Reliability of the rapid antigen test for diagnosis of SARS-CoV-2 in Tunisia.

Background: Early and accurate diagnosis is crucial for preventing the spread of SARS-CoV-2 infection. The rapid antigen test was developed for testing infection, and it was necessary to assess its performance before widespread use in Tunisia. Aim: To evaluate the effectiveness of a rapid antigen test for the detection of SARS-CoV-2 in nasopharyngeal swabs in Tunisia. Methods: Nasopharyngeal samples were taken from COVID-19 suspected cases between October and December 2020 and tested using the Standard Q COVID-19 Ag test (SD-Biosensor, Republic of Korea) and real-time reverse transcription polymerase chain reaction (RT­PCR). Results: Overall, 4539 patients were tested. Of the total study population (N = 4539), 82.5% of positive samples remained positive with the rapid antigen test, while 20.2% (470/2321) of samples that were negative with rapid antigen test were confirmed positive with RT-PCR, giving a negative predictive value of 79.8% for the rapid antigen test. The sensitivity and negative predictive value of the rapid antigen test were 70.2% and 65.8%, respectively. These results improved to 96.4% and 92.8%, respectively, when considering the cycle threshold value by RT-PCR below 25. Conclusion: Although the rapid antigen test was less sensitive than RT-PCR, its ability to rapidly detect individuals with high viral loads makes it suitable for use during an epidemic.

Design and Development of an Antigen Test for SARS-CoV-2 Nucleocapsid Protein to Validate the Viral Quality Assurance Panels.

The continuing mutability of the SARS-CoV-2 virus can result in failures of diagnostic assays. To address this, we describe a generalizable bioinformatics-to-biology pipeline developed for the calibration and quality assurance of inactivated SARS-CoV-2 variant panels provided to Radical Acceleration of Diagnostics programs (RADx)-radical program awardees. A heuristic genetic analysis based on variant-defining mutations demonstrated the lowest genetic variance in the Nucleocapsid protein (Np)-C-terminal domain (CTD) across all SARS-CoV-2 variants. We then employed the Shannon entropy method on (Np) sequences collected from the major variants, verifying the CTD with lower entropy (less prone to mutations) than other Np regions. Polyclonal and monoclonal antibodies were raised against this target CTD antigen and used to develop an Enzyme-linked immunoassay (ELISA) test for SARS-CoV-2. Blinded Viral Quality Assurance (VQA) panels comprised of UV-inactivated SARS-CoV-2 variants (XBB.1.5, BF.7, BA.1, B.1.617.2, and WA1) and distractor respiratory viruses (CoV 229E, CoV OC43, RSV A2, RSV B, IAV H1N1, and IBV) were assembled by the RADx-rad Diagnostics core and tested using the ELISA described here. The assay tested positive for all variants with high sensitivity (limit of detection: 1.72-8.78 ng/mL) and negative for the distractor virus panel. Epitope mapping for the monoclonal antibodies identified a 20 amino acid antigenic peptide on the Np-CTD that an in-silico program also predicted for the highest antigenicity. This work provides a template for a bioinformatics pipeline to select genetic regions with a low propensity for mutation (low Shannon entropy) to develop robust 'pan-variant' antigen-based assays for viruses prone to high mutational rates.

Validity of at-home rapid antigen lateral flow assay and artificial intelligence read to detect SARS-CoV-2.

BACKGROUND: The gold standard for COVID-19 diagnosis-reverse-transcriptase polymerase chain reaction (RT-PCR)- is expensive and often slow to yield results whereas lateral flow tests can lack sensitivity. METHODS: We tested a rapid, lateral flow antigen (LFA) assay with artificial intelligence read (LFAIR) in subjects from COVID-19 treatment trials (N = 37; daily tests for 5 days) and from a population-based study (N = 88; single test). LFAIR was compared to RT-PCR from same-day samples. RESULTS: Using each participant's first sample, LFAIR showed 86.2% sensitivity (95% CI 73.6%-98.8) and 94.3% specificity (88.8%-99.7%) compared to RT-PCR. Adjusting for days since symptom onset and repeat testing, sensitivity was 97.8% (89.9%-99.5%) on the first symptomatic day and decreased with each additional day. Sensitivity improved with artificial intelligence (AI) read (86.2%) compared to the human eye (71.4%). CONCLUSION: LFAIR showed improved accuracy compared to LFA alone. particularly early in infection.

