Humoral Immune Response Diversity to Different COVID-19 Vaccines: Implications for the "Green Pass" Policy

Abstract

In the COVID-19 pandemic year 2021, several countries have implemented a vaccine certificate policy, the "Green Pass Policy" (GPP), to reduce virus spread and to allow safe relaxation of COVID-19 restrictions and reopening of social and economic activities. The rationale for the GPP is based on the assumption that vaccinated people should maintain a certain degree of immunity to SARS-CoV-2. Here we describe and compare, for the first time, the humoral immune response to mRNA-1273, BNT162b2, Ad26.COV2.S, and ChAdOx1 nCoV-19 vaccines in terms of antibody titer elicited, neutralizing activity, and epitope reactogenicity among 369 individuals aged 19 to 94 years. In parallel, we also considered the use of a rapid test for the determination of neutralizing antibodies as a tool to guide policymakers in defining booster vaccination strategies and eligibility for Green Pass. Our analysis demonstrates that the titer of antibodies directed towards the receptor-binding domain (RBD) of SARS-CoV-2 Spike is significantly associated with age and vaccine type. Moreover, natural COVID-19 infection combined with vaccination results, on average, in higher antibody titer and higher neutralizing activity as compared to fully vaccinated individuals without prior COVID-19. We also found that levels of anti-Spike RBD antibodies are not always strictly associated with the extent of inhibition of RBD-ACE2 binding, as we could observe different neutralizing activities in sera with similar anti-RBD concentrations. Finally, we evaluated the reactivity to four synthetic peptides derived from Spike protein on a randomly selected serum sample and observed that similar to SARS-CoV-2 infection, vaccination elicits a heterogeneous antibody response with qualitative individual features. On the basis of our results, the use of rapid devices to detect the presence of neutralizing antibodies, even on a large scale and repeatedly over time, appears helpful in determining the duration of the humoral protection elicited by vaccination. These aspects and their implications for the GPP are discussed.

Keywords: Green Pass Policy; SARS-CoV-2; anti-RBD antibody titer; humoral immune response; neutralizing antibodies; vaccine.

Figure 1

(A) Electrochemiluminescence immunoassay (ECLIA)-based determination of the titer of specific antibodies towards the receptor-binding domain (RBD) of SARS-CoV-2 Spike expressed as median UI/ml ± IQR and represented on log10 scale, evaluated in serum from 369 individuals who received different vaccination. Statistically significant variations of medians were assessed by Kruskal–Wallis method for non-parametric data (p < 0.0001), followed by Dunn’s multiple comparisons test. (B) Distribution of serum anti-RBD antibody titers according to age among the study population and (C) within each vaccination group. Spearman’s correlation r and p-values have been calculated for each group (statistical significance for p < 0.05). Trendline is represented in red.

Figure 2

Spearman’s correlation analysis of serum anti-receptor-binding domain (anti-RBD) antibody titers and neutralizing activity in 70 participants in the study (22 vaccinated with BNT162b2, 9 with mRNA-1273, 27 with ChAdOx1 nCov19, 9 with Ad26.COV2.S, 1 COVID-19 convalescent, and 2 with mixed vaccines). (A) Correlation plot of anti-RBD antibody titers versus neutralizing activity (percentage inhibition of RBD-ACE2 binding) assessed through the cPass™ ELISA-based assay. (B) Correlation plot of anti-RBD antibody titers versus neutralizing activity assessed through IgG/Neutralizing Antibody Rapid Test. (C) Correlation plot of neutralizing activity evaluated through cPass™ ELISA-based assay and IgG/Neutralizing Antibody Rapid Test cassettes. Trendlines, Spearman’s r, and p-values are also represented (statistical significance for p < 0.05).

Figure 3

Neutralizing activity evaluated by cPass™ ELISA-based SARS-CoV-2 Neutralization Antibody Detection Kit in 70 sera from differently vaccinated individuals. Serum samples were considered positive when ≥30% inhibition was measured, as shown by the red line in the graph. (A) Percentage inhibition of receptor-binding domain–angiotensin-converting enzyme 2 (RBD-ACE2) binding within different vaccination groups (see also Table 3 ). Statistical significance was assessed by ANOVA following Tukey’s multiple comparisons test, **p < 0.005, *p < 0.05. (B) Comparison of neutralizing activity in sera from individuals who received adenoviral DNA-based vaccines and mRNA-based vaccines. Statistical significance was assessed by unpaired t-test, *p = 0.0016.

Figure 4

(A) Correlation of anti-receptor-binding domain (anti-RBD) antibody titer to rapid test scores for IgG/Neutralizing antibodies in sera from 180 participants in the study and within each vaccination group: (B) BNT162b2, (C) ChAdOx1 nCov19, (D) Ad26.COV2.S, (E) mRNA-1273, (F) COVID19 + vaccine, and (G) others. Trendlines, Spearman’s r, and p-values are also reported (statistical significance for p < 0.05).

Figure 5

(A) Distribution of anti-receptor-binding domain (anti-RBD) antibody titers in AbNeu-positive and AbNeu-negative sera from 180 participants in the study and within each vaccination group: (B) BNT162b2, (C) ChAdOx1 nCov19, (D) Ad26.COV2.S, (E) mRNA-1273, (F) COVID19 + vaccine, and (G) others. Data are represented as scatter plot with median UI/ml ± IQR on log10 scale. Mann–Whitney test was performed to assess statistical significance of differences between medians (see Table 5 ). **p-value <0.0001; *p-value <0.01; ns, not specific.