Longitudinal analysis of clinical serology assay performance and neutralising antibody levels in COVID19 convalescents

To be remembered: B cell memory response against SARS-CoV-2 and its variants in vaccinated and unvaccinated individuals

COVID-19 disease has plagued the world economy and affected the overall well-being and life of most of the people. Natural infection as well as vaccination leads to the development of an immune response against the pathogen. This involves the production of antibodies, which can neutralize the virus during future challenges. In addition, the development of cellular immune memory with memory B and T cells provides long-lasting protection. The longevity of the immune response has been a subject of intensive research in this field. The extent of immunity conferred by different forms of vaccination or natural infections remained debatable for long. Hence, understanding the effectiveness of these responses among different groups of people can assist government organizations in making informed policy decisions. In this article, based on the publicly available data, we have reviewed the memory response generated by some of the vaccines against SARS-CoV-2 and its variants, particularly B cell memory in different groups of individuals.

COVID-19 Infection, Vaccination, and Antibody Levels: Investigating Correlations through a Cohort Study

AIM

The objective of this study was to explore the potential correlation between COVID-19 infection or vaccination and levels of anti-nucleocapsid (anti-N) and anti-spike (anti-S) antibodies.

METHODS

Among 6050 healthcare workers at the Ege University Hospital, a cohort study with 162 participants divided into three arms with 54 participants each was conducted. The three groups were selected as follows: those diagnosed with COVID-19 and not vaccinated (group 1), those diagnosed with COVID-19 and subsequently vaccinated with CoronaVac (group 2), and those not diagnosed with COVID-19 but vaccinated with two doses of CoronaVac (group 3). Antibody levels measured at the sixth month of follow-up were defined as the primary outcome.

RESULTS

At the sixth month, all serum samples tested positive for anti-S. Anti-S levels were found to be significantly higher in group 2 than in the other groups (p < 0.001). There were no differences in antibody levels between groups 1 and 3 (p = 0.080). Average antibody levels were found to be lower in office workers and males. Anti-N antibodies were found to be positive in 85.1% of subjects at the sixth month. In group 2, anti-N antibodies were detected in all samples at the sixth month. Anti-N antibody levels were not significantly different between groups 1 and 2 (p = 0.165). Groups 1 and 2 had significantly higher antibody levels than group 3 (p < 0.001).

CONCLUSIONS

Vaccination or infection provide protection for at least 6 months. Those who have previously been diagnosed with COVID-19 do not need to be vaccinated in the early period before their antibody levels decrease.

Antibody tests for identification of current and past infection with SARS-CoV-2

BACKGROUND

The diagnostic challenges associated with the COVID-19 pandemic resulted in rapid development of diagnostic test methods for detecting SARS-CoV-2 infection. Serology tests to detect the presence of antibodies to SARS-CoV-2 enable detection of past infection and may detect cases of SARS-CoV-2 infection that were missed by earlier diagnostic tests. Understanding the diagnostic accuracy of serology tests for SARS-CoV-2 infection may enable development of effective diagnostic and management pathways, inform public health management decisions and understanding of SARS-CoV-2 epidemiology.

OBJECTIVES

To assess the accuracy of antibody tests, firstly, to determine if a person presenting in the community, or in primary or secondary care has current SARS-CoV-2 infection according to time after onset of infection and, secondly, to determine if a person has previously been infected with SARS-CoV-2. Sources of heterogeneity investigated included: timing of test, test method, SARS-CoV-2 antigen used, test brand, and reference standard for non-SARS-CoV-2 cases.

SEARCH METHODS

The COVID-19 Open Access Project living evidence database from the University of Bern (which includes daily updates from PubMed and Embase and preprints from medRxiv and bioRxiv) was searched on 30 September 2020. We included additional publications from the Evidence for Policy and Practice Information and Co-ordinating Centre (EPPI-Centre) 'COVID-19: Living map of the evidence' and the Norwegian Institute of Public Health 'NIPH systematic and living map on COVID-19 evidence'. We did not apply language restrictions.

SELECTION CRITERIA

We included test accuracy studies of any design that evaluated commercially produced serology tests, targeting IgG, IgM, IgA alone, or in combination. Studies must have provided data for sensitivity, that could be allocated to a predefined time period after onset of symptoms, or after a positive RT-PCR test. Small studies with fewer than 25 SARS-CoV-2 infection cases were excluded. We included any reference standard to define the presence or absence of SARS-CoV-2 (including reverse transcription polymerase chain reaction tests (RT-PCR), clinical diagnostic criteria, and pre-pandemic samples).

DATA COLLECTION AND ANALYSIS

We use standard screening procedures with three reviewers. Quality assessment (using the QUADAS-2 tool) and numeric study results were extracted independently by two people. Other study characteristics were extracted by one reviewer and checked by a second. We present sensitivity and specificity with 95% confidence intervals (CIs) for each test and, for meta-analysis, we fitted univariate random-effects logistic regression models for sensitivity by eligible time period and for specificity by reference standard group. Heterogeneity was investigated by including indicator variables in the random-effects logistic regression models. We tabulated results by test manufacturer and summarised results for tests that were evaluated in 200 or more samples and that met a modification of UK Medicines and Healthcare products Regulatory Agency (MHRA) target performance criteria.

