Serum Neutralization Against SARS-CoV-2 Variants Is Heterogenic and Depends on Vaccination Regimen

Should SARS-CoV-2 serological testing be used in the decision to deliver a COVID-19 vaccine booster? A pro-con assessment

Anti-SARS-CoV-2 vaccination has saved millions of lives in the past few years. To maintain a high level of protection, particularly in at-risk populations, booster doses are recommended to counter the waning of circulating antibody levels over time and the continuous emergence of immune escape variants of concern (VOCs). As anti-spike serology is now widely available, it may be considered a useful tool to identify individuals needing an additional vaccine dose, i.e., to screen certain populations to identify those whose plasma antibody levels are too low to provide protection. However, no recommendations are currently available on this topic. We reviewed the relevant supporting and opposing arguments, including areas of uncertainty, and concluded that in most populations, spike serology should not be used to decide about the administration of a booster dose. The main counterarguments are as follows: correlates of protection are imperfectly characterised, essentially owing to the emergence of VOCs; spike serology has an intrinsic inability to comprehensively reflect the whole immune memory; and booster vaccines are now VOC-adapted, while the commonly available commercial serological assays explore antibodies against the original virus.

COVID-19 antibody responses in individuals with natural immunity and with vaccination-induced immunity: a systematic review and meta-analysis

BACKGROUND

The COVID-19 pandemic has caused a large mortality and morbidity burden globally. For individuals, a strong immune response is the most effective means to block SARS-CoV-2 infection. To inform clinical case management of COVID-19, development of improved vaccines, and public health policy, a better understanding of antibody response dynamics and duration following SARS-CoV-2 infection and after vaccination is imperatively needed.

METHODS

We systematically analyzed antibody response rates in naturally infected COVID-19 patients and vaccinated individuals. Specifically, we searched all published and pre-published literature between 1 December 2019 and 31 July 2023 using MeSH terms and "all field" terms comprising "COVID-19" or "SARS-CoV-2," and "antibody response" or "immunity response" or "humoral immune." We included experimental and observational studies that provided antibody positivity rates following natural COVID-19 infection or vaccination. A total of 44 studies reporting antibody positivity rate changes over time were included.

RESULTS

The meta-analysis showed that within the first week after COVID-19 symptom onset/diagnosis or vaccination, antibody response rates in vaccinated individuals were lower than those in infected patients (p < 0.01), but no significant difference was observed from the second week to the sixth month. IgG, IgA, and IgM positivity rates increased during the first 3 weeks; thereafter, IgG positivity rates were maintained at a relatively high level, while the IgM seroconversion rate dropped.

CONCLUSIONS

Antibody production following vaccination might not occur as quickly or strongly as after natural infection, and the IgM antibody response was less persistent than the IgG response.

Evaluation of immune response to SARS-CoV-2 Omicron sublineages six months after different vaccination regimens in Italy

The Omicron variant is the most divergent, displaying more mutations than previous SARS-CoV-2 variants, particularly in the gene that encodes the spike protein. This study aimed to assess the persistence of neutralizing antibodies towards the SARS-CoV-2 Omicron sublineages (BA.2, BA.5, BQ.1, XBB and XBB1.5) six months after the third dose in different vaccination regimens. Subjects who received 3 doses of mRNA vaccine retained their neutralization activity against BA.2 and BA.5, even though 56.3% and 66.7% showed a ≥ 2-fold reduction in the neutralizing antibody titre, respectively. Subjects who had received the adenovirus-based vaccine plus a booster dose of mRNA vaccine retained their neutralization activity especially against BA.2. With regard to BQ.1, XBB and XBB.1.5, the majority of the subjects showed a ≥ 2-fold reduction in neutralizing antibody titre, with the greatest evasion being observed in the case of XBB. Overall, our results provide further evidence that triple homologous/heterologous vaccination and hybrid immunity result in detectable neutralizing antibodies against the ancestral virus; however, emerging Omicron sublineages, such as XBB and XBB.1.5, show a great evasive capacity, which compromises the effectiveness of current COVID-19 vaccines.

Immunity of Heterologously and Homologously Boosted or Convalescent Individuals Against Omicron BA.1, BA.2, and BA.4/5 Variants

BACKGROUND

The emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variants BA.1, BA.2, and BA.4/5 demonstrate higher transmission and infection rates than previous variants of concern. To evaluate effectiveness of heterologous and homologous booster vaccination, we directly compared cellular and humoral immune responses as well as neutralizing capacity against replication-competent SARS-CoV-2 wild type, Delta, and Omicron variants BA.1, BA.2, and BA.4/5.

