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We aimed to review the existing literature on the different types of neutralization assays and international standards for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We comprehensively summarized the serological assays for detecting neutralizing antibodies against SARS-CoV-2 and demonstrated the importance of an international standard for calibrating the measurement of neutralizing antibodies. Following the coronavirus disease outbreak in December 2019, there was an urgent demand to detect neutralizing antibodies in patients or vaccinated people to monitor disease outcomes and determine vaccine efficacy. Therefore, many approaches were developed to detect neutralizing antibodies against SARS-CoV-2, such as microneutralization assay, SARS-CoV-2 pseudotype virus assay, enzyme-linked immunosorbent assay (ELISA), and rapid lateral flow assay. Given the many types of serological assays for quantifying the neutralizing antibody titer, the comparison of different assay results is a challenge. In 2020, the World Health Organization proposed the first international standard as a common unit to define neutralizing antibody titer and antibody responses against SARS-CoV-2. These standards are useful for comparing the results of different assays and laboratories.

This study investigated the detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) neutralizing antibodies following inoculation with the coronavirus disease (COVID-19) vaccine. From June to July 2021, 127 participants who had completed COVID-19 vaccination (inactivated SARS-CoV-2 vaccine, 64; CoronaVac, 61; CanSino, 2) were recruited and tested using SARS-CoV-2 neutralizing antibody kits. The positive detection rate (inhibition of neutralizing antibodies ≥ 30%) was calculated and stratified according to population characteristics and inoculation time. The positive rate of neutralizing antibody was 47.22% (17/36) in men and 53.85% (49/91) in women, and 54.55% (24/44) in BMI ≥ 24 and 50.60% (42/83) in BMI < 24. Age was stratified as 20–29, 30–39, 40–49, and ≥50; positive detection rates of SARS-CoV-2 neutralizing antibodies were observed in 60.00% (24/40), 50.00% (21/42), 48.39% (15/31), and 42.86% (6/14), respectively, but with no significant difference (x2 = 1.724, p = 0.632). Among 127 vaccinated participants, 66 (51.97%) were positive. The positive detection rate was 63.93% (39/61) with CoronaVac and 42.19% (27/64) with the inactivated SARS-CoV-2 vaccine (significance x2 = 5.927, p = 0.015). Multivariate analysis revealed a significant difference in vaccination times, with average vaccination weeks in the positive and negative groups of 11.57 ± 6.48 and 17.87 ± 9.17, respectively (t= −4.501, p < 0.001). The positive neutralizing antibody rate was 100.00%, 60.00%, 58.33%, 55.56%, 43.14%, 28.57%, and 0.00% at 2–4, 5–8, 9–12, 13–16,17–20, 21–24, and >24 weeks, respectively (x2 = 18.030, p = 0.006). Neutralizing antibodies were detected after COVID-19 inoculation, with differences relating to inoculation timing. This study provides a reference for vaccine evaluation and follow-up immunization strengthening.

To combat the COVID-19 pandemic, vaccines against SARS-CoV-2 are now given to protect populations worldwide. The level of neutralizing antibodies following the vaccination will evolve with time and vary between individuals. Immunoassays quantifying immunoglobulins against the viral spike (S) protein in serum/plasma have been developed, but the need for venous blood samples could limit the frequency and scale of control in populations. The use of a quantitative dried blood spot (DBS) that can be self-collected would simplify this monitoring. The objective of this study was to determine whether a quantitative DBS device (Capitainer qDBS 10 µL) could be used in combination with an Elecsys anti-SARS-CoV-2 S immunoassay from Roche to follow the development and persistence of anti-S antibodies. This objective was carried out through two clinical studies. The first study investigated 14 volunteers who received two doses of the Comirnaty (Pfizer) vaccine. The levels of anti-S antibodies and the progression over time post-vaccination were studied for three months. The level of produced antibodies varied between subjects, but a similar trend was observed. The anti-S antibodies were highly stimulated by the second dose (×100) and peaked two weeks later. The antibody levels subsequently decreased and three months later were down to 65%. DBS proved to be sufficiently sensitive for use in evaluating the immune status against SARS-CoV-2 over a prolonged time. The second cohort was composed of 200 random patients from a clinical chemistry department in Stockholm. In this cohort, we had no information on previous COVID-19 infections or vaccination. Nevertheless, 87% of the subjects had anti-S immunoglobulins over 0.8 U/mL, and the bias between plasma and DBS proved to be variable, as was also seen in the first vaccination study.