MAIN RESULTS

We included 178 separate studies (described in 177 study reports, with 45 as pre-prints) providing 527 test evaluations. The studies included 64,688 samples including 25,724 from people with confirmed SARS-CoV-2; most compared the accuracy of two or more assays (102/178, 57%). Participants with confirmed SARS-CoV-2 infection were most commonly hospital inpatients (78/178, 44%), and pre-pandemic samples were used by 45% (81/178) to estimate specificity. Over two-thirds of studies recruited participants based on known SARS-CoV-2 infection status (123/178, 69%). All studies were conducted prior to the introduction of SARS-CoV-2 vaccines and present data for naturally acquired antibody responses. Seventy-nine percent (141/178) of studies reported sensitivity by week after symptom onset and 66% (117/178) for convalescent phase infection. Studies evaluated enzyme-linked immunosorbent assays (ELISA) (165/527; 31%), chemiluminescent assays (CLIA) (167/527; 32%) or lateral flow assays (LFA) (188/527; 36%). Risk of bias was high because of participant selection (172, 97%); application and interpretation of the index test (35, 20%); weaknesses in the reference standard (38, 21%); and issues related to participant flow and timing (148, 82%). We judged that there were high concerns about the applicability of the evidence related to participants in 170 (96%) studies, and about the applicability of the reference standard in 162 (91%) studies. Average sensitivities for current SARS-CoV-2 infection increased by week after onset for all target antibodies. Average sensitivity for the combination of either IgG or IgM was 41.1% in week one (95% CI 38.1 to 44.2; 103 evaluations; 3881 samples, 1593 cases), 74.9% in week two (95% CI 72.4 to 77.3; 96 evaluations, 3948 samples, 2904 cases) and 88.0% by week three after onset of symptoms (95% CI 86.3 to 89.5; 103 evaluations, 2929 samples, 2571 cases). Average sensitivity during the convalescent phase of infection (up to a maximum of 100 days since onset of symptoms, where reported) was 89.8% for IgG (95% CI 88.5 to 90.9; 253 evaluations, 16,846 samples, 14,183 cases), 92.9% for IgG or IgM combined (95% CI 91.0 to 94.4; 108 evaluations, 3571 samples, 3206 cases) and 94.3% for total antibodies (95% CI 92.8 to 95.5; 58 evaluations, 7063 samples, 6652 cases). Average sensitivities for IgM alone followed a similar pattern but were of a lower test accuracy in every time slot. Average specificities were consistently high and precise, particularly for pre-pandemic samples which provide the least biased estimates of specificity (ranging from 98.6% for IgM to 99.8% for total antibodies). Subgroup analyses suggested small differences in sensitivity and specificity by test technology however heterogeneity in study results, timing of sample collection, and smaller sample numbers in some groups made comparisons difficult. For IgG, CLIAs were the most sensitive (convalescent-phase infection) and specific (pre-pandemic samples) compared to both ELISAs and LFAs (P < 0.001 for differences across test methods). The antigen(s) used (whether from the Spike-protein or nucleocapsid) appeared to have some effect on average sensitivity in the first weeks after onset but there was no clear evidence of an effect during convalescent-phase infection. Investigations of test performance by brand showed considerable variation in sensitivity between tests, and in results between studies evaluating the same test. For tests that were evaluated in 200 or more samples, the lower bound of the 95% CI for sensitivity was 90% or more for only a small number of tests (IgG, n = 5; IgG or IgM, n = 1; total antibodies, n = 4). More test brands met the MHRA minimum criteria for specificity of 98% or above (IgG, n = 16; IgG or IgM, n = 5; total antibodies, n = 7). Seven assays met the specified criteria for both sensitivity and specificity. In a low-prevalence (2%) setting, where antibody testing is used to diagnose COVID-19 in people with symptoms but who have had a negative PCR test, we would anticipate that 1 (1 to 2) case would be missed and 8 (5 to 15) would be falsely positive in 1000 people undergoing IgG or IgM testing in week three after onset of SARS-CoV-2 infection. In a seroprevalence survey, where prevalence of prior infection is 50%, we would anticipate that 51 (46 to 58) cases would be missed and 6 (5 to 7) would be falsely positive in 1000 people having IgG tests during the convalescent phase (21 to 100 days post-symptom onset or post-positive PCR) of SARS-CoV-2 infection.

AUTHORS' CONCLUSIONS

Some antibody tests could be a useful diagnostic tool for those in whom molecular- or antigen-based tests have failed to detect the SARS-CoV-2 virus, including in those with ongoing symptoms of acute infection (from week three onwards) or those presenting with post-acute sequelae of COVID-19. However, antibody tests have an increasing likelihood of detecting an immune response to infection as time since onset of infection progresses and have demonstrated adequate performance for detection of prior infection for sero-epidemiological purposes. The applicability of results for detection of vaccination-induced antibodies is uncertain.

