T cell tests look for evidence of the immune response to Covid-19. The aim is to answer the question: have I been exposed to Covid-19 and produced an immune response? The T cell test is intended to detect a T cell-mediated immune response to SARS-CoV-2, which is different to serology which looks for an antibody response. T cells and antibodies are two arms of the adaptive immune response. The detection and measurement of each can provide different information about disease activity and an individual’s personal immunity. Antibodies may not be present in everyone who gets infected with SARS-CoV-2 and they may not be detectible after a long period of time. Evidence of a T cell response in the blood suggests previous exposure to the SARS-CoV-2 virus.
If current infection is suspected or you want to see if you are carrying covid-19 now, you need to use an antigen test (PCR test).
We do not yet know if the presence of SARS-CoV-2 specific T cells gives protection and makes you immune, nor how long any immunity will last.