an infection has resolved [
23]. For example, in seasonal coronavirus infections, antibodies start to decline at about a week after infection and typically only last for about a year [
24]. It should also be noted that memory T and B cells are formed after infection [
25,
26]; these can be reactivated when another infection with the same virus occurs and could provide long-lasting immunity. A preliminary study that has not yet undergone peer review has shown that memory T and B cells were found in patients with mild COVID-19 symptoms who had recovered and that these cells persisted, suggesting the potential for longer-term immunity [
27].
SARS-CoV-2-specific memory T cells have also been detected in exposed seronegative healthy individuals (relatives of confirmed cases), which may indicate asymptomatic infection. One study has shown that ~93% of “exposed asymptomatic” individuals had a T cell response to SARS-CoV-2, despite seropositivity in only 60% of cases [
28]. Asymptomatic infections may therefore be more common, and antibody testing alone may underestimate the true prevalence of the infection or population immunity. SARS-CoV-2-specific T cells were found in most of the convalescent patients in this study, which is a promising sign that infection may give rise to immunity [
29]