Evaluation of the analytical performance of anti-SARS-CoV-2 antibody test kits distributed or developed in Japan.

Background: With the spread of COVID-19, anti-SARS-CoV-2 antibody tests have been utilized. Herein we evaluated the analytical performance of anti-SARS-CoV-2 antibody test kits using a new reference standard prepared from COVID-19 patient sera. Methods: Fifty-seven kits in total (16 immunochromatography types, 11 ELISA types and 30 types for automated analyzers) were examined. By measuring serially diluted reference standards, the maximum dilution factor showing a positive result and its precision were investigated. Results: The measured cut-off titers varied largely depending on the antibody kit; however, the variability was small, with the titers obtained by each kit being within twofold in most cases. Conclusion: The current results suggest that a suitable kit should be selected depending on the intended purpose.

Determining the reliability of rapid SARS-CoV-2 antigen detection in fully vaccinated individuals.

BACKGROUND: Rapid antigen detection tests (RADT) are commonly used as SARS-CoV-2 diagnostic tests both by medical professionals and laypeople. However, the performance of RADT in vaccinated individuals has not been fully investigated. OBJECTIVES: RT-qPCR and rapid antigen detection testing were performed to evaluate the performance of the Standard Q COVID-19 Ag Test in detecting SARS-CoV-2 breakthrough infections in vaccinated individuals. STUDY DESIGN: Two swab specimens, one for RT-qPCR and one for RADT, were collected from vaccinated individuals in an outpatient clinic. For comparison of RADT performance in vaccinated and unvaccinated individuals, a dataset already published by this group was used as reference. RESULTS: During the delta wave, a total of 696 samples were tested with both RT-qPCR and RADT that included 692 (99.4%) samples from vaccinated individuals. Of these, 76 (11.0%) samples were detected SARS-CoV-2 positive by RT-qPCR and 45 (6.5%) samples by the Standard Q COVID-19 Ag test. Stratified by Ct values, sensitivity of the RADT was 100.0%, 94.4% and 81.1% for Ct ≤ 20 (n=18), Ct ≤ 25 (n=36) and Ct ≤ 30 (n=53), respectively. Samples with Ct values ≥ 30 (n=23) were not detected. Overall RADT specificity was 99.7% and symptom status did not affect RADT performance. Notably, RADT detected 4 out of 4 samples of probable Omicron variant infection based on single nucleotide polymorphism analysis. CONCLUSION: Our results show that RADT testing remains a valuable tool in detecting breakthrough infections with high viral RNA loads.

Evaluation of the Performance of a Multiplexed Serological Assay in the Detection of SARS-CoV-2 Infections in a Predominantly Vaccinated Population.

SARS-CoV-2 seroprevalence studies may be complicated by vaccination efforts. It is important to characterize the ability of serology methods to correctly distinguish prior infection from postvaccination seroreactivity. We report the performance of the Meso Scale Discovery (MSD) V-PLEX COVID-19 Coronavirus Panel 2 IgG assay. Using serum samples from a prospective cohort of paramedics, we calculated the performance of the V-PLEX nucleocapsid ("N") assay to classify prior SARS-CoV-2 infections, defined as a (i) history of a positive SARS-CoV-2 PCR test or (ii) positive serology results using the Roche Elecsys total nucleocapsid anti-SARS-Cov-2 assay. We calculated sensitivity and specificity at the optimal threshold (defined by the highest Youden index). We compared subgroups based on vaccination status, and between models that excluded prior infections 3 to 12 months before sample collection. Of 1119 participants, 914 (81.7%) were vaccinated and 60 (5.4%) had evidence of a preceding SARS-CoV-2 infection. Overall and within vaccinated and unvaccinated subgroups, the optimal thresholds were 828 AU/mL, 827 AU/mL, and 1324 AU/mL; with sensitivities of 0.95 (95% CI: 0.94 to 0.96), 0.95 (0.94 to 0.96), 0.94 (0.92 to 0.96) and specificities of 0.88 (0.86 to 0.90), 0.87 (0.85 to 0.89), and 0.94 (0.89 to 0.98), respectively. N-assay specificity was significantly better in unvaccinated (versus vaccinated) individuals (P = 0.005). Overall optimal thresholds based on the AUC values were higher for samples from unvaccinated participants, especially when examining infections within the preceding 9 months (5855 versus 1704 AU/mL). Overall, V-PLEX nucleocapsid assay cutoff values were higher among unvaccinated individuals. Specificity was also significantly higher among unvaccinated individuals. Different thresholds were required to achieve optimal test performance, especially for detecting SARS-CoV-2 infections within the preceding 9 months. IMPORTANCE Among a cohort of adult paramedics in Canada, we investigated the performance of nucleocapsid (N) antibody detection (measured with a V-PLEX assay) to identify previous COVID-19 infections and compared differences among vaccinated and unvaccinated. Our data indicate that vaccinated and unvaccinated groups require different thresholds to achieve optimal test performance, especially for detecting COVID-19 within the preceding 9 months. Overall, specificity was significantly higher among unvaccinated, compared to vaccinated individuals.

