Antibody response in elderly vaccinated four times with an mRNA anti-COVID-19 vaccine

Abstract

The humoral response after the fourth dose of a mRNA vaccine against COVID-19 has not been adequately described in elderly recipients, particularly those not exposed previously to SARS-CoV-2. Serum anti-RBD IgG levels (Abbott SARS-CoV-2 IgG II Quant assay) and neutralizing capacities (spike SARS-CoV-2 pseudovirus Wuhan and Omicron BA.1 variant) were measured after the third and fourth doses of a COVID-19 mRNA vaccine among 46 elderly residents (median age 85 years [IQR 81; 89]) of an assisted living facility. Among participants never infected by SARS-CoV-2, the mean serum IgG levels against RBD (2025 BAU/ml), 99 days after the fourth vaccine, was as high as 76 days after the third vaccine (1987 BAU/ml), and significantly higher (p = 0.030) when the latter were corrected for elapsed time. Neutralizing antibody levels against the historical Wuhan strain were significantly higher (Mean 1046 vs 1573; p = 0.002) and broader (against Omicron) (Mean 170 vs 375; p = 0.018), following the fourth vaccine. The six individuals with an Omicron breakthrough infection mounted strong immune responses for anti-RBD and neutralizing antibodies against the Omicron variant indicating that the fourth vaccine dose did not prevent a specific adaptation of the immune response. These findings point out the value of continued vaccine boosting in the elderly population.

Figure 1

Anti-RBD IgG response in uninfected residents. Anti-RBD IgG were measured in samples taken 76 and 99 days (median) after third (post third vaccine) and fourth (post fourth vaccine) vaccines, respectively, by Chemiluminescent Microparticle ImmunoAssay (CMIA) SARS-CoV-2 IgG II Quant (Abbott, IL, USA), expressed as binding antibody units (BAU) per ml. Each triangle represents one individual. There was initially no significant difference in mean antibody levels between blood sample taken after the third and the fourth vaccines. After time correction of values post third vaccine (20% lower), the increase between third and fourth vaccines became significant. Mean values are indicated below the x-axis. Bars represent 95%CI around the mean. ns: p = 0.8587; *p = 0.0301.

Figure 2

Anti-RBD IgG titers distribution according to Frailty of the whole cohort. There was no difference in antibody titers between frailty categories for all three samples (Kruskal–Wallis p = 0.648, 0.158, 0.802, respectively, for the entire cohort; p = 0.611, 0.189, 0.376, respectively, for patients who were not infected by SARS-CoV-2, Supplementary Fig. S2). Each circle represents one individual. Bars (horizontal) represent the median values, dashes represent first (lower) and third (upper) quartiles. In white the second blood sample data, in grey—the third blood sample data. ns: Kruskal–Wallis test p > 0.05. Frailty score were calculated according to the literature^–.

Figure 3

Wild type and Omicron neutralizing antibodies after the third and fourth vaccines in non-infected residents. WT: wild type SARS-CoV-2. Omicron: BA.1 variant. Neutralization capacity [NT₅₀] is expressed as a function of reciprocal values of sera dilutions on a log₁₀ scale. Starting at a final 1:60 dilution of the sera, the participants were assessed for both WT and Omicron neutralization antibodies by SARS-CoV-2 spike-pseudotyped VSV-GFP-ΔG reporter assay on Vero-E6 cells. Sera with NT₅₀ that could not be calculated at the 1:60 dilutions were graded 2 (LOD) for graphical representations. Post 3rd vaccine: blood sampling was done at a median time of 76 days after the third vaccine. Post 4th blood: sampling was done at a median time of 99 days after the fourth vaccine. Each point corresponds to on individual. Bars represent 95%CI around the mean. **WT paired p-value = 0.0023; *Omicron paired p-value = 0.0179.

Figure 4

Neutralizing antibodies and frailty scores of the whole cohort. There was no difference in NT₅₀ against WT between frailty categories for blood samples two (in white) or three (in grey) (Kruskal–Wallis p = 0.125, 0.975, respectively, for the entire cohort; p = 0.195, 0.863, respectively, for patients who were not infected by SARS-CoV-2, Supplementary Fig. S3). Likewise there was no difference in NT₅₀ against Omicron between frailty categories for blood samples two or three (Kruskal–Wallis p = 0.279, 0.964, respectively, for the entire cohort; p = 0.245, 0.725, respectively, for patients who were not infected by SARS-CoV-2, Supplementary Fig. S3). White rectangle: second blood sampling at 76 days (median) after the third vaccine. Grey rectangle: third blood sampling after 99 days (median) after the fourth vaccine. Each circle represents one individual. Horizontal bars represent the median values, lower and upper borders of the boxes represent the first and third quartile, respectively. Whiskers represent the minimal (lower) and maximal (upper) values. ns: Kruskal–Wallis test p > 0.05. Frailty score were calculated according to the literature^–.

Figure 5

WT and Omicron neutralizing antibodies in residents before and after breakthrough infections. Serum neutralization in breakthrough infections was assessed for both wild type SARS-CoV-2 (WT) and BA.1 variant (Omicron) neutralization capacity by SARS-CoV-2 spike-pseudotyped VSV-GFP-ΔG reporter assay on Vero-E6 cells. NT5₅₀ is expressed as a function of reciprocal values of sera dilutions on a log2 scale. Each circle or square represents one individual. Post 3rd vaccine: blood sampling was done at a median time of 76 days after the third vaccine. Post 4th blood: sampling was done at a median time of 99 days after the fourth vaccine. Mean values are indicated below the x-axis. *WT paired p-value = 0.0222; ns: Omicron (BA.1) paired p-value = 0.0531 but the median difference was highly significant (Wilcoxon signed ranks test p = 0.028) (Table 2).