Long-term adaptive response in COVID-19 vaccine recipients and the effect of a booster dose

Abstract

We examined the immune response in subjects previously infected with SARS-CoV2 and infection-naïve 9 months after primary 2-dose COVID-19 mRNA vaccination and 3 months after the booster dose in a longitudinal cohort of healthcare workers. Nine months after primary vaccination, previously infected subjects exhibited higher residual antibody levels, with significant neutralizing activity against distinct variants compared to infection-naïve subjects. The higher humoral response was associated with higher levels of receptor binding domain (RBD)-specific IgG⁺ and IgA⁺ memory B cells. The booster dose increased neither neutralizing activity, nor the B and T cell frequencies. Conversely, infection-naïve subjects needed the booster to achieve comparable levels of neutralizing antibodies as those found in previously infected subjects after primary vaccination. The neutralizing titer correlated with anti-RBD IFNγ producing T cells, in the face of sustained B cell response. Notably, pre-pandemic samples showed high Omicron cross-reactivity. These data show the importance of the booster dose in reinforcing immunological memory and increasing circulating antibodies in infection-naïve subjects.

Keywords: COVID-19; Delta; Omicron; SARS-CoV-2; T and B cells; mRNA vaccine; neutralizing antibodies.

Figure 1

Humoral response and neutralizing activity of infection-naïve and previously infected vaccine recipients over time. (A) Schematic representation depicting the study design. Drawings were created using BioRender. #p-value=0.049 vs infection-naïve subjects. (B) Evaluation of neutralizing anti-RBD IgG in infection-naïve and previously infected vaccine recipients at baseline (T1), 19 days after primary vaccination (T2), 9 months after primary vaccination (T3), and 3 months after the booster dose (T4). *p-value<0.0001 vs infection-naïve subjects; #p-value<0.0001 vs the respective T1. (C) Representative images and quantification of lentiviral construct infection in Vero E6 cells overexpressing human angiotensin converting enzyme 2 (ACE2). Nuclei are counterstained with Hoechst. Scale bar: 50 μm. *p-value<0.001 vs Bald, #p-value<0.01; ##p-value<0.001 vs Delta. (D, E) Quantification of neutralizing activity of sera against (D) B.1.617.2 (Delta) and (E) B.1.1.529.1 (Omicron) at T3 and T4. *p-value<0.0001 vs infection-naïve subjects; #p-value<0.0001 vs the respective T3. The sample size (n) for each panel is indicated in brackets.

Figure 2

Analysis of B cell frequency in infection-naïve and previously infected vaccine recipients over time. (A) Representative flow cytometry pseudocolor plots of spike-specific B cells in pre-pandemic healthy donor (HD), in an infection-naïve and a previously infected subject. (B, C) Percentages of tetramer⁺ on CD19⁺ B cells (B) and the percentages of memory CD27⁺, IgG⁺CD27⁺ and IgA⁺CD27⁺ B cells on tetramer⁺ B cells (C) are shown for the spike and RBD protein of the Wuhan Hu-1 SARS-CoV-2 and in response to the B.1.1.529.1 (Omicron) spike protein in infection-naïve and previously infected subjects at 9 months after primary vaccination (T3) and 3 months after the booster dose (T4), as well as for pre-pandemic HD. *p-value<0.05 vs infection-naïve and previously infected subjects at T3 and T4; #p-value<0.05 vs infection-naïve subjects at T3. The sample size (n) for all B cell analyses is indicated in brackets in panel (B).

Figure 3

Analysis of effector T cell response in infection-naïve and previously infected vaccine recipients over time. (A) Frequency of IFNγ producing T cells in response to peptide pools of the spike and RBD protein of the Wuhan Hu-1 SARS-CoV-2 and in response to the B.1.1.529.1 (Omicron) spike protein in infection-naïve and previously infected subjects at 9 months after primary vaccination (T3) and 3 months after the booster dose (T4), as well as in pre-pandemic healthy donors (HD). Representative ELISPOT wells are shown on the right. Horizontal lines indicate median values; *p-value<0.005 vs of infection-naïve and previously infected subjects at T3 and T4. (B) Correlation of anti-RBD antibody levels with the frequency of spike-specific (left panels) or RBD-specific (right panels) IFNγ producing T cells at T4 in the two study groups. The sample size (n) for all T cell analyses is indicated in brackets in panel (A).