Immunogenicity and Durability of Antibody Responses to Homologous and Heterologous Vaccinations with BNT162b2 and ChAdOx1 Vaccines for COVID-19

Vaccine-induced SARS-CoV-2 antibody response: the comparability of S1-specific binding assays depends on epitope and isotype discrimination

Background

Quantification of the SARS-CoV-2-specific immune response by serological immunoassays is critical for the management of the COVID-19 pandemic. In particular, neutralizing antibody titers to the viral spike (S) protein have been proposed as a correlate of protection (CoP). The WHO established the First International Standard (WHO IS) for anti-SARS-CoV-2 immunoglobulin (Ig) (NIBSC 20/136) to harmonize binding assays with the same antigen specificity by assigning the same unitage in binding antibody units (BAU)/ml.

Method

In this study, we analyzed the S1-specific antibody response in a cohort of healthcare workers in Germany (n = 76) during a three-dose vaccination course over 8.5 months. Subjects received either heterologous or homologous prime-boost vaccination with ChAdOx1 nCoV-19 (AstraZeneca) and BNT162b2 (Pfizer-BioNTech) or three doses of BNT162b2. Antibodies were quantified using three anti-S1 binding assays (ELISA, ECLIA, and PETIA) harmonized to the WHO IS. Serum levels of neutralizing antibodies were determined using a surrogate virus neutralization test (sVNT). Binding assays were compared using Spearman's rank correlation and Passing-Bablok regression.

Findings

All assays showed good correlation and similar antibody kinetics correlating with neutralizing potential. However, the assays show large proportional differences in BAU/ml. ECLIA and PETIA, which detect total antibodies against the receptor- binding domain (RBD) within the S1 subunit, interact similarly with the convalescent plasma-derived WHO IS but differently with vaccine serum, indicating a high sensitivity to the IgG/IgM/IgA ratio.

Conclusion

All three binding assays allow monitoring of the antibody response in COVID-19-vaccinated individuals. However, the assay-specific differences hinder the definition of a common protective threshold in BAU/ml. Our results highlight the need for the thoughtful use of conversion factors and consideration of method-specific differences. To improve the management of future pandemics and harmonize total antibody assays, we should strive for reference material with a well-characterized Ig isotype composition.

A Systematic Evaluation of the SARS-CoV-2 Vaccine-Induced Anti-S-RBD-Ig Response in a Population of Health Care Workers

In the wake of the COVID-19 pandemic, the novel class of mRNA vaccines has been granted first-time approval for active immunization against SARS-CoV-2 alongside the already established viral vector-based vaccines. In this prospective single-center study, we set out to determine the vaccine-induced humoral immune response in a population of 1512 health care employees after the second and third vaccination, respectively. Anti-SARS-CoV-2 receptor-binding domain (RBD) and nucleocapsid antigen antibody concentrations were assessed using commercially available immunoassays. We could show that, in particular, young study subjects aged below 30 years, as well as those with a prior SARS-CoV-2 infection, developed significantly higher antibody concentrations. Our data further suggest that being in physically close contact with formerly SARS-CoV-2-positive people positively affects the post-vaccination response. Surprisingly, study subjects with a BMI > 30 produced the highest anti-S-RBD Ig antibody levels if they had recently received their third vaccination. Also, heterologous dual vaccine regimens consisting of a BNT162b2 and ChAdOx1 n-CoV-19, a homologous triple combination of BNT162b2, and an application of mRNA-1273 as the third vaccine, were most efficient at eliciting a humoral immune response. Our study substantiates existing evidence, but beyond that, scrutinizes the impact of vaccine agents and their respective combinations, as well as different time intervals on humoral immunogenicity.

Impact of timing and combination of different BNT162b2 and ChAdOx1-S COVID-19 basic and booster vaccinations on humoral immunogenicity and reactogenicity in adults

In this single-center observational study with 1,206 participants, we prospectively evaluated SARS-CoV-2-antibodies (anti-S RBD) and vaccine-related adverse drug reactions (ADR) after basic and booster immunization with BNT162b2- and ChAdOx1-S-vaccines in four vaccination protocols: Homologous BNT162b2-schedule with second vaccination at either three or six weeks, homologous ChAdOx1-S-vaccination or heterologous ChAdOx1-S/BNT162b2-schedule, each at 12 weeks. All participants received a BNT162b2 booster. Blood samples for anti-S RBD analysis were obtained multiple times over a period of four weeks to six months after basic vaccination, immediately before, and up to three months after booster vaccination. After basic vaccination, the homologous ChAdOx1-S-group showed the lowest anti-S RBD levels over six months, while the heterologous BNT162b2-ChAdOx1-S-group demonstrated the highest anti-S levels, but failed to reach level of significance compared with the homologous BNT162b2-groups. Antibody levels were higher after an extended vaccination interval with BNT162b2. A BNT162b2 booster increased anti-S-levels 11- to 91-fold in all groups, with the homologous ChAdOx1-S-cohort demonstrated the highest increase in antibody levels. No severe or serious ADR were observed. The findings suggest that a heterologous vaccination schedule or prolonged vaccination interval induces robust humoral immunogenicity with good tolerability. Extending the time to boost-immunization is key to both improving antibody induction and reducing ADR rate.

Heterologous Vector-mRNA Based SARS-CoV-2 Vaccination Strategy Appears Superior to a Homologous Vector-Based Vaccination Scheme in German Healthcare Workers Regarding Humoral SARS-CoV-2 Response Indicating a High Boosting Effect by mRNA Vaccines

BACKGROUND

Longitudinal humoral SARS-CoV-2 (severe acute respiratory syndrome coronavirus type 2) immunity for up to 15 months due to vaccination, the efficacy of vaccination strategies (homologous, vector-vector versus heterologous, vector-mRNA), the influence of vaccination side effects, and the infection rate in German healthcare workers need to be investigated.

METHODS

In this study, 103 individuals vaccinated against SARS-CoV-2 were enrolled to examine their anti-SARS-CoV-2 anti-N- and anti-RBD/S1-Ig levels. A total of 415 blood samples in lithium heparin tubes were prospectively obtained, and a structured survey regarding medical history, type of vaccine, and vaccination reactions was conducted.

RESULTS

All participants demonstrated a humoral immune response, among whom no values decreased below the positivity cutoff. Five to six months after the third vaccination, three participants showed anti-RBD/S1 antibodies of less than 1000 U/mL. We observed higher levels for heterologous mRNA-/vector-based combinations compared to pure vector-based vaccination after the second vaccination, which is harmonized after a third vaccination with the mRNA-vaccine only in both cohorts. The incidence of vaccine breakthrough in a highly exposed cohort was 60.3%.

CONCLUSION

Sustained long-term humoral immunity was observed, indicating the superiority of a heterologous mRNA-/vector-based combination compared to pure vector-based vaccination. There was longevity of anti-RBD/S1 antibodies of at least 4 and up to 7 months without external stimulus. Regarding vaccination reactogenity, the occurrence of local symptoms as pain at the injection site was increased after the first mRNA application compared to the vector-vector cohort with a general decrease in adverse events at later vaccination time points. Overall, a correlation between the humoral vaccination response and vaccination side effects was not observed. Despite the high prevalence of vaccine breakthroughs, these only occurred in the later course of the study when more infectious variants, which are, however, associated with milder courses, were present. These results provide insights into vaccine-related serologic responses, and the study should be expanded using additional vaccine doses and novel variants in the future.