Introduction
Neutralizing antibodies against omicron variants and subvariants, which represents a strong correlate of protection from SARS-CoV-2 infection, have been showed to significantly increase after bivalent booster administration 1-4. Accumulating evidence suggests that T cell response, i.e. helper CD4+ and cytotoxic CD8+ T cells, plays a key role in the protection against severe disease 5. In contrast to neutralizing antibodies, T cells are more resilient against highly mutated emerging variants, with >80% of epitopes conserved among T cells 5,6. Currently, the long-term kinetics of the humoral and cellular immunity has been poorly explored.