Prior vaccination promotes early activation of memory T cells and enhances immune responses during SARS-CoV-2 breakthrough infection

SARS-CoV-2 breakthrough infections enhance T cell response magnitude, breadth, and epitope repertoire

Little is known about the effect of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2 or SARS2) vaccine breakthrough infections (BTIs) on the magnitude and breadth of the T cell repertoire after exposure to different variants. We studied samples from individuals who experienced symptomatic BTIs during Delta or Omicron waves. In the pre-BTI samples, 30% of the donors exhibited substantial immune memory against non-S (spike) SARS2 antigens, consistent with previous undiagnosed asymptomatic SARS2 infections. Following symptomatic BTI, we observed (1) enhanced S-specific CD4 and CD8 T cell responses in donors without previous asymptomatic infection, (2) expansion of CD4 and CD8 T cell responses to non-S targets (M, N, and nsps) independent of SARS2 variant, and (3) generation of novel epitopes recognizing variant-specific mutations. These variant-specific T cell responses accounted for 9%-15% of the total epitope repertoire. Overall, BTIs boost vaccine-induced immune responses by increasing the magnitude and by broadening the repertoire of T cell antigens and epitopes recognized.

T cell hybrid immunity against SARS-CoV-2 in children: a longitudinal study

BACKGROUND

Hybrid immunity to SARS-CoV-2, resulting from both vaccination and natural infection, remains insufficiently understood in paediatric populations, despite increasing rates of breakthrough infections among vaccinated children.

METHODS

We conducted a prospective longitudinal study to investigate the magnitude, specificity, and cytokine profile of antigen-specific T cell responses elicited by breakthrough SARS-CoV-2 infection in a cohort of mRNA-vaccinated children (n = 29) aged 5-11. This longitudinal analysis involved six distinct time points spanning a 16-month period post-vaccination, during which we analysed a total of 159 blood samples. All children who were followed for at least 12 months (n = 26) experienced a breakthrough infection. We conducted cytokine release assays using minimal blood samples, and we verified the cellular origin of these responses through intracellular cytokine staining.

FINDINGS

After breakthrough infection, children who had received mRNA vaccines showed enhanced Th1 responses specific to Spike peptides. Additionally, their Spike-specific T cells exhibited a distinctive enrichment of CD4+ IFN-γ+IL10+ cells, a characteristic akin to adults with hybrid immunity. Importantly, vaccination did not impede the development of multi-specific T cell responses targeting Membrane, Nucleoprotein, and ORF3a/7/8 antigens.

INTERPRETATION

Children, previously primed with a Spike-based mRNA vaccine and experiencing either symptomatic or asymptomatic breakthrough infection, retained the ability to enhance and diversify Th1/IL-10 antigen-specific T cell responses against multiple SARS-CoV-2 proteins. These findings mirror characteristics associated with hybrid cellular immunity in adults, known to confer resistance against severe COVID-19.

FUNDING

This study was funded by the National Medical Research Council (NMRC) Singapore (COVID19RF-0019, MOH-000019, MOH-000535, OFLCG19May-0034 and MOH-OFYIRG19nov-0002).

Immune responses and reinfection of SARS-CoV-2 Omicron variant in patients with lung cancer

A significant Omicron wave emerged in China in December 2022. To explore the duration of humoral and cellular response postinfection and the efficacy of hybrid immunity in preventing Omicron reinfection in patients with lung cancer, a total of 447 patients were included in the longitudinal study after the Omicron wave from March 2023 to August 2023. Humoral responses were measured at pre-Omicron wave, 3 months and 7 months postinfection. The detected severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) specific antibodies including total antibodies, anti-receptor binding domain (RBD) specific IgG, and neutralizing antibodies against SARS-CoV-2 wild type (WT) and BA.4/5 variant. T cell responses against SARS-CoV-2 WT and Omicron variant were evaluated in 101 patients by ELISpot at 3 months postinfection. The results showed that Omicron-infected symptoms were mild, while fatigue (30.2%), shortness of breath (34.0%) and persistent cough (23.6%) were long-lasting, and vaccines showed efficacy against fever in lung cancer patients. Humoral responses were higher in full or booster vaccinated patients than those unvaccinated (p < .05 for all four antibodies), and the enhanced response persisted for at least 7 months. T cell response to Omicron was higher than WT peptides (21.3 vs. 16.0 SFUs/106 PBMCs, p = .0093). Moreover, 38 (9.74%) patients were reinfected, which had lower antibody responses than non-reinfected patients (all p < .05), and those patients of unvaccinated at late stage receiving anti-cancer immunotherapy alone were at high risk of reinfection. Collectively, these data demonstrate the Omicron infection induces a high and durable immune response in vaccinated patients with lung cancer, which protects vaccinated patients from reinfection.

