SARS-CoV-2 inactivated vaccines have shown remarkable efficacy in clinical trials, especially in reducing severe illness and casualty. However, the waning of humoral immunity over time has raised concern over the durability of immune memory following vaccination. Thus, we conducted a nonrandomized trial among the healthcare workers (HCWs) to investigate the long-term sustainability of SARS-CoV-2-specific B cells and T cells stimulated by inactivated vaccines and the potential need for a third booster dose. Although neutralizing antibodies elicited by the standard two-dose vaccination schedule dropped from a peak of 29.3 arbitrary units (AU)/mL to 8.8 AU/mL 5 months after the second vaccination, spike-specific memory B and T cells were still detectable, forming the basis for a quick recall response. As expected, the faded humoral immune response was vigorously elevated to 63.6 AU/mL by 7.2 folds 1 week after the third dose along with abundant spike-specific circulating follicular helper T cells in parallel. Meanwhile, spike-specific CD4+ and CD8+ T cells were also robustly elevated by 5.9 and 2.7 folds respectively. Robust expansion of memory pools by the third dose potentiated greater durability of protective immune responses. Another key finding in this trial was that HCWs with low serological response to two doses were not truly “non-responders” but fully equipped with immune memory that could be quickly recalled by a third dose even 5 months after the second vaccination. Collectively, these data provide insights into the generation of long-term immunological memory by the inactivated vaccine, which could be rapidly recalled and further boosted by a third dose.

SARS-CoV-2 inactivated vaccines have shown remarkable efficacy in clinical trials, especially in reducing severe illness and casualty. However, the waning of humoral immunity over time has raised concern over the durability of immune memory following vaccination. Thus, we conducted a non-randomized trial among the healthcare professionals (HCWs) to investigate the long-term sustainability of SARS-CoV-2-specific B cells and T cells stimulated by inactivated vaccine and the potential need for a third booster dose for the HCWs. Although neutralizing antibodies elicited by the standard two-dose vaccination schedule dropped from a peak of 31.2 AU/ml to 9.2 AU/ml 5 months after the second vaccination, spike-specific memory B and T cells were still detectable, forming the basis for a quick recall response. As expected, the faded humoral immune response was vigorously elevated to 66.8 AU/ml by 7.2 folds 1 week after the third dose along with abundant spike-specific circulating follicular helper T cells in parallel. Meanwhile, spike-specific CD4+ and CD8+ T cells were also robustly elevated by 5.9 and 2.7 folds respectively. Robust expansion of memory pools by the third dose potentiated greater durability of protective immune responses. Another key finding in this trial was that HCWs with low serological response to 2 doses were not truly no responders but fully equipped with immune memory that could be quickly recalled by a third dose even 5 months after the second vaccination. Collectively, these data provide insights into the generation of long-term immunological memory by the inactivated vaccine, which has implications for future booster strategies that the frontline HCWs, individuals with low serological response to 2 dose of vaccine and immune compromised patients could benefit from a third dose of inactivated vaccine.

The utilization of dendritic cell (DC) vaccines is a promising approach in cancer immunotherapy, and the modification of DCs for the expression of tumor‐associated antigens is critical for successful cancer immunotherapy. A safe and efficient method for delivering DNA/RNA into DCs without inducing maturation is beneficial to achieve successful DC transformation for cell vaccine applications, yet remains challenging. This work presents a nanochannel electro‐injection (NEI) system for the safe and efficient delivery of a variety of nucleic acid molecules into DCs. The device is based on track‐etched nanochannel membrane as key components, where the nano‐sized channels localize the electric field on the cell membrane, enabling lower voltage (<30 V) for cell electroporation. The pulse conditions of NEI are examined so that the transfection efficiency (>70%) and biosafety (viability >85%) on delivering fluorescent dyes, plasmid DNA, messenger RNA, and circular RNA (circRNA) into DC2.4 are optimized. Primary mouse bone marrow DC can also be transfected with circRNA with 68.3% efficiency, but without remarkably affecting cellular viability or inducing DC maturation. These results suggest that NEI can be a safe and efficient transfection platform for in vitro transformation of DCs and possesses a promising potential for developing DC vaccines against cancer.

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.

No abstract