SARS-CoV-2 antigen lateral flow tests for detecting infectious people: linked data analysis.

OBJECTIVES: To investigate the proportion of lateral flow tests (LFTs) that produce negative results in those with a high risk of infectiousness from SARS-CoV-2, to investigate the impact of the stage and severity of disease, and to compare predictions made by influential mathematical models with findings of empirical studies. DESIGN: Linked data analysis combining empirical evidence of the accuracy of the Innova LFT, the probability of positive viral culture or transmission to secondary cases, and the distribution of viral loads of SARS-CoV-2 in individuals in different settings. SETTING: Testing of individuals with symptoms attending NHS Test-and-Trace centres across the UK, residents without symptoms attending municipal mass testing centres in Liverpool, and students without symptoms screened at the University of Birmingham. PARTICIPANTS: Evidence for the sensitivity of the Innova LFT, based on 70 individuals with SARS-CoV-2 and LFT results. Infectiousness was based on viral culture rates on 246 samples (176 people with SARS-CoV-2) and secondary cases among 2 474 066 contacts; distributions of cycle threshold (Ct) values from 231 497 index individuals attending NHS Test-and-Trace centres; 70 people with SARS-CoV-2 detected in Liverpool and 62 people with SARS-CoV-2 in Birmingham (54 imputed). MAIN OUTCOME MEASURES: The predicted proportions who were missed by LFT and viral culture positive and missed by LFT and sources of secondary cases, in each of the three settings. Predictions were compared with those made by mathematical models. RESULTS: The analysis predicted that of those with a viral culture positive result, Innova would miss 20% attending an NHS Test-and-Trace centre, 29% without symptoms attending municipal mass testing, and 81% attending university screen testing without symptoms, along with 38%, 47%, and 90% of sources of secondary cases. In comparison, two mathematical models underestimated the numbers of missed infectious individuals (8%, 10%, and 32% in the three settings for one model, whereas the assumptions from the second model made it impossible to miss an infectious individual). Owing to the paucity of usable data, the inputs to the analyses are from limited sources. CONCLUSIONS: The proportion of infectious people with SARS-CoV-2 missed by LFTs is substantial enough to be of clinical importance. The proportion missed varied between settings because of different viral load distributions and is likely to be highest in those without symptoms. Key models have substantially overestimated the sensitivity of LFTs compared with empirical data. An urgent need exists for additional robust well designed and reported empirical studies from intended use settings to inform evidence based policy.

Development of a quantitative COVID-19 multiplex assay and its use for serological surveillance in a low SARS-CoV-2 incidence community.

A serological COVID-19 Multiplex Assay was developed and validated using serum samples from convalescent patients and those collected prior to the 2020 pandemic. After initial testing of multiple potential antigens, the SARS-CoV-2 nucleocapsid protein (NP) and receptor-binding domain (RBD) of the spike protein were selected for the human COVID-19 Multiplex Assay. A comparison of synthesized and mammalian expressed RBD proteins revealed clear advantages of mammalian expression. Antibodies directed against NP strongly correlated with SARS-CoV-2 virus neutralization assay titers (rsp = 0.726), while anti-RBD correlation was moderate (rsp = 0.436). Pan-Ig, IgG, IgA, and IgM against NP and RBD antigens were evaluated on the validation sample sets. Detection of NP and RBD specific IgG and IgA had outstanding performance (AUC > 0.90) for distinguishing patients from controls, but the dynamic range of the IgG assay was substantially greater. The COVID-19 Multiplex Assay was utilized to identify seroprevalence to SARS-CoV-2 in people living in a low-incidence community in Ithaca, NY. Samples were taken from a cohort of healthy volunteers (n = 332) in early June 2020. Only two volunteers had a positive result on a COVID-19 PCR test performed prior to serum sampling. Serological testing revealed an exposure rate of at least 1.2% (NP) or as high as 5.7% (RBD), higher than the measured incidence rate of 0.16% in the county at that time. This highly sensitive and quantitative assay can be used for monitoring community exposure rates and duration of immune response following both infection and vaccination.