An Omicron-specific, self-amplifying mRNA booster vaccine for COVID-19: a phase 2/3 randomized trial

Here we conducted a multicenter open-label, randomized phase 2 and 3 study to assess the safety and immunogenicity of a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron-specific (BA.1/B.1.1.529), monovalent, thermostable, self-amplifying mRNA vaccine, GEMCOVAC-OM, when administered intradermally as a booster in healthy adults who had received two doses of BBV152 or ChAdOx1 nCoV-19. GEMCOVAC-OM was well tolerated with no related serious adverse events in both phase 2 and phase 3. In phase 2, the safety and immunogenicity of GEMCOVAC-OM was compared with our prototype mRNA vaccine GEMCOVAC-19 (D614G variant-specific) in 140 participants. At day 29 after vaccination, there was a significant rise in anti-spike (BA.1) IgG antibodies with GEMCOVAC-OM (P < 0.0001) and GEMCOVAC-19 (P < 0.0001). However, the IgG titers (primary endpoint) and seroconversion were higher with GEMCOVAC-OM (P < 0.0001). In phase 3, GEMCOVAC-OM was compared with ChAdOx1 nCoV-19 in 3,140 participants (safety cohort), which included an immunogenicity cohort of 420 participants. At day 29, neutralizing antibody titers against the BA.1 variant of SARS-CoV-2 were significantly higher than baseline in the GEMCOVAC-OM arm (P < 0.0001), but not in the ChAdOx1 nCoV-19 arm (P = 0.1490). GEMCOVAC-OM was noninferior (primary endpoint) and superior to ChAdOx1 nCoV-19 in terms of neutralizing antibody titers and seroconversion rate (lower bound 95% confidence interval of least square geometric mean ratio >1 and difference in seroconversion >0% for superiority). At day 29, anti-spike IgG antibodies and seroconversion (secondary endpoints) were significantly higher with GEMCOVAC-OM (P < 0.0001). These results demonstrate that GEMCOVAC-OM is safe and boosts immune responses against the B.1.1.529 variant. Clinical Trial Registry India identifier: CTRI/2022/10/046475 .

Correlates of protection against symptomatic SARS-CoV-2 in vaccinated children

The paucity of information on longevity of vaccine-induced immune responses and uncertainty of the correlates of protection hinder the development of evidence-based COVID-19 vaccination policies for new birth cohorts. Here, to address these knowledge gaps, we conducted a cohort study of healthy 5-12-year-olds vaccinated with BNT162b2. We serially measured binding and neutralizing antibody titers (nAbs), spike-specific memory B cell (MBC) and spike-reactive T cell responses over 1 year. We found that children mounted antibody, MBC and T cell responses after two doses of BNT162b2, with higher antibody and T cell responses than adults 6 months after vaccination. A booster (third) dose only improved antibody titers without impacting MBC and T cell responses. Among children with hybrid immunity, nAbs and T cell responses were highest in those infected after two vaccine doses. Binding IgG titers, MBC and T cell responses were predictive, with T cells being the most important predictor of protection against symptomatic infection before hybrid immunity; nAbs only correlated with protection after hybrid immunity. The stable MBC and T cell responses over time suggest sustained protection against symptomatic SARS-CoV-2 infection, even when nAbs wane. Booster vaccinations do not confer additional immunological protection to healthy children.

Vaccine adjuvants: Tailoring innate recognition to send the right message

Adjuvants play pivotal roles in vaccine development, enhancing immunization efficacy through prolonged retention and sustained release of antigen, lymph node targeting, and regulation of dendritic cell activation. Adjuvant-induced activation of innate immunity is achieved via diverse mechanisms: for example, adjuvants can serve as direct ligands for pathogen recognition receptors or as inducers of cell stress and death, leading to the release of immunostimulatory-damage-associated molecular patterns. Adjuvant systems increasingly stimulate multiple innate pathways to induce greater potency. Increased understanding of the principles dictating adjuvant-induced innate immunity will subsequently lead to programming specific types of adaptive immune responses. This tailored optimization is fundamental to next-generation vaccines capable of inducing robust and sustained adaptive immune memory across different cohorts.

