Objectives: Identifying immune-protective biomarkers is crucial for the effective management and mitigation of current and future COVID-19 outbreaks, particularly in preventing or counteracting the immune evasion exhibited by the Omicron variants. The emergence of SARS-CoV-2 variants, especially those within the Omicron lineage, has highlighted their capacity to evade neutralizing antibodies, emphasizing the need to understand the role of antibody-dependent cell-mediated cytotoxicity (ADCC) in combating these infections. Methods: This study, conducted in Qichun City, Hubei province, from December 2021 to March 2023, involved 50 healthy Chinese adults who had received two doses of inactivated vaccines and had subsequently experienced mild infections with the Omicron BA.5 variant. Blood samples from these 50 healthy Chinese adults were collected at six distinct time points: at baseline and at the 1st, 3rd, 6th, and 9th months following the third dose of the inactivated vaccine, as well as 3 months post-breakthrough infection. Their sera were analyzed to assess ADCC and neutralization effects. Results: The results indicated that the antibodies elicited by the inactivated SARS-CoV-2 vaccine targeted the spike protein, exhibiting both pre-existing neutralizing and ADCC activities against Omicron variants BA.5 and XBB.1.5. Notably, the ADCC activity demonstrated greater stability compared to that of the neutralizing effects, persisting for at least 15 months post-vaccination, and could be augmented by additional vaccine doses and breakthrough infections. The ADCC effect associated with hybrid immunity effectively targets a spectrum of prospective Omicron variants, including BA.2.86, CH.1.1, EG.5.1, and JN.1. Conclusions: In light of its stability and broad-spectrum efficacy, we recommend the use of the ADCC effect as a biomarker for assessing protective immunity and guiding the development of vaccines and monoclonal antibodies.

Background: The impact of spike protein mutations on T cell responses, particularly in SARS-CoV-2 Omicron variants, remains incompletely elucidated. Methods: In this study, DNA vaccines encoding the spike protein of both the ancestral virus and Omicron variants were developed and administered in conjunction with a Th1-type adjuvant to BALB/c mice. Cross-reactive T cell responses to the spike proteins were assessed in the splenocytes of these mice using IFN-γ ELISPOT assays. Additionally, flow cytometry (FACS) was utilized to evaluate IFN-γ⁺ CD4⁺ and CD8⁺ T cell responses to various peptides covering the entire spike protein sequence. Results: Our study demonstrated that only a limited number of mice, along with a minor subset of their splenocytes vaccinated with the DNA vaccine targeting the original spike protein, exhibited weak cross-reactivity with Omicron variants. This observation underscores the differences in T cell epitopes between Omicron variants and the prototype spikes, indicating that at least 50 and 37 mutations in Omicron variants contribute to their evasion of CD4⁺ and CD8⁺ T cell responses, respectively. Conversely, DNA vaccines encoding spike proteins from Omicron variants successfully elicited strong cross-reactive T cell responses in the immunized mice. In particular, Vaccines targeting the BA.1, BA.5, XBB.1.5, and JN.1 variants demonstrated the most robust and comprehensive T cell immune responses among Omicron variants. This efficacy is attributed to the presence of less than eleven T cell immune evasion mutations in their spike proteins, alongside numerous mutations that enhance T cell responses. Conclusions: These findings underscore the imperative to update WHO emergency vaccine policies and contribute to the development of more effective vaccines and immunization strategies, to better control the infections caused by emerging Omicron variants.