BAFF has a critical role in B-cell survival, maturation and function, which makes its pathway a prime therapeutic target for various autoimmune diseases, such as systemic lupus erythematosus (SLE), rheumatoid arthritis and Sjögren’s syndrome. A cell-based assay that measures the functional activity of BAFF is required for many high throughput purposes, such as lead target screening and BAFF quantification. We report here the development of a sensitive BAFF responsive cell line via stable transfection of the BAFFR-TNFR1 hybrid receptor into monkey kidney epithelial COS-1 cells. The cellular response to BAFF can be detected by measuring the secretion of IL-8. This BAFF bioassay is not only reproducible and sensitive, but also responsive to a wide concentration range of BAFF stimulation in sera from various species. This cell line is useful in the development of sensitive bioassays to measure the levels of bioactive BAFF, inhibition of BAFF and neutralizing antibodies against any BAFF pathway-mediated therapeutic proteins.

This chapter contains sections titled: Introduction Interferon‐Beta Treatment of Multiple Sclerosis Cell‐Based Assays for Detecting Neutralizing Antibodies to IFN‐β Neutralizing‐Antibody Titer and Clinical Relevance Rheumatoid Arthritis TNF‐α‐Inhibitor Treatment of Rheumatoid Arthritis Cell‐Based Assay Approaches for Detecting Neutralizing Antibodies to TNF‐α Inhibitors References

This chapter contains sections titled: Introduction Assay Platforms for Detection of Neutralizing Antibodies Competitive Ligand‐Binding Assays Based on Electrochemiluminescence Detection Comparative Analysis of Performance of Cell‐Based Bioassays and Non‐Cell‐Based Competitive Ligand‐Binding Assays Summary Acknowledgments References

This chapter contains sections titled: Introduction Markers of Type‐I Interferon Bioactivity Type‐I Interferon Biomarkers as Indicators of Neutralizing Antibodies Summary References

This chapter contains sections titled: Introduction An Introduction to Real‐Time RT‐PCR Experimental: Neutralizing Antibodies Measured by Real‐Time RT‐PCR Assays Technical Issues Practical Issues References

This chapter contains sections titled: Introduction Immunogenicity Testing Strategy Assay Formats: Cell‐Based Versus Ligand‐Binding Immunoassays Development of a Cell‐Based Assay for Detection of Neutralizing Antibodies Optimization of a Cell‐Based Assay for Detection of Neutralizing Antibodies Validation of a Cell‐Based Assay for Detection of Neutralizing Antibodies Standardization Conclusion References

Long-term kinetics of the neutralizing antibody (NAb) response against Omicron using 1/10 intradermal (ID) mRNA vaccination as a booster following a complete series of inactivated vaccines, as well as its safety are still limited. Therefore, the objective of this study was to compare local and systemic reactions, NAb levels against Omicron BA.2 and BA.4/5 after four weeks of boosting, and durability of NAb against BA.2 and BA.4/5 after 12 and 24 weeks of mRNA-1273 vaccine boosting among the 1/10 ID, 1/5 ID, and full-dose IM groups in subjects who received a standard primary series of the BBIBP-CorV vaccine. Two-week side effects and baseline, 4-week, 12-week and 24-week NAb levels against wild type, BA.2 and BA.4/5 Omicron among 3 groups were compared. There were 140 participants with 46, 47, and 47 subjects in 1/10 ID, 1/5 ID, and IM groups, respectively. The 1/10 ID-induced localized pain is less common than IM and 1/5 ID. Systemic reactions were lower than IM and comparable to 1/5 ID. BA.2 NAb was indifferent from the other 2 groups, except 2.5-fold lower than IM at 12 weeks. BA.2 NAb in 1/10 ID was higher than the cut-off level throughout the 24-week study period whereas BA.4/5 NAb at 24 weeks was below the NAb detection threshold and significantly lower than IM group. The 1/10 ID mRNA-1273 boosting after BBIBP-CorV priming was safe and induced above-threshold NAb against BA.2 and BA.4/5 for at least 12 weeks. This study was registered to the Thai Clinical Trials Registry with study ID: TCTR20210822002. •The 1/10 ID mRNA-1273 boosting after BBIBP-CorV priming was safe and induced NAb.•Local pain is less common than both IM and 1/5 ID vaccination.•Non-aching local reactions are more common than IM but less common than 1/5 ID.•Systemic reactions are comparable with 1/5 ID but less common than IM vaccination.