COVID-19 Serology Control Panel Using the Dried-Tube Specimen Method.

The dried-tube specimen (DTS) procedure was used to develop the COVID-19 serology control panel (CSCP). The DTS offers the benefit of shipping materials without a cold chain, allowing for greater access without deterioration of material integrity. Samples in the panel were sourced from COVID-19 convalescent persons from March to May 2020. The immunoglobulin subtypes (total Ig, IgM, and IgG) and their respective reactivity to severe acute respiratory syndrome coronavirus 2 nucleocapsid, spike, and receptor-binding domain antigens of the samples were delineated and compared with the WHO International Standard to elucidate the exact binding antibody units of each CSCP sample and ensure the CSCP provides adequate reactivity for different types of serological test platforms. We distribute the CSCP as a kit with five coded tubes to laboratories around the world to be used to compare test kits for external quality assurance, for harmonizing laboratory testing, and for use as training materials for laboratory workers.

Paper-based microfluidic chip for rapid detection of SARS-CoV-2 N protein.

This research has developed a method for rapid detection of SARS-CoV-2 N protein on a paper-based microfluidic chip. The chitosan-glutaraldehyde cross-linking method is used to fix the coated antibody, and the sandwich enzyme-linked immunosorbent method is used to achieve the specific detection of the target antigen. The system studied the influence of coating antibody concentration and enzyme-labeled antibody concentration on target antigen detection. According to the average gray value measured under different N protein concentrations, the standard curve of the method was established and the sensitivity was tested, and its linear regression was obtained. The equation is y = 9.8286x+137.6, R2 = 0.9772 > 0.90, which shows a high degree of fit. When the concentration of coating antibody and enzyme-labeled antibody were 1 µg/mL and 2 µg/mL, P > 0.05, the difference was not statistically significant, so the lower concentration of 1 µg/mL was chosen as the coating antibody concentration. The results show that the minimum concentration of N protein that can be detected by this method is 8 µg/mL, and the minimum concentration of coating antibody and enzyme-labeled antibody is 1 µg/mL, which has the characteristics of high sensitivity and good repeatability.

Association of Self-reported COVID-19 Infection and SARS-CoV-2 Serology Test Results With Persistent Physical Symptoms Among French Adults During the COVID-19 Pandemic.

Importance: After an infection by SARS-CoV-2, many patients present with persistent physical symptoms that may impair their quality of life. Beliefs regarding the causes of these symptoms may influence their perception and promote maladaptive health behaviors. Objective: To examine the associations of self-reported COVID-19 infection and SARS-CoV-2 serology test results with persistent physical symptoms (eg, fatigue, breathlessness, or impaired attention) in the general population during the COVID-19 pandemic. Design, Setting, and Participants: Participants in this cross-sectional analysis were 26 823 individuals from the French population-based CONSTANCES cohort, included between 2012 and 2019, who took part in the nested SAPRIS and SAPRIS-SERO surveys. Between May and November 2020, an enzyme-linked immunosorbent assay was used to detect anti-SARS-CoV-2 antibodies. Between December 2020 and January 2021, the participants reported whether they believed they had experienced COVID-19 infection and had physical symptoms during the previous 4 weeks that had persisted for at least 8 weeks. Participants who reported having an initial COVID-19 infection only after completing the serology test were excluded. Main Outcomes and Measures: Logistic regressions for each persistent symptom as the outcome were computed in models including both self-reported COVID-19 infection and serology test results and adjusting for age, sex, income, and educational level. Results: Of 35 852 volunteers invited to participate in the study, 26 823 (74.8%) with complete data were included in the present study (mean [SD] age, 49.4 [12.9] years; 13 731 women [51.2%]). Self-reported infection was positively associated with persistent physical symptoms, with odds ratios ranging from 1.39 (95% CI, 1.03-1.86) to 16.37 (95% CI, 10.21-26.24) except for hearing impairment (odds ratio, 1.45; 95% CI, 0.82-2.55) and sleep problems (odds ratio, 1.14; 95% CI, 0.89-1.46). A serology test result positive for SARS-COV-2 was positively associated only with persistent anosmia (odds ratio, 2.72; 95% CI, 1.66-4.46), even when restricting the analyses to participants who attributed their symptoms to COVID-19 infection. Further adjusting for self-rated health or depressive symptoms yielded similar results. There was no significant interaction between belief and serology test results. Conclusions and Relevance: The findings of this cross-sectional analysis of a large, population-based French cohort suggest that persistent physical symptoms after COVID-19 infection may be associated more with the belief in having been infected with SARS-CoV-2 than with having laboratory-confirmed COVID-19 infection. Further research in this area should consider underlying mechanisms that may not be specific to the SARS-CoV-2 virus. A medical evaluation of these patients may be needed to prevent symptoms due to another disease being erroneously attributed to "long COVID."

