Long-Term Dynamic of Anti-TrimericS and Anti-RBD Antibodies in Naive and COVID-19 Recovered mRNA-1273 Vaccine Recipients.

OBJECTIVE: Patients and physicians are increasingly requesting their clinical laboratory to provide SARS-CoV-2 serology interpretation. Our study aimed to assess the evolution of SARS-CoV-2 antibodies in Moderna-vaccinated health care workers. METHODS: We analyzed the evolution of mRNA-1273 (Moderna)-elicited antibodies by 2 high-throughput assays, TrimericS IgG (Diasorin) and SARS-CoV-2 IgG-II (Abbott). RESULTS: After the first injection, the COVID-19-recovered vaccinees showed a serological response as strong as that observed 1 month after the second injection in participants without COVID-19 history. Although remaining above the positivity thresholds, the TrimericS immunoglobulin G (IgG) and anti-RBD (receptor-binding domain) IgG levels fell considerably between 1 and 7 months postvaccination, dropping to 10.6% and 13% for the COVID-19 recovered subgroup and to 11.7% and 9.3% for the COVID-19 naive subgroup. CONCLUSION: Regardless of the test used, a decrease in circulating anti-SARS-CoV-2 IgG levels should be expected a few months after vaccination. As this decline does not preclude the efficacy of immune response, caution is necessary when interpretating postvaccination serological data.

Nationwide Harmonization Effort for Semi-Quantitative Reporting of SARS-CoV-2 PCR Test Results in Belgium.

From early 2020, a high demand for SARS-CoV-2 tests was driven by several testing indications, including asymptomatic cases, resulting in the massive roll-out of PCR assays to combat the pandemic. Considering the dynamic of viral shedding during the course of infection, the demand to report cycle threshold (Ct) values rapidly emerged. As Ct values can be affected by a number of factors, we considered that harmonization of semi-quantitative PCR results across laboratories would avoid potential divergent interpretations, particularly in the absence of clinical or serological information. A proposal to harmonize reporting of test results was drafted by the National Reference Centre (NRC) UZ/KU Leuven, distinguishing four categories of positivity based on RNA copies/mL. Pre-quantified control material was shipped to 124 laboratories with instructions to setup a standard curve to define thresholds per assay. For each assay, the mean Ct value and corresponding standard deviation was calculated per target gene, for the three concentrations (107, 105 and 103 copies/mL) that determine the classification. The results of 17 assays are summarized. This harmonization effort allowed to ensure that all Belgian laboratories would report positive PCR results in the same semi-quantitative manner to clinicians and to the national database which feeds contact tracing interventions.

An original multiplex method to assess five different SARS-CoV-2 antibodies.

OBJECTIVES: Accurate SARS-CoV-2 serological assays are urgently needed to help diagnose infection, determine past exposure of populations and assess the response to future vaccines. The study aims at assessing the performance of the multiplex D-tek COVIDOT 5 IgG assay for the detection of SARS-CoV-2 IgG antibodies (N, S1+S2, S1, S2 and RBD). METHODS: Sensitivity and dynamic trend to seropositivity were evaluated in 218 samples obtained from 46 rRT-PCR confirmed COVID-19 patients. Non-SARS-CoV-2 sera (n=118) collected before the COVID-19 pandemic with a potential cross-reaction to the SARS-CoV-2 immunoassay were included in the specificity analysis. RESULTS: A gradual dynamic trend since symptom onset was observed for all IgG antibodies. Sensitivities before day 14 were suboptimal. At ≥21 days, sensitivities reached 100% (93.4-100%) for N, S1+S2, S2 and RBD-directed IgG and 96.3% (87.3-99.6%) for S1-directed IgG. In 42 out of 46 patients (91.3%), all five antibodies were detected at ≥14 days. The four remaining patients had between 2 and 4 positive antibodies at their respective maximal follow-up period. The specificity was 100 % for S1+S2, S2 and RBD, 98.3% for N and 92.4% (86.0-96.5%) for S1-directed IgG. The combined use of antigens increases the early sensitivity whilst enforcing high specificity. CONCLUSIONS: Sensitivities at ≥21 days and specificities were excellent, especially for N, S1+S2, S2 and RBD-directed IgG. Caution is however required when interpreting single S1-directed reactivities. Using a multiplex assay complies with the orthogonal testing algorithm of the CDC and allows a better and critical interpretation of the serological status of a patient.