Antibodies to COVID-19
Antibody (IgG) to S1 subunit of the spike protein remains relatively stable for 6 months plus

Cellular immunity to COVID-19
Memory B cells, cells producing antibodies, were more abundant at 6 months post symptom onset than at 1 month post symptom onset

T cell immunity ( CD4 and CD8 T cells) to SARS-CoV-2/COVID-19 declined with a half life of 3-5 months