Immunological imprinting shapes the specificity of human antibody responses against SARS-CoV-2 variants

The spike glycoprotein of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) continues to accumulate substitutions, leading to breakthrough infections of vaccinated individuals. It remains unclear if exposures to antigenically distant SARS-CoV-2 variants can overcome memory B cell biases established by initial SARS-CoV-2 encounters. We determined the specificity and functionality of antibody and B cell responses following exposure to BA.5 and XBB variants in individuals who received ancestral SARS-CoV-2 mRNA vaccines. BA.5 exposures elicited antibody responses that targeted epitopes conserved between the BA.5 and ancestral spike. XBB exposures also elicited antibody responses that primarily targeted epitopes conserved between the XBB.1.5 and ancestral spike. However, unlike BA.5, a single XBB exposure elicited low frequencies of XBB.1.5-specific antibodies and B cells in some individuals. Pre-existing cross-reactive B cells and antibodies were correlated with stronger overall responses to XBB but weaker XBB-specific responses, suggesting that baseline immunity influences the activation of variant-specific SARS-CoV-2 responses.

Reverse development of vaccines against antimicrobial-resistant pathogens

Vaccine R&D is typically a lengthy process taking >10 years. However, vaccines still fail in clinical development because of unreliable animal models or absent immunological correlates of protection. Without a correlate of protection, phase-1 and -2 studies of safety and immunogenicity can fail to predict phase-3 efficacy. Indeed, the history of vaccine development is replete with promising phase-1 and -2 results and failed phase-3 efficacy trials. To avoid this misfortune, we present Reverse Vaccine Development for vaccines against antimicrobial-resistant (AMR) pathogens. In this approach, instead of evaluating efficacy in phase 3, proof-of-principle efficacy is evaluated as early as possible in a population with a high incidence of disease, which may differ from the population intended for registration, and can be a controlled human infection population. To identify a correlate of protection in these populations, the vaccine-elicited immune response is compared between protected and unprotected subjects. If a correlate is identified, it can help to refine the vaccine dosage, schedule, and formulation, and facilitate the assessment of vaccine efficacy in other populations with different attack rates, subject characteristics, and disease manifestations. This may be the only way to provide life-saving vaccines to populations affected by AMR-pathogen diseases at incidences that are typically low and unsuited to phase-3 efficacy trials. The availability of a correlate of protection early in clinical development can potentially prevent failures of large phase-3 trials and unnecessary exposures of populations to inefficacious vaccines that have resulted in disinvestment in the development of vaccines against AMR pathogens.

Cross-Reactivity Assessment of Vaccine-Derived SARS-CoV-2 T Cell Responses against BA.2.86 and JN.1

The SARS-CoV-2 Omicron sub-variants BA.2.86 and JN.1 contain multiple mutations in the spike protein that were not present in previous variants of concern and Omicron sub-variants. Preliminary research suggests that these variants reduce the neutralizing capability of antibodies induced by vaccines, which is particularly significant for JN.1. This raises concern as many widely deployed COVID-19 vaccines are based on the spike protein of the ancestral Wuhan strain of SARS-CoV-2. While T cell responses have been shown to be robust against previous SARS-CoV-2 variants, less is known about the impact of mutations in BA.2.86 and JN.1 on T cell responses. We evaluate the effect of mutations specific to BA.2.86 and JN.1 on experimentally determined T cell epitopes derived from the spike protein of the ancestral Wuhan strain and the spike protein of the XBB.1.5 strain that has been recommended as a booster vaccine. Our data suggest that BA.2.86 and JN.1 affect numerous T cell epitopes in spike compared to previous variants; however, the widespread loss of T cell recognition against these variants is unlikely.

