Abstract
BBIBP-CorV exerts efficient protection against SARS-CoV-2 infection. However, waning vaccine-induced humoral immune responses after two-dose vaccination have significantly undermined durable immuno-protection. In this study, we have demonstrated that although anti-spike (S) antibody responses in BBIBP-CorV vaccinees exhibited three serotypes after 6 months, including de novo sero-negative, sero-positive, and sero-decay features, S-specific interferon-γ release as well as Th1 cytokine production in CD4(+) and CD8(+) T cells were comparable, especially in vaccinees without detectable neutralizing antibodies. Notably, regardless of dramatic increases in humoral immunity after booster vaccination, T cell responses targeting S protein from either wild type or Omicron remained stable before and after booster vaccination in all three serotype vaccinees. No severe cases were observed even in the sero-decay group during the Omicron epidemic in Shanghai. Our results thus illustrate that unlike fluctuating humoral responses, viral-specific T cell responses are extremely stable after booster vaccination. Sustained T cell responses might be dedicated to the rapid restoration of antibody responses after booster vaccination.