Factors influencing the performance of rapid SARS-CoV-2 antigen tests under field condition.

BACKGROUND: Globally, real-time reverse transcription-polymerase chain reaction (rRT-PCR) is the reference detection technique for SARS-CoV-2, which is expensive, time consuming, and requires trained laboratory personnel. Thus, a cost-effective, rapid antigen test is urgently needed. This study evaluated the performance of the rapid antigen tests (RATs) for SARS-CoV-2 compared with rRT-PCR, considering different influencing factors. METHODS: We enrolled a total of 214 symptomatic individuals with known COVID-19 status using rRT-PCR. We collected and tested paired nasopharyngeal (NP) and nasal swab (NS) specimens (collected from same individual) using rRT-PCR and RATs (InTec and SD Biosensor). We assessed the performance of RATs considering specimen types, viral load, the onset of symptoms, and presenting symptoms. RESULTS: We included 214 paired specimens (112 NP and 100 NS SARS-CoV-2 rRT-PCR positive) to the analysis. For NP specimens, the average sensitivity, specificity, and accuracy of the RATs were 87.5%, 98.6%, and 92.8%, respectively, when compared with rRT-PCR. While for NS, the overall kit performance was slightly lower than that of NP (sensitivity 79.0%, specificity 96.1%, and accuracy 88.3%). We observed a progressive decline in the performance of RATs with increased Ct values (decreased viral load). Moreover, the RAT sensitivity using NP specimens decreased over the time of the onset of symptoms. CONCLUSION: The RATs showed strong performance under field conditions and fulfilled the minimum performance limit for rapid antigen detection kits recommended by World Health Organization. The best performance of the RATs can be achieved within the first week of the onset of symptoms with high viral load.

Analytical characterization of the SARS-CoV-2 EURM-017 reference material.

BACKGROUND: Current serological methods for SARS-CoV-2 lack adequate standardization to a universal standard reference material. Standardization will allow comparison of results across various lab-developed and commercial assays and publications. SARS-CoV-2 EURM-017 is human sera reference material containing antibodies directed against SARS-CoV-2 proteins, S1/S2 (full-length spike [S]), S1 receptor-binding domain (S1 RBD), S1, S2, and nucleocapsid (N) protein. The goal of this study was to characterize five antigen-specific serum fractions in EURM-017 for standardization of serology assays. METHODS: Five antigen-specific serum fractions were affinity purified, quantified, and PRNT50 titers compared. Standardization methods were established for two anti-S1 RBD (IgG and Total Ig) and one N protein assay. For the anti-S1 RBD assays, standardization involved determining assay index values for serial dilutions of S1-RBD anti-sera. Index values for the anti-S1 RBD IgG assay and PRNT50 titers were determined for 44 symptomatic COVID-19 patient sera. The index values were converted to EURM-017 ug/mL. RESULTS: Anti-sera protein content was as follows: S1 (17.7 µg/mL), S1 RBD (17.4 µg/mL), S1/S2 (full-length S) (34.1 µg/mL), S2 (29.7 µg/mL), and N protein (72.5 µg/mL). S1 anti-serum had the highest neutralization activity. A standardization method for S1 RBD anti-serum and an anti-S1 RBD IgG assay yielded the linear equation (y = 0.75x-0.10; y = index, x=µg/mL anti-serum). Patient sample index values for the S1-RBD IgG assay correlated well with PRNT50 titers (Pearson r = 0.84). Using the equation above, patient index values were converted to standardized µg/mL. CONCLUSIONS: Standardization of different lab-developed and commercial assays to EURM-017 antigen-specific anti-sera will allow comparison of results across studies globally due to traceability to a single standard reference material.