Memory T cells effectively recognize the SARS-CoV-2 hypermutated BA.2.86 variant

T cells are critical in mediating the early control of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) breakthrough infection. However, it remains unknown whether memory T cells can effectively cross-recognize new SARS-CoV-2 variants with a broad array of mutations, such as the emergent hypermutated BA.2.86 variant. Here, we report in two separate cohorts, including healthy controls and individuals with chronic lymphocytic leukemia, that SARS-CoV-2 spike-specific CD4+ and CD8+ T cells induced by prior infection or vaccination demonstrate resilient immune recognition of BA.2.86. In both cohorts, we found largely preserved SARS-CoV-2 spike-specific CD4+ and CD8+ T cell magnitudes against mutated spike epitopes of BA.2.86. Functional analysis confirmed that both cytokine expression and proliferative capacity of SARS-CoV-2 spike-specific T cells to BA.2.86-mutated spike epitopes are similarly sustained. In summary, our findings indicate that memory CD4+ and CD8+ T cells continue to provide cell-mediated immune recognition to highly mutated emerging variants such as BA.2.86.

Post-pandemic memory T cell response to SARS-CoV-2 is durable, broadly targeted, and cross-reactive to the hypermutated BA.2.86 variant

Ongoing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) evolution has given rise to recombinant Omicron lineages that dominate globally (XBB.1), as well as the emergence of hypermutated variants (BA.2.86). In this context, durable and cross-reactive T cell immune memory is critical for continued protection against severe COVID-19. We examined T cell responses to SARS-CoV-2 approximately 1.5 years since Omicron first emerged. We describe sustained CD4+ and CD8+ spike-specific T cell memory responses in healthcare workers in South Africa (n = 39) who were vaccinated and experienced at least one SARS-CoV-2 infection. Spike-specific T cells are highly cross-reactive with all Omicron variants tested, including BA.2.86. Abundant nucleocapsid and membrane-specific T cells are detectable in most participants. The bulk of SARS-CoV-2-specific T cell responses have an early-differentiated phenotype, explaining their persistent nature. Overall, hybrid immunity leads to the accumulation of spike and non-spike T cells evident 3.5 years after the start of the pandemic, with preserved recognition of highly mutated SARS-CoV-2 variants.

In search of a pan-coronavirus vaccine: next-generation vaccine design and immune mechanisms

Members of the coronaviridae family are endemic to human populations and have caused several epidemics and pandemics in recent history. In this review, we will discuss the feasibility of and progress toward the ultimate goal of creating a pan-coronavirus vaccine that can protect against infection and disease by all members of the coronavirus family. We will detail the unmet clinical need associated with the continued transmission of SARS-CoV-2, MERS-CoV and the four seasonal coronaviruses (HCoV-OC43, NL63, HKU1 and 229E) in humans and the potential for future zoonotic coronaviruses. We will highlight how first-generation SARS-CoV-2 vaccines and natural history studies have greatly increased our understanding of effective antiviral immunity to coronaviruses and have informed next-generation vaccine design. We will then consider the ideal properties of a pan-coronavirus vaccine and propose a blueprint for the type of immunity that may offer cross-protection. Finally, we will describe a subset of the diverse technologies and novel approaches being pursued with the goal of developing broadly or universally protective vaccines for coronaviruses.

Silent battles: immune responses in asymptomatic SARS-CoV-2 infection

SARS-CoV-2 infections manifest with a broad spectrum of presentations, ranging from asymptomatic infections to severe pneumonia and fatal outcomes. This review centers on asymptomatic infections, a widely reported phenomenon that has substantially contributed to the rapid spread of the pandemic. In such asymptomatic infections, we focus on the role of innate, humoral, and cellular immunity. Notably, asymptomatic infections are characterized by an early and robust innate immune response, particularly a swift type 1 IFN reaction, alongside a rapid and broad induction of SARS-CoV-2-specific T cells. Often, antibody levels tend to be lower or undetectable after asymptomatic infections, suggesting that the rapid control of viral replication by innate and cellular responses might impede the full triggering of humoral immunity. Even if antibody levels are present in the early convalescent phase, they wane rapidly below serological detection limits, particularly following asymptomatic infection. Consequently, prevalence studies reliant solely on serological assays likely underestimate the extent of community exposure to the virus.

