ABSTRACT
A recent MMWR reported that the effectiveness of a 3rd dose of SARS-CoV-2 mRNA vaccine waned quickly in the Omicron-predominant period. Similarly, a substantial decline of immune responses induced by a 3rd dose of inactivated vaccines was also observed in our study. In response to the fast waning immune response and the great threat of Omicron variant of concern (VOC) to frontline healthcare workers (HCWs), 38 HCWs who were in our previous cohort investigating responses to the first three doses of inactivated vaccines participated in the current study and volunteered to receive a 4th homologous booster. Here, we demonstrated that the 4th dose is safe and capable of recalling waned immune responses 6 months after the 3rd dose. However, a greater suppression on the induction of overall Neutralizing antibodies (NAbs) and NAbs targeting the receptor-binding domain (RBD) was found in participants with stronger immune responses after the 3rd dose. As a result, a stepwise elevation of RBD-NAbs from the 1st to the 3rd vaccination achieved a "turning point". The peak RBD-NAbs level induced by the 4th dose was inferior to the peak of the 3rd dose. Accompanied with reduced induction of RBD-NAbs, the immune system shifted responses to the nucleocapsid protein (NP) and the N-terminal domain (NTD) of the spike protein. Although NTD directed antibodies are capable of neutralization, they only compensated the loss of RBD-NAbs to ancestral SARS-CoV-2 virus but not to the Omicron variant due to a substantial conformational change of Omicron NTD. This longitudinal clinical study monitored the immune response of the same cohort for every doses, shaping a relationship between the trajectory of immune focus and the dynamics of the neutralizing potency against the evolving virus. Our data reveal that immune responses could not be endlessly elevated, while suppression of heightened immune responses focusing on one subunit together with a shift of immune responses to other subunits would occur after repeated vaccination. Thus, an updated vaccine with more diverse epitopes capable of inducing NAbs against VOCs would be a future direction for boosters.