Analysis of Serological Biomarkers of SARS-CoV-2 Infection in Convalescent Samples From Severe, Moderate and Mild COVID-19 Cases.

Precision monitoring of antibody responses during the COVID-19 pandemic is increasingly important during large scale vaccine rollout and rise in prevalence of Severe Acute Respiratory Syndrome-related Coronavirus-2 (SARS-CoV-2) variants of concern (VOC). Equally important is defining Correlates of Protection (CoP) for SARS-CoV-2 infection and COVID-19 disease. Data from epidemiological studies and vaccine trials identified virus neutralising antibodies (Nab) and SARS-CoV-2 antigen-specific (notably RBD and S) binding antibodies as candidate CoP. In this study, we used the World Health Organisation (WHO) international standard to benchmark neutralising antibody responses and a large panel of binding antibody assays to compare convalescent sera obtained from: a) COVID-19 patients; b) SARS-CoV-2 seropositive healthcare workers (HCW) and c) seronegative HCW. The ultimate aim of this study is to identify biomarkers of humoral immunity that could be used to differentiate severe from mild or asymptomatic SARS-CoV-2 infections. Some of these biomarkers could be used to define CoP in further serological studies using samples from vaccination breakthrough and/or re-infection cases. Whenever suitable, the antibody levels of the samples studied were expressed in International Units (IU) for virus neutralisation assays or in Binding Antibody Units (BAU) for ELISA tests. In this work we used commercial and non-commercial antibody binding assays; a lateral flow test for detection of SARS-CoV-2-specific IgG/IgM; a high throughput multiplexed particle flow cytometry assay for SARS-CoV-2 Spike (S), Nucleocapsid (N) and Receptor Binding Domain (RBD) proteins); a multiplex antigen semi-automated immuno-blotting assay measuring IgM, IgA and IgG; a pseudotyped microneutralisation test (pMN) and an electroporation-dependent neutralisation assay (EDNA). Our results indicate that overall, severe COVID-19 patients showed statistically significantly higher levels of SARS-CoV-2-specific neutralising antibodies (average 1029 IU/ml) than those observed in seropositive HCW with mild or asymptomatic infections (379 IU/ml) and that clinical severity scoring, based on WHO guidelines was tightly correlated with neutralisation and RBD/S antibodies. In addition, there was a positive correlation between severity, N-antibody assays and intracellular virus neutralisation.

Sensitivity and potential utility of SARS-CoV-2 rapid antigen and nucleic acid amplification tests in the context of an elimination approach.

AIM: To assess the sensitivity and potential utility of five RATs and the IDNow, Liat and Oxsed nucleic acid amplification tests (NAATs) in our population. METHOD: 39 retrospective and contrived SARS-CoV-2 positive samples were tested in parallel by standard RT-PCR and RAT. A second group of 44 samples was tested by standard RT-PCR, rapid RT-PCR and two isothermal NAAT assays. Limit of detection was compared at RT-PCR cycle thresholds for all assays. RESULTS: We found that the Cobas Liat RT-PCR had 100% concordance with conventional RT-PCR, whereas the sensitivity of other rapid NAAT assays was less at lower viral loads indicated by Cts >30 (p=0.042) and the RATs at Cts >25 (p<0.001). When applied to New Zealand testing scenarios, IDNow or Oxsed NAAT could miss up to 12% and RATs up to 44.3% of COVID-19 cases compared with the RT-PCR currently used at our laboratory. CONCLUSION: We found that the POC Cobas Liat, a platform that delivers a sample answer in 20 minutes, demonstrated equivalent performance to standard RT-PCR. However, the RATs and isothermal NAAT assays demonstrated reduced sensitivity, limiting their utility in New Zealand's currently very low prevalence setting.