B-cell and antibody responses to SARS-CoV-2: infection, vaccination, and hybrid immunity

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 2019 prompted scientific, medical, and biotech communities to investigate infection- and vaccine-induced immune responses in the context of this pathogen. B-cell and antibody responses are at the center of these investigations, as neutralizing antibodies (nAbs) are an important correlate of protection (COP) from infection and the primary target of SARS-CoV-2 vaccine modalities. In addition to absolute levels, nAb longevity, neutralization breadth, immunoglobulin isotype and subtype composition, and presence at mucosal sites have become important topics for scientists and health policy makers. The recent pandemic was and still is a unique setting in which to study de novo and memory B-cell (MBC) and antibody responses in the dynamic interplay of infection- and vaccine-induced immunity. It also provided an opportunity to explore new vaccine platforms, such as mRNA or adenoviral vector vaccines, in unprecedented cohort sizes. Combined with the technological advances of recent years, this situation has provided detailed mechanistic insights into the development of B-cell and antibody responses but also revealed some unexpected findings. In this review, we summarize the key findings of the last 2.5 years regarding infection- and vaccine-induced B-cell immunity, which we believe are of significant value not only in the context of SARS-CoV-2 but also for future vaccination approaches in endemic and pandemic settings.

Rapid and high throughput assessment of cellular immunity against SARS-CoV-2 based on the ex vivo activation of genes in leukocyte assay with whole blood

During the novel coronavirus outbreak and vaccine development, antibody production garnered major focus as the primary immunogenic response. However, cellular immunity's recent demonstration of comparable or greater significance in controlling infection demands the re-evaluation of the importance of T-cell immunity in SARS-CoV-2 infection. Here, we developed a novel assay, the ex vivo activation of genes in leukocytes (EAGL), which employs short-term whole blood stimulation with the LeukoComplete™ system, to measure ex vivo SARS-CoV-2-specific T cell responses (cellular immunity). This assay measures upregulated mRNA expression related to leukocyte activation 4 h after antigen stimulation. LeukoComplete™ system uses whole blood samples, eliminating the need for pretreatment before analysis. Furthermore, this system's high reproducibility is ensured through a series of operations from mRNA extraction to cDNA synthesis on a 96-well plate. In the performance evaluation using fresh blood from previously SARS-CoV-2-infected and COVID-19-vaccinated individuals, the EAGL assay had a comparable sensitivity and specificity to the ELISpot assay (EAGL: 1.000/1.000; ELISpot: 0.900/0.973). As a simple, high-throughput assay, the EAGL assay is also a quantitative test that is useful in studies with large sample numbers, such as monitoring new vaccine efficacies against novel coronaviruses or epidemiologic studies that require cellular immune testing during viral infection.

Immunological imprinting shapes the specificity of human antibody responses against SARS-CoV-2 variants

The spike glycoprotein of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) continues to accumulate substitutions, leading to breakthrough infections of vaccinated individuals and prompting the development of updated booster vaccines. Here, we determined the specificity and functionality of antibody and B cell responses following exposure to BA.5 and XBB variants in individuals who received ancestral SARS-CoV-2 mRNA vaccines. BA.5 exposures elicited antibody responses that primarily targeted epitopes conserved between the BA.5 and ancestral spike, with poor reactivity to the XBB.1.5 variant. XBB exposures also elicited antibody responses that targeted epitopes conserved between the XBB.1.5 and ancestral spike. However, unlike BA.5, a single XBB exposure elicited low levels of XBB.1.5-specific antibodies and B cells in some individuals. Pre-existing cross-reactive B cells and antibodies were correlated with stronger overall responses to XBB but weaker XBB-specific responses, suggesting that baseline immunity influences the activation of variant-specific SARS-CoV-2 responses.

Extraordinary Titer and Broad Anti-SARS-CoV-2 Neutralization Induced by Stabilized RBD Nanoparticles from Strain BA.5

The receptor-binding domain (RBD) of the SARS-CoV-2 spike is a primary target of neutralizing antibodies and a key component of licensed vaccines. Substantial mutations in RBD, however, enable current variants to escape immunogenicity generated by vaccination with the ancestral (WA1) strain. Here, we produce and assess self-assembling nanoparticles displaying RBDs from WA1 and BA.5 strains by using the SpyTag:SpyCatcher system for coupling. We observed both WA1- and BA.5-RBD nanoparticles to degrade substantially after a few days at 37 °C. Incorporation of nine RBD-stabilizing mutations, however, increased yield ~five-fold and stability such that more than 50% of either the WA1- or BA.5-RBD nanoparticle was retained after one week at 37 °C. Murine immunizations revealed that the stabilized RBD-nanoparticles induced ~100-fold higher autologous neutralization titers than the prefusion-stabilized (S2P) spike at a 2 μg dose. Even at a 25-fold lower dose where S2P-induced neutralization titers were below the detection limit, the stabilized BA.5-RBD nanoparticle induced homologous titers of 12,795 ID50 and heterologous titers against WA1 of 1767 ID50. Assessment against a panel of β-coronavirus variants revealed both the stabilized BA.5-RBD nanoparticle and the stabilized WA1-BA.5-(mosaic)-RBD nanoparticle to elicit much higher neutralization breadth than the stabilized WA1-RBD nanoparticle. The extraordinary titer and high neutralization breadth elicited by stabilized RBD nanoparticles from strain BA.5 make them strong candidates for next-generation COVID-19 vaccines.

