Comparative Immune Response after Vaccination with SOBERANA® 02 and SOBERANA® plus Heterologous Scheme and Natural Infection in Young Children

Abstract

(1) Background: In children, SARS-CoV-2 infection is mostly accompanied by mild COVID-19 symptoms. However, multisystem inflammatory syndrome (MIS-C) and long-term sequelae are often severe complications. Therefore, the protection of the pediatric population against SARS-CoV-2 with effective vaccines is particularly important. Here, we compare the humoral and cellular immune responses elicited in children (n = 15, aged 5-11 years) vaccinated with the RBD-based vaccines SOBERANA^® 02 and SOBERANA^® Plus combined in a heterologous scheme with those from children (n = 10, aged 4-11 years) who recovered from mild symptomatic COVID-19. (2) Methods: Blood samples were taken 14 days after the last dose for vaccinated children and 45-60 days after the infection diagnosis for COVID-19 recovered children. Anti-RBD IgG and ACE2-RBD inhibition were assessed by ELISA; IgA, cytokines, and cytotoxic-related proteins were determined by multiplex assays. Total B and T cell subpopulations and IFN-γ release were measured by multiparametric flow cytometry using a large panel of antibodies after in vitro stimulation with S1 peptides. (3) Results: Significant higher levels of specific anti-RBD IgG and IgA and ACE2-RBD inhibition capacity were found in vaccinated children in comparison to COVID-19 recovered children. Th1-like and Th2-like CD4⁺ T cells were also significantly higher in vaccinated subjects. IFN-γ secretion was higher in central memory CD4⁺ T cells of COVID-19 recovered children, but no differences between both groups were found in the CD4⁺ and CD8⁺ T cell effector, terminal effector, and naïve T cell subpopulations. In contrast to low levels of IL-4, high levels of IL-2, IL-6, IFN-γ, and IL-10 suggest a predominant Th1 cell polarization. Cytotoxic-related proteins granzyme A and B, perforin, and granulin were also found in the supernatant after S1 stimulation in both vaccinated and recovered children. (4) Conclusions: Vaccination with the heterologous scheme of SOBERANA^® 02/SOBERANA^® Plus induces a stronger antibody and cellular immune response compared to natural infections in young children.

Keywords: COVID-19; SARS-CoV-2; SARS-CoV-2 RBD IgG; children; memory T cell; vaccine.

Figure 1

Spike specific antibody response in vaccinated and COVID-19 recovered children. (a) Experimental design. Blood samples were collected from vaccinated children (n = 15) 14 days after the last (third) dose of the SOBERANA^® 02/SOBERANA^® Plus heterologous scheme, and from children (control, n = 10) who recovered from mild COVID-19 (45–60 days after the infection diagnosis). (b) Spike IgG antibodies expressed in BAU/mL. (c) Concentration of anti-RBD IgA antibodies in pg/mL. (d) Serum-induced inhibition of the RBD–hACE2 interaction. Mean ± SEM are shown together with the p-values from the Mann–Whitney non-parametric t test.

Figure 2

Neutralizing antibody response in vaccinated and COVID-19 recovered children. Mean ± SEM are shown together with the p-values from the Mann–Whitney non-parametric t test.

Figure 3

Analysis of B cell frequency in vaccinated and COVID-19 recovered children. (a) Representative gate of RBD-specific B cells in recovered and vaccinated children. (b) Percentages of RBD tetramer⁺ on CD19⁺ B cells. (c) Percentages of IgG⁺CD27⁺, IgM⁺CD27⁺, and IgA⁺CD27⁺ on RBD⁺ B cells. Mean ± SEM are shown together with the p-values from the Mann–Whitney non-parametric t test.

Figure 4

Composition of the T helper cell subpopulation in vaccinated and COVID-19 recovered children. PBMCs from vaccinated and recovered children (control) were isolated and stained with antibody cocktails for CD4⁺ T cell subpopulations. The frequencies of Th1 CD4⁺ T cells (CD4⁺CXCR3⁺CCR6⁻) are shown. (a) Th2 CD4⁺ T cells (CD4⁺CCR4⁺CCR6⁻), (b) Th17 CD4⁺ T cells (CD4⁺CCR4⁺CCR6⁺), (c) Th1/Th17 cells (CD4⁺CXCR3⁺CCR6⁺), and (d) CD4⁺ T cells for individual children. The data are also shown as mean ± SEM and the p-values from the Mann–Whitney non-parametric t test.

Figure 5

Percentage of IFN-γ secreting CD4⁺ and CD8⁺ memory T cells in vaccinated and COVID-19 recovered children. PBMCs from convalescent and vaccinated children were left unstimulated or were stimulated with S1 SARS-CoV-2 peptide pool, then the T cell-specific response was evaluated as secretion of IFN-γ within the CD4⁺ (a) and CD8⁺ (b) T cell subpopulations with respect to their memory phenotype. The bars show the mean ± SEM values and the p-values from the Mann–Whitney non-parametric t test. N: naive; CM: central memory; EM: effector memory; EMRA: terminal differentiated effector memory.

Figure 6

Spike-specific cytokine secretion in vitro from vaccinated and COVID-19 recovered children. PBMCs from convalescent and vaccinated children were stimulated with a S1 SARS-CoV-2 peptide pool. Cell supernatants were analyzed using a multiplex cytokine secretion assay. (a) Cytokines; (b) cytotoxicity-related molecules. Each dot identifies an individual child; the bars indicate mean ± SEM and p-values from the Mann–Whitney non-parametric t test.