Kinetics and persistence of cellular and humoral immune responses to SARS-CoV-2 vaccine in healthcare workers with or without prior COVID-19

Abstract

SARS-CoV-2 vaccines are highly efficient against severe forms of the disease, hospitalization and death. Nevertheless, insufficient protection against several circulating viral variants might suggest waning immunity and the need for an additional vaccine dose. We conducted a longitudinal study on the kinetics and persistence of immune responses in healthcare workers vaccinated with two doses of BNT162b2 mRNA vaccine with or without prior SARS-CoV-2 infection. No new infections were diagnosed during follow-up. At 6 months, post-vaccination or post-infection, despite a downward trend in the level of anti-S IgG antibodies, the neutralizing activity does not decrease significantly, remaining higher than 75% (85.14% for subjects with natural infection, 88.82% for vaccinated after prior infection and 78.37% for vaccinated only). In a live-virus neutralization assay, the highest neutralization titres were present at baseline and at 6 months follow-up in persons vaccinated after prior infection. Anti-S IgA levels showed a significant descending trend in vaccinated subjects (p < 0.05) after 14 weeks. Cellular immune responses are present even in vaccinated participants with declining antibody levels (index ratio 1.1-3) or low neutralizing activity (30%-40%) at 6 months, although with lower T-cell stimulation index (p = 0.046) and IFN-γ secretion (p = 0.0007) compared to those with preserved humoral responses.

Keywords: IFN-γ; SARS-CoV-2 infection; antibody levels; immune response post-infection; immune response post-vaccination; mRNA vaccine; neutralizing antibodies; spike-specific CD4+ T cell; spike-specific CD8+ T cell.

FIGURE 1

Schematic representation of the groups investigated for the longitudinal immune response

FIGURE 2

Changes in antibody levels over time. Sequential analysis of isotypic antibody responses (anti‐S IgG, IgA and anti‐NCP IgM) to SARS‐CoV‐2 for plasma samples collected over a 6 months period. Groups description: natural SARS‐CoV‐2 infection without vaccine (I), with prior SARS‐CoV‐2 infection and vaccine (I + V), and vaccinated without prior infection (V). *p < 0.05, **p < 0.01 and ***p < 0.001

FIGURE 3

Dynamic of neutralizing antibody using both a classic and a surrogate virus neutralization test. The neutralizing antibody titres and capacity of different groups were compared over time. (A) The titres were measured by whole‐virus replication assay and are expressed as geometrical mean titre (GMT). (B) Results from surrogate virus neutralization test. The neutralizing capacity is expressed as percentage. A cut‐off value ≥30% was used as positive result. (C) Correlations between values obtained in the ELISA surrogate neutralization and the virus neutralization. Groups description: natural SARS‐CoV‐2 infection without vaccine (I), with prior SARS‐CoV‐2 infection and vaccine (I + V), and vaccinated without prior infection (V). Horizontal lines indicate median values

FIGURE 4

Neutralizing activity positively correlates with the level of anti‐S IgG and IgA antibodies. Correlation of plasma anti‐S IgG, IgA and neutralizing capacity of anti‐S antibodies in different groups of individuals analysed by ELISA. Data were analysed using nonlinear regression and two‐tailed Pearson r correlation coefficient. Significant correlations (p > 0.05) were presented on graphs via trendlines. Each colour is specific to a time point, according to figure legend

FIGURE 5

SARS‐CoV‐2‐specific TCD4, TCD8 cells, as well as B and NK cells, were analysed after ex vivo stimulation of PBMCs with PepTivator for 20 h. Individual representative cases showing variation of cellular response specific for SARS‐CoV‐2 vaccinated subjects. (A) A representative sample from a good responder with high Ab titre and neutralizing capacity (GR). (B) A representative sample from a low responder with low Ab titre and neutralizing capacity (LR)

FIGURE 6

Interferon secretion post–SARS‐CoV‐2‐specific stimulation. (A) Comparative assessment between good responders (GR) with high Ab titre and neutralizing capacity, and low responders (LR) with low Ab titre and neutralizing capacity. (B) Comparative analysis between categories according to infection with or without vaccination. (C) Positive correlation between IFN‐γ secretion and neutralizing capacity