Primary and Recall Immune Responses to SARS-CoV-2 in Breakthrough Infection

Breakthrough infections in SARS-CoV-2 vaccinated individuals are an ideal circumstance for the simultaneous exploration of both the vaccine-induced memory reaction to the spike (S) protein and the primary response to the membrane (M) and nucleocapsid (N) proteins generated by natural infection. We monitored 15 healthcare workers who had been vaccinated with two doses of Pfizer BioNTech BNT162b2 and were then later infected with the SARS-CoV-2 B.1.617.2. (Delta) variant, analysing the antiviral humoral and cellular immune responses. Natural infection determined an immediate and sharp rise in anti-RBD antibody titres and in the frequency of both S-specific antibody secreting cells (ASCs) and memory B lymphocytes. T cells responded promptly to infection by activating and expanding already at 2-5 days. S-specific memory and emerging M- and N-specific T cells both expressed high levels of activation markers and showed effector capacity with similar kinetics but with different magnitude. The results show that natural infection with SARS-CoV-2 in vaccinated individuals induces fully functional and rapidly expanding T and B lymphocytes in concert with the emergence of novel virus-specific T cells. This swift and punctual response also covers viral variants and captures a paradigmatic case of a healthy adaptive immune reaction to infection with a mutating virus.

COVID-19 Vaccines for Optimizing Immunity in the Upper Respiratory Tract

Rapid development and deployment of vaccines greatly reduced mortality and morbidity during the COVID-19 pandemic. The most widely used COVID-19 vaccines approved by national regulatory authorities require intramuscular administration. SARS-CoV-2 initially infects the upper respiratory tract, where the infection can be eliminated with little or no symptoms by an effective immune response. Failure to eliminate SARS-CoV-2 in the upper respiratory tract results in lower respiratory tract infections that can lead to severe disease and death. Presently used intramuscularly administered COVID-19 vaccines are effective in reducing severe disease and mortality, but are not entirely able to prevent asymptomatic and mild infections as well as person-to-person transmission of the virus. Individual and population differences also influence susceptibility to infection and the propensity to develop severe disease. This article provides a perspective on the nature and the mode of delivery of COVID-19 vaccines that can optimize protective immunity in the upper respiratory tract to reduce infections and virus transmission as well as severe disease.

Immunity Induced by Inactivated SARS-CoV-2 Vaccine: Breadth, Durability, Potency, and Specificity in a Healthcare Worker Cohort

Vaccination has proven to be highly effective against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but the long-term immunogenicity and the functional preserved immune responses of vaccines are needed to inform evolving evidence-based guidelines for boosting schedules. We enrolled 205 healthcare workers into a cohort study; all had received three doses of BBIBP-CorV (China Sinopharm Bio-Beijing Company, Beijing, China) inactivated vaccine. We assessed SARS-CoV-2 specific binding antibodies, neutralizing antibodies, and peripheral T and B cell responses. We demonstrated that more robust antibody responses to SARS-CoV-2 were elicited by booster immunization compared with primary vaccination. Neutralizing antibody titers to SARS-CoV-2 Omicron BA.1 were also efficiently elevated post-homologous vaccine booster despite being in a lower titer compared with the prototype stain. In addition to S-specific humoral and cellular immunity, BBIBP-CorV also induced N-specific antibody and effector T cell responses. The third-dose vaccination led to further expansion of critical polyfunctional T cell responses, likely an essential element for vaccine protection. In particular, a functional role for Tfh cell subsets in immunity was suggested by the correlation between both CD4+ Tfh and CD8+ Tfh with total antibody, IgG, B cell responses, and neutralizing antibodies. Our study details the humoral and cellular responses generated by the BBIBP-CorV booster vaccination in a seven-month follow-up study. There is a clear immunologic boosting value of homologous inactivated SARS-CoV-2 vaccine boosters, a consideration for future vaccine strategies.