The kinetics of IgG subclasses and contributions to neutralizing activity against SARS-CoV-2 wild-type strain and variants in healthy adults immunized with inactivated vaccine

Abstract

Neutralizing antibody is an important indicator of vaccine efficacy, of which IgG is the main component. IgG can be divided into four subclasses. Up to now, studies analysing the humoral response to SARS-CoV-2 vaccination have mostly focused on measuring total IgG, and the contribution of specific IgG subclasses remains elusive. The aim of this study is to investigate the kinetics of neutralizing antibodies and IgG subclasses, and to explore their relationships in people vaccinated with inactivated COVID-19 vaccine. We conducted a prospective cohort study in 174 healthy adults aged 18-59 years old who were administrated 2 doses of CoronaVac 14 days apart and a booster dose 1 year after the primary immunization, and followed up for 15 months. Blood samples were collected at various time points after primary and booster immunization. We used live SARS-CoV-2 virus neutralizing assay to determine neutralizing ability against the wild-type strain and 4 variants (Beta, Gamma, Delta and Omicron) and ELISA to quantify SARS-CoV-2 RBD-specific IgG subclasses. The results showed that the 2-dose primary immunization only achieved low neutralizing ability, while a booster shot can significantly enhance neutralizing ability against the wild-type strain, Beta, Gamma, Delta and Omicron variants. IgG1 and IgG3 were the most abundant serum antibodies, and IgG2 and IgG4 were hardly detected at any time. The ratio of IgG1/IgG3 was positively associated with the neutralization ability. The underlying mechanism requires further exploration.

Keywords: IgG subclasses; SARS-CoV-2 inactivated vaccine; neutralization ability; variants.

FIGURE 1

Study design and the sample collection

FIGURE 2

Neutralizing antibodies against the wild‐type strain and variants of SARS‐CoV‐2 after 2‐dose primary immunization. The results of neutralization assays against SARS‐CoV‐2 wild‐type strain (a), Beta variant (b), Gamma variant (c), Delta variant (d), and Omicron variant (e). Each dot represents the neutralizing antibody titre of an individual. The numbers indicated above the bars are the geometric mean titres (GMT), and the error bars indicate the 95% confidence intervals (CI) of GMT. The dotted horizontal line represents the seropositivity threshold of 1:8. The titres lower than the limit of detection (1:4) are presented as half the limit of detection (1:2)

FIGURE 3

Neutralizing antibodies against the wild‐type strain and variants of SARS‐CoV‐2 after the booster shot. (a–e) show the results of the different neutralization assays against SARS‐CoV‐2 wild‐type strains (a), Beta variant (b), Gamma variant (c), Delta variant (d), and Omicron variant (e), while (f) shows the results at 3 months for the 5 strains. Each dot represents the neutralizing antibody titre of an individual. The numbers above the bars are GMTs, and the error bars indicate the 95% CIs of GMT. The dotted horizontal line represents the seropositivity threshold of 1:8. The titres lower than the limit of detection (1:4) are presented as half the limit of detection (1:2). Bonferroni's multiple comparison test was used to compare the titres of neutralizing antibodies against different strains in (f). ***p < 0.001, ****p < 0.0001, ns: not statistically significant. No multiple comparison adjustment has been done

FIGURE 4

IgG subclasses at the different time points after 2‐dose primary immunization and the booster shot. The levels of IgG1 (a), IgG2 (b), IgG3 (c) and IgG4 (d) after 2‐dose primary immunization and the booster shot. Each dot represents the IgG subclass level of an individual. The numbers above the bars are the mean levels, and the error bars indicate the 95% CIs. The dotted horizontal line represents the seropositive threshold of 20

FIGURE 5

IgG1 and IgG3 distribution after 2‐dose primary immunization. IgG1, IgG3 and IgG1/IgG3 ratio was grouped by titres of the neutralizing antibodies at 1 month (a), 3 months (b), 6 months (c) and 12 months (d) after 2‐dose primary immunization. The dotted line represents the linear trend between the IgG1/IgG3 ratio and titres of the neutralizing antibodies. Based on each individual's neutralizing antibody titre, the seropositive threshold of 1:8, and the serial dilution multiplier of 2 in the neutralizing assay, the participants were divided into several mutually exclusive groups

FIGURE 6

IgG1 and IgG3 distribution after the booster shot within 1 month. IgG1, IgG3 and IgG1/IgG3 ratio grouped by titres of the different neutralizing antibodies after the booster shot within 1 month. The dotted line reprints the linear trend between the IgG1/IgG3 ratio and titres of neutralizing antibodies. The grouping rationale similar to Figure 5 also applies here

FIGURE 7

The association between IgG subclasses and neutralizing antibodies against the wild‐type strain and variants at 3 months after the booster shot. IgG1, IgG3 and IgG1/IgG3 ratio grouped by the titres of neutralizing antibodies against the wild‐type strain (a), Beta (b), Gamma (c), Delta (d) and Omicron(e) variants. The dotted line represents the linear trend between the IgG1/IgG3 ratio and the titres of different neutralizing antibodies. The grouping rationale similar to Figure 5 applies here