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OBJECTIVE The dynamic adaptive immune responses elicited by the inactivated virus vaccine, CoronaVac, remain elusive. METHODS In a prospective cohort of 100 SARS-CoV-2 naïve healthcare professionals who received two doses of CoronaVac, we analyzed SARS-CoV-2-specific humoral and cellular responses at four different timepoints, including before vaccination (T1), 2 weeks after the first dose (T2), 2 weeks after the booster dose (T3), and 8-10 weeks post the booster dose (T4). SARS-CoV-2-specific antibodies, serum neutralizing activities, peripheral B cells, CD4 + and CD8 + T cells, and their memory subsets were simultaneously measured in this cohort. RESULTS SARS-CoV-2 Spike-specific IgG responses reached the peak (geometric mean titer [GMT] 54827, 30969-97065) after two doses and rapidly declined (GMT 502, 212-1190) at T4, whereas suboptimal IgA responses were detected (GMT 5, 2-9). Spike-specific circulating B cells (0.60%, 0.46-0.73% of total B cells) and memory B cells (1.18%, 0.92-1.44% of total memory B cells) were effectively induced at T3 and sustained over time (0.33%, 0.23-0.43%; 0.87%, 0.05-1.67%, respectively). SARS-CoV-2-specific circulating CD4 + T cells (0.57%, 0.47-0.66%) and CD8 + T cells (1.29%, 1.04-1.54%) were detected at T3. At T4, 0.78% (0.43-1.20%) of memory CD4+ T cells and 0.68% (0.29-1.30%) of memory CD8+ T cells were identified as SARS-CoV-2 specific, while 0.62% (0.51-0.75%) of CD4 + T cells and 0.47% (0.38-0.58%) of CD8 + T cells were SARS-CoV-2 specific terminally differentiated effector memory cells. Furthermore, age and interval between doses affected the magnitude of CoronaVac induced immune responses. SARS-CoV-2 memory CD4 + T cells was strongly associated with both RBD-specific memory B cells ( r =0.87, p <0.0001) and SARS-CoV-2 specific memory CD8 + T cells ( r =0.48, p <0.0001). CONCLUSIONS CoronaVac induced robust circulating and memory B cells and T cell responses. Our study offers new insight into the underlying immunobiology of inactivated virus vaccines in humans and may have implications for vaccine strategies in the future.

The waning humoral immunity and emerging contagious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants resulted in the necessity of the booster vaccination of coronavirus disease 2019 (COVID-19). The inactivated vaccine, CoronaVac, is the most widely supplied COVID-19 vaccine globally. Whether the CoronaVac booster elicited adaptive responses that cross-recognize SARS-CoV-2 variants of concern (VoCs) among 77 healthy subjects receiving the third dose of CoronaVac were explored. After the boost, remarkable elevated spike-specific IgG and IgA responses, as well as boosted neutralization activities, were observed, despite 3.0-fold and 5.9-fold reduced neutralization activities against Delta and Omicron strains compared to that of the ancestral strain. Furthermore, the booster dose induced potent B cells and memory B cells that cross-bound receptor-binding domain (RBD) proteins derived from VoCs, while Delta and Omicron RBD-specific memory B cell recognitions were reduced by 2.7-fold and 4.2-fold compared to that of ancestral strain, respectively. Consistently, spike-specific circulating follicular helper T cells (cTfh) significantly increased and remained stable after the boost, with a predominant expansion towards cTfh17 subpopulations. Moreover, SARS-CoV-2-specific CD4 + and CD8 + T cells peaked and sustained after the booster. Notably, CD4 + and CD8 + T cell recognition of VoC spike was largely preserved compared to the ancestral strain. Individuals without generating Delta or Omicron neutralization activities had comparable levels of CD4 + and CD8 + T cells responses as those with detectable neutralizing activities. Our study demonstrated that the CoronaVac booster induced broad and potent adaptive immune responses that could be effective in controlling SARS-CoV-2 Delta and Omicron variants.