Correlation of vaccine‐elicited antibody levels and neutralizing activities against SARS‐CoV‐2 and its variants

Both Pfizer-BNT162b2 and Moderna-mRNA-1273 vaccines can elicit an effective immune response against SARS-CoV-2 infection. However, the elicited serum antibody levels vary substantially and longitudinally decrease after vaccination. We examined the correlation of vaccination-induced IgG levels and neutralization titers against newly emerged variants remains and demonstrate a significant reduction of neutralization activities against the variants (B.1.1.7, B.1.525, and B.1.351) in Pfizer or Moderna vaccined sera. There was a significant and positive correlation between serum IgG levels and ID50 titers for not only SARS-CoV-2 WT but also the variants. These findings indicate that a high level of anti-spike IgG may offer better protection against infection from SARS-CoV-2 and its variants. Therefore, it is necessary to longitudinally monitor specific serum IgG level for evaluating the protective efficacy of the vaccines against SARS-CoV-2 and its new variants.


Dear Editor,
The COVID-19 vaccines (Pfizer-BNT162b2 and Moderna-mRNA-1273) can elicit an effective immune response against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. 1,2 However, titers of elicited serum antibody to spike protein of the virus differ among vaccinated individuals and decline after vaccination. 3,4 Additionally, the ability of the vaccines to protect against newly emerged variants needs to be further elucidated. Therefore, it is important to understand the correlation between levels of vaccination-induced antibody and neutralizing activity against SARS-CoV-2, including the variants.
Previous authors have investigated the subject of vaccine efficacy against SARS-CoV-2 variants using both clinical and in-vitro models. For example, a study by Bernal et al. comparing the B.1.617.2 and B.1.1.7 variants using clinical data noted only modest differences in BNT162b2 and ChAdOx1 vaccine's effectiveness. 5 Abu-Raddad et al. used a similar test-negative case-control design and found that the BNT162b2 vaccine's effectiveness was reduced against B.1.351, but noted that protection against severe disease was still robust. 6 An in-vitro study using BNT162b2-elicited serum by Liu (Table S2). This study was approved by Temple University Institutional Review Board (IRB; IRB #28021) and the informed consent forms were signed by all study subjects.
We measured the serum titers of specific IgG antibodies to SARS-CoV-2 spike S1 by an enzyme-linked immunosorbent assay and demonstrated that all subjects vaccinated with either Pfizer or Moderna vaccine had detectable levels of serum IgG. The Pfizer group IgG titers ranged from 1.05 × 10 4 to 1.68 × 10 5 ng/ml, and the Moderna group IgG titers ranged from 2.01 × 10 4 to 1.70 × 10 5 ng/ml ( Figure 1A). Given that the distribution of the IgG titers was left-skewed ( Figure 1B), we reported the GMTs, with the geometric standard deviation factor (GSDF) and the 95% CI of the GMT. For the Pfizer group, the GMT was 6.12 × 10 4 ng/ml (GSDF = 2.24, 95% confidence interval [CI] = 4.53 × 10 4 -8.27 × 10 4 ng/ml). For the Moderna group, the GMT was 9.24 × 10 4 ng/ml (GSDF = 1.90, 95% CI = 6.78 × 10 4 -1.26 × 10 4 ng/ml). The Wilcoxon rank-sum test p-value comparing both groups' IgG titers was 0.0906, and thus the difference in post-vaccination titers between Pfizer and Moderna recipients was not statistically significant (p > 0.05).
We then performed a recombinant vesicular stomatitis virus-based SARS-CoV-2 neutralization assay to determine the serum neutralization titer (50% inhibitory dilution, ID 50 , units are 1/dilution) of all vaccinated subjects.  The neutralizing infectivity of the pseudoviruses was evaluated in sera at dilutions ranging from 1:50 to 1:36 450. We showed that sera from all vaccinated subjects had neutralizing activity and that there was no statistical difference (p = .144) in serum neutralizing activity (ID 50 ) against SARS-CoV-2 WT between Pfizer and Moderna ( Figure 1C). For the Pfizer group, the WT ID 50 ranged from 732 to 3.00 × 10 4 , and for the Moderna group, the WT ID 50 ranged from 2.43 × 10 3 to 2.67 × 10 4 ( Figure 1C). As the ID 50 values are left-skewed for all variants ( Figure 1D-H), we have also reported the GMT of the ID 50 values for each group with the GSDF and 95% CI of the GMT. The Pfizer group WT ID 50 had a GMT of 6.74 × 10 3 (GSDF = 2.42, 95% CI = 4.84 × 10 3 -9.38 × 10 3 ), and the Moderna group ID 50 had a GMT of 9.67 × 10 3 (GSDF = 1.92, 95% CI = 7.07 × 10 3 -1.32 × 10 4 ). Therefore, based on the large GSDF, there is substantial variation in the neutralizing ID 50 values for both groups of vaccine recipients for WT SARS-CoV-2. It is worth noting that the level of neutralizing antibodies that confers immunity in-vivo against SARS-CoV-2 is not well known, and so it is possible that even those donors with low ID 50 values (and therefore, comparatively poorer neutralizing ability) would still have sufficient antibody titers to negate or diminish viral infection and its associated sequelae.
Although sera from all vaccinated subjects could neutralize the pseudoviruses bearing spike proteins of variants, neutralizing titers were lower when compared to SARS-CoV-2 WT spike protein. In Pfizer-vaccinated sera, there was a significant decrease of GMTs for D614G (-1. In conclusion, we demonstrated that all study participants vaccinated with either Pfizer or Moderna vaccine were able to produce effective antibodies against spike proteins of both SARS-CoV-2 WT and the variants (Table S3). There was a several-fold reduction in GMTs of ID 50 against the variants (UK-B.1.1.7, UK-B.1.525 and SA-B.1.351) in sera, as compared to those against SARS-CoV-2 WT. However, sera at low dilutions were equally effective in neutralizing both SARS-CoV-2 and the variants. Importantly, we demonstrated that among all vaccinated subjects, there was an overall positive correlation between serum IgG levels and ID 50 titers for not only SARS-CoV-2 WT but also the variants ( Figure 2C). This finding suggests that the level of IgG titer may be a correlate of immunity. Therefore, it is necessary to longitudinally monitor specific serum IgG levels for evaluating the protective efficacy of vaccines against SARS-CoV-2 and its new variants.
Limitations of the study include the small sample size, the lack of live SARS-CoV-2 neutralization assays, and the fact that the timing of sampling after the second dose of mRNA vaccine was not well controlled (ranging from 22 to 68 days)

C O N F L I C T O F I N T E R E S T
The authors declare that they have no conflict of interest.

A D D I T I O N A L C O N T R I B U T I O N S
We thank all the voluntary vaccinated subjects for providing clinical samples and the members of the Neurology Clinic in Temple Hospital for collecting blood samples. We also thank Guangxiang Luo, PhD (University of Alabama), for providing Hela/ACE2-11 cells. None of these contributors received any compensation for their help in carrying out the study.