METHODS

Peripheral blood mononuclear cells and serum samples from 137 participants were investigated, in 3 major groups. Individuals in the first group were vaccinated twice with ChAdOx1 and boosted with a messenger RNA (mRNA) vaccine (BNT162b2 or mRNA-1273); the second group included triple mRNA--vaccinated participants, and the third group, twice-vaccinated and convalescent individuals.

RESULTS

Vaccination and convalescence resulted in the highest SARS-CoV-2-specific antibody levels, stronger T-cell responses, and best neutralization against wild type, Delta Omicron BA.2, and BA.4/5, while a combination of ChAdOx1 and BNT162b2 vaccination elevated neutralizing capacity against Omicron BA.1. In addition, heterologous booster regimens, compared with homologous regimens, showed higher efficacy against Omicron BA.2 as well as BA.4/5.

CONCLUSIONS

We showed that twice-vaccinated and convalescent individuals demonstrated the strongest immunity against Omicron BA.2 and BA.4/5 variant, followed by those receiving heterologous and homologous booster vaccine regimens.

Intravesical BCG in bladder cancer induces innate immune responses against SARS-CoV-2

BCG is the most efficient adjuvant therapy for high-risk, non-muscle-invasive bladder cancer (NMIBC). Both innate and adaptive immune responses have been implicated in BCG-mediated effects. BCG vaccination can boost innate immune responses via trained immunity (TI), resulting in an increased resistance to respiratory viral infections. Here we evaluated for the first time whether intravesical application of BCG triggers increased immunity against SARS-CoV-2 in patients with high-risk NMIBC. Serum and peripheral blood mononuclear cells (PBMCs) from heparinized whole blood samples of 11 unvaccinated SARS-CoV-2-naïve high-risk NMIBC patients were collected at baseline and during BCG treatment in a pre-COVID-19 era. To examine B-cell or T cell-dependent adaptive immunity against SARS-CoV-2, sera were tested for the presence of SARS-CoV-2 neutralizing antibodies. Using a SARS-CoV-2 peptide pool, virus-specific T cells were quantified via IFNγ ELISpot assays. To analyze innate immune responses, mRNA and protein expression levels of pro- and anti-inflammatory cytokines were measured after a 24-hour stimulation of PBMCs with either BCG or SARS-CoV-2 wildtype. ATAC- sequencing was performed to identify a potential epigenetic reprogramming in immune cells. We neither identified SARS-CoV-2 neutralizing antibodies nor SARS-CoV-2- reactive T cells, indicating that intravesical BCG did not induce adaptive immunity against SARS-CoV-2. However, a significant increase in mRNA as well as protein expression of IL-1β, IL-6 and TNFα, which are key cytokines of trained immunity, could be observed after at least four intravesical BCG instillations. Genomic regions in the proximity of TI genes (TLR2, IGF1R, AKT1, MTOR, MAPK14, HSP90AA1) were more accessible during BCG compared to baseline. Although intravesical BCG did not induce adaptive immune responses, repetitive intravesical instillations of BCG induced circulating innate immune cells that produce TI cytokines also in response to SARS-CoV-2.

Utility of Roche Elecsys anti-SARS-CoV-2 S in ascertaining post-vaccine neutralizing antibodies

With widespread global COVID-19 vaccine coverage, a scalable, cost-effective, and standardized tool to ascertain post-vaccine immunity is a dire need. Neither clinical evaluations of vaccine efficacy, nor live virus antibody neutralization assays fulfill these criteria. Commercially available anti-S binding immunological assays have the potential to fill this gap, but need to be systematically evaluated for their utility to serve as surrogates for the aforementioned, widely accepted tools of determining vaccine efficacy. In this study, we evaluated an anti-S binding immunological assay (Roche Elecsys Anti-SARS-CoV-2 S) by utilizing two hundred and fifty-five archived serum specimens, either pre-pandemic, or those exposed to natural infections or vaccines with their neutralizing titers pre-determined through a live virus, pseudotyped antibody neutralization assay. Roche Elecsys Anti-SARS-CoV-2 S demonstrated good sensitivity (98%) and specificity (99%), just as has been reported in some other previously conducted studies using this assay. Only a mild correlation, however, with the live virus pseudotyped lentivirus antibody neutralization assay (Spearman's r = 0.26) was observed. We conclude that, as such, Elecsys Anti-SARS-CoV-2 S has a high sensitivity and specificity for detecting anti-SARS-CoV-2 S proteins, though the assay does not always correlate well with live virus assays for quantitative outcomes.