Long-Term Humoral Immune Response against SARS-CoV-2 after Natural Infection and Subsequent Vaccination According to WHO International Binding Antibody Units (BAU/mL).

The new WHO reference standard allows for the definition of serum antibodies against various SARS-CoV-2 antigens in terms of binding antibody units (BAU/mL) and thus to compare the results of different ELISA systems. In this study, the concentration of antibodies (ABs) against both the S- and the N-protein of SARS-CoV-2 as well as serum neutralization activity were evaluated in three patients after a mild course of COVID-19. Serum samples were collected frequently during a period of over one year. Furthermore, in two individuals, the effects of an additional vaccination with a mRNA vaccine containing the S1-RBD sequence on these antibodies were examined. After natural infection, the antibodies (IgA, IgG) against the S1-protein remained elevated above the established cut-off to positivity (S-IgA 60 BAU/mL and S-IgG 50 BAU/mL, respectively) for over a year in all patients, while this was not the case for ABs against the N-protein (cut-off N-IgG 40 BAU/mL, N-IgA 256 BAU/mL). Sera from all patients retained the ability to neutralize SARS-CoV-2 for more than a year. Vaccination resulted in a rapid boost of antibodies to S1-protein but, as expected, not to the N-protein. Most likely, the wide use of the WHO reference preparation will be very useful in determining the individual immune status of patients after an infection with SARS-CoV-2 or after vaccination.

COVID-19 Seropositive Rates between the Waves, Israel.

BACKGROUND: Israel has experienced three waves of coronavirus disease-2019 (COVID-19) infection since late February 2020, with lockdown and other measures employed to contain infection rates. In cooperation with the Israel Ministry of Health, serological testing was conducted by all four health maintenance organizations (HMO) in order to estimate national infection rates and the proportion of previously undetected disease. OBJECTIVES: To estimate the proportion of the population that was seropositive, identify factors associated with seropositive outcome, and approximate the proportion of residents that were asymptomatic. METHODS: Seroconversion rates (IgG) were measured in a representative sample of over 17,000 members of Maccabi Healthcare Services. Direct standardization was used to estimate the seropositive rates for COVID-19 infection for members of the HMO. Rates were adjusted for sensitivity and specificity of the testing products used. In addition to blood sampling, respondents were asked to complete a digital survey regarding potential exposures and symptoms experienced. RESULTS: It was estimated that 1.9% of the adult HMO population was seropositive 4 months after the first infected person was identified in the country. Seroconversion was associated with travel abroad and exposure to infected individuals. Loss of smell and taste, fever, cough, and fatigue are associated with infection. Of those found to be seropositive for COVID-19, 160 (59%) had a prior negative polymerase chain reaction (PCR) or no PCR test at all. CONCLUSIONS: Adult seropositive rates of infection were low relative to other countries. The findings suggest that early initiatives to limit infection entry and spread were effective.

The WHO International Standard for COVID-19 serological tests: towards harmonization of anti-spike assays.

BACKGROUND AND AIMS: SARS-CoV-2 antibody assays are relevant in managing the COVID-19 pandemic, providing valuable data on the immunization status of the population. However, current serology tests are highly variable, due to their different characteristics and to the lack of reference materials. The aim of the World Health Organization (WHO) first International Standard (IS) for anti-SARS-CoV-2 immunoglobulin is to harmonize humoral immune response assessment after natural infection or vaccination, and recommend reporting the results for binding activity in Binding Antibody Units (BAU). MATERIALS AND METHODS: This study analyzed six commercial quantitative anti-SARS-CoV-2 S-protein assays in a head-to-head comparison, using the manufacturers' conversion factors for the WHO IS to obtain BAU/mL values. RESULTS: Our data showed good alignment up to 1000 BAU/mL, then began to disperse, exhibiting some discrepancies. Moreover, correlations among methods varied with Cohen's Kappa ranging from 0.580 to 1.00, with the lowest agreement values for kits using different target antigens or different antibody isotypes, making it clear that the laboratory report should include this information. Values expressed as BAU/ml showed a reduced between-assays variability compared to AU/ml (median coefficients of variation 0.38 and 0.68, respectively; p < 0.001). CONCLUSION: On the basis of these data at present anti-SARS CoV-2 serological assays' results are not interchangeable, and, more importantly, individual immune monitoring should be performed with the same method.