Inactivated Vaccine-Induced SARS-CoV-2 Variant-Specific Immunity in Children

Abstract

Multiple vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have been evaluated in clinical trials. However, trials addressing the immune response in the pediatric population are scarce. The inactivated vaccine CoronaVac has been shown to be safe and immunogenic in a phase 1/2 clinical trial in a pediatric cohort in China. Here, we report interim safety and immunogenicity results of a phase 3 clinical trial for CoronaVac in healthy children and adolescents in Chile. Participants 3 to 17 years old received two doses of CoronaVac in a 4-week interval until 31 December 2021. Local and systemic adverse reactions were registered for volunteers who received one or two doses of CoronaVac. Whole-blood samples were collected from a subgroup of 148 participants for humoral and cellular immunity analyses. The main adverse reaction reported after the first and second doses was pain at the injection site. Four weeks after the second dose, an increase in neutralizing antibody titer was observed in subjects relative to their baseline visit. Similar results were found for activation of specific CD4+ T cells. Neutralizing antibodies were identified against the Delta and Omicron variants. However, these titers were lower than those for the D614G strain. Importantly, comparable CD4+ T cell responses were detected against these variants of concern. Therefore, CoronaVac is safe and immunogenic in subjects 3 to 17 years old, inducing neutralizing antibody secretion and activating CD4+ T cells against SARS-CoV-2 and its variants. (This study has been registered at ClinicalTrials.gov under no. NCT04992260.) IMPORTANCE This work evaluated the immune response induced by two doses of CoronaVac separated by 4 weeks in healthy children and adolescents in Chile. To date, few studies have described the effects of CoronaVac in the pediatric population. Therefore, it is essential to generate knowledge regarding the protection of vaccines in this population. Along these lines, we reported the anti-S humoral response and cellular immune response to several SARS-CoV-2 proteins that have been published and recently studied. Here, we show that a vaccination schedule consisting of two doses separated by 4 weeks induces the secretion of neutralizing antibodies against SARS-CoV-2. Furthermore, CoronaVac induces the activation of CD4+ T cells upon stimulation with peptides from the proteome of SARS-CoV-2. These results indicate that, even though the neutralizing antibody response induced by vaccination decreases against the Delta and Omicron variants, the cellular response against these variants is comparable to the response against the ancestral strain D614G, even being significantly higher against Omicron.

Keywords: COVID-19; CoronaVac; SARS-CoV-2; immunogenicity; pediatric; phase 3 clinical trial; safety; vaccines; variants of concern.

Conflict of interest statement

The authors declare a conflict of interest. XM and QQX are SINOVAC Biotech employees and contributed to the conceptualization of the study and did not participate in the analysis or interpretation of the data presented in the manuscript. A.S. is a consultant for Gritstone, Flow Pharma, Arcturus, Immunoscape, CellCarta, OxfordImmunotech and Avalia. La Jolla Institute for Immunology (LJI) has filed for patent protection for various aspects of T cell epitope and vaccine design work. All other authors declare no conflict of interest.

Figures

FIG 1

Study profile, enrolled participants, and cohort included in this study from September 10th to December 31st 2021. (A) Timeline of the vaccination schedule and sample collection. Text in red denotes time points at which blood draws occurred. (B) Study profile of subjects that received 1 dose (orange boxes) and 2 doses (light blue boxes) until December 31st 2021 by age and safety group.

FIG 2

Frequency and duration of nonimmediate local and systemic adverse events by dose and age group. Shown are the total number of nonimmediate local AEs presented by 653 participants in the 3- to 11-year age group and 46 participants in the 12- to 17-year age group after their first dose of CoronaVac (A) or 336 participants in the 3- to 11-year age group and 45 participants in the 12- to 17-year age group after their second dose of CoronaVac (B). (C and D) Total number of systemic AEs presented by 653 participants in the 3- to 11-year age group and 46 participants in the 12- to 17-year age group after the first (C) or the second (D) dose of CoronaVac. Black bars correspond to the 3- to 11-year age group, and white bars correspond to the 12- to 17-year age group. The values on the bars represented as percentages correspond to the number of the AEs evaluated over the total number of participants of that age range.

FIG 3

CoronaVac immunization induces anti-S1 RBD antibodies with neutralizing capacities in children and adolescents after two vaccine doses. (A) Total IgG anti-S1 RBD antibodies were detected by chemiluminescence in the plasma of participants immunized with CoronaVac. Results were obtained from 25 participants in the 3- to 11-year age group and 36 participants in the 12- to 17-year age group. (B) Neutralizing antibodies were detected in plasma using a surrogate viral neutralization test (sVNT), which quantifies the interaction between S1 RBD and hACE2 on ELISA plates. Results were obtained from 55 participants in the 3- to 11-year age group and 36 participants in the 12- to 17-year age group. Data are represented as WHO international units per milliliter. (C) Neutralizing antibody titers in plasma were evaluated using a conventional virus neutralizing test (cVNT) in 27 children in the 3- to 11-year age group and 34 participants in the 12- to 17-year age group. A comparison between the 3- to 11-year age group and the 12- to 17-year age group was performed for total IgG anti-S1 RBD antibodies (D), sVNT (E), and cVNT (F). Seroresponse was calculated as the ratio of the geometric means of the 2nd dose plus 4 weeks to baseline (G). The numbers above each set of individual data points show the geometric mean, and the error bars indicate the 95% confidence interval (CI). A two-way ANOVA was followed by Šídák's multiple-comparison test (A to C) and unpaired t test (D to F) on the log10-transformed data to evaluate differences. ***, P < 0.001; ns, nonsignificant.

FIG 4

Changes in activation-induced marker (AIM) expression in CD4+ T cells and memory AIM+ CD4+ T cells specific for SARS-CoV-2 after two doses of CoronaVac in children and adolescents. AIM+ CD4+ T cells were quantified in peripheral blood mononuclear cells (PBMCs) from participants that received two doses of CoronaVac by flow cytometry upon stimulation with megapools (MPs) of peptides derived from SARS-CoV-2 proteins. The percentages of activated AIM+ CD4+ T cells (OX40+, CD137+) were determined upon stimulation for 24 h with MPs S, R, M, and N in baseline samples and with samples obtained 4 weeks after the second dose. Data from flow cytometry were normalized against dimethyl sulfoxide (DMSO). Percentages of AIM+ CD4+ T cells against MPs S, R, M, and N were obtained from a total of 51 participants in the 3- to 11-year age group and 38 participants in the 12- to 17-year age group (A). Memory AIM+ CD4+ T cells were quantified in PBMCs from participants that received two doses of CoronaVac upon stimulation with MPs of peptides derived from SARS-CoV-2 proteins. The percentages of memory-activated AIM+ CD4+ T cells (OX40+, CD137+, CD45RA−, CCR7+/−) were determined upon stimulation for 24 h with MPs S, R, M, and N in samples obtained at baseline and 4 weeks after the second dose. Data from flow cytometry were normalized against DMSO. Memory AIM+ CD4+ T cells against the MPs S, R, M, and N were obtained from a total of 51 participants in the 3- to 11-year age group and 38 participants in the 12- to 17-year age group (B). The numbers above each individual point data set represent the mean, and the error bars are the 95% CI. A two-way ANOVA followed by Šídák's multiple-comparison test was used to assess differences in panels A and B. *, P < 0.05; **, P < 0.01; ***, P < 0.001; n.s., nonsignificant.

FIG 5

Quantification of circulating neutralizing antibodies and AIM+ CD4+ T cells against SARS-CoV-2 variants Delta and Omicron in participants that received two doses of CoronaVac. (A) Neutralizing antibodies were detected in the plasma of 88 participants 4 weeks after the second dose of CoronaVac, using a pseudotyped virus neutralization test (pVNT). Data are expressed as the reciprocal of the highest dilution preventing 80% of the infection (ID80). Numbers above the bars show the GMT, and the error bars indicate the 95% CI. (B) Neutralizing antibody levels between 52 participants in the 3- to 11-year age group and 36 participants in the 12- to 17-year age group against D614G, Delta, and Omicron variants are shown. (C and D) AIM+ CD4+ T cells against the variants Delta and Omicron were measured by flow cytometry. (C) Results were obtained from a total of 59 participants. (D) Results shown by age group (29 participants from the 3- to 11-year age group and 30 participants from the 12- to 17-year age group). The numbers in red (decrease) or blue (increase) next to the statistics show the fold change of the response against the variant relative to the WT or D614G strain. (E) The correlations between neutralizing antibody titers and AIM+ percentages against the Omicron variant were evaluated. The Pearson correlation coefficient (r) and the P value are indicated in each scatterplot. A one-way ANOVA followed by Dunnett’s multiple-comparison test (A and C) and two-way ANOVA followed by Šídák’s multiple-comparison test (B and D) were performed to assess differences. *, P < 0.05; ***, P < 0.001; n.s., nonsignificant.