Nationally representative results on SARS-CoV-2 seroprevalence and testing in Germany at the end of 2020

Pre-vaccine SARS-CoV-2 seroprevalence data from Germany are scarce outside hotspots, and socioeconomic disparities remained largely unexplored. The nationwide representative RKI-SOEP study (15,122 participants, 18-99 years, 54% women) investigated seroprevalence and testing in a supplementary wave of the Socio-Economic-Panel conducted predominantly in October-November 2020. Self-collected oral-nasal swabs were PCR-positive in 0.4% and Euroimmun anti-SARS-CoV-2-S1-IgG ELISA from dry-capillary-blood antibody-positive in 1.3% (95% CI 0.9-1.7%, population-weighted, corrected for sensitivity = 0.811, specificity = 0.997). Seroprevalence was 1.7% (95% CI 1.2-2.3%) when additionally correcting for antibody decay. Overall infection prevalence including self-reports was 2.1%. We estimate 45% (95% CI 21-60%) undetected cases and lower detection in socioeconomically deprived districts. Prior SARS-CoV-2 testing was reported by 18% from the lower educational group vs. 25% and 26% from the medium and high educational group (p < 0.001, global test over three categories). Symptom-triggered test frequency was similar across educational groups. Routine testing was more common in low-educated adults, whereas travel-related testing and testing after contact with infected persons was more common in highly educated groups. This countrywide very low pre-vaccine seroprevalence in Germany at the end of 2020 can serve to evaluate the containment strategy. Our findings on social disparities indicate improvement potential in pandemic planning for people in socially disadvantaged circumstances.

Indicators of recent COVID-19 infection status: findings from a large occupational cohort of staff and postgraduate research students from a UK university

BACKGROUND

Researchers conducting cohort studies may wish to investigate the effect of episodes of COVID-19 illness on participants. A definitive diagnosis of COVID-19 is not always available, so studies have to rely on proxy indicators. This paper seeks to contribute evidence that may assist the use and interpretation of these COVID-indicators.

METHODS

We described five potential COVID-indicators: self-reported core symptoms, a symptom algorithm; self-reported suspicion of COVID-19; self-reported external results; and home antibody testing based on a 'lateral flow' antibody (IgG/IgM) test cassette. Included were staff and postgraduate research students at a large London university who volunteered for the study and were living in the UK in June 2020. Excluded were those who did not return a valid antibody test result. We provide descriptive statistics of prevalence and overlap of the five indicators.

RESULTS

Core symptoms were the most common COVID-indicator (770/1882 participants positive, 41%), followed by suspicion of COVID-19 (n = 509/1882, 27%), a positive symptom algorithm (n = 298/1882, 16%), study antibody lateral flow positive (n = 124/1882, 7%) and a positive external test result (n = 39/1882, 2%), thus a 20-fold difference between least and most common. Meeting any one indicator increased the likelihood of all others, with concordance between 65 and 94%. Report of a low suspicion of having had COVID-19 predicted a negative antibody test in 98%, but positive suspicion predicted a positive antibody test in only 20%. Those who reported previous external antibody tests were more likely to have received a positive result from the external test (24%) than the study test (15%).

CONCLUSIONS

Our results support the use of proxy indicators of past COVID-19, with the caveat that none is perfect. Differences from previous antibody studies, most significantly in lower proportions of participants positive for antibodies, may be partly due to a decline in antibody detection over time. Subsequent to our study, vaccination may have further complicated the interpretation of COVID-indicators, only strengthening the need to critically evaluate what criteria should be used to define COVID-19 cases when designing studies and interpreting study results.

SARS-CoV-2 anti-nucleocapsid assay performance in healthcare workers at baseline and 6 months

INTRODUCTION

Serological SARS-CoV-2 assays have an important role in guiding the pandemic response. This research aimed to compare the performance of 2 antinucleocapsid assays.

METHODS

Serum from 49 HCWs was analysed at baseline and 6 months using the Abbott diagnostics SARS-CoV-2 IgG assay and the Roche Diagnostics Elecsys Anti-SARS-CoV-2 total antibody assay.

RESULTS

At baseline, 14/49 participants (29%) demonstrated antibody reactivity using the Abbott assay. At 6 months, 4/14 participants (29%) continued to demonstrate reactivity. A total of 14/49 (29%) participants had detectable antibodies at baseline using the Roche assay. In total, 13/14 (93%) of participants demonstrated antibody reactivity at 6 months. The Abbott assay showed a statistically significant difference in the signal-to-threshold values of baseline reactive samples when repeated at 6 months (p = 0.001). This was not seen with the Roche assay (p = 0.51).

CONCLUSION

In this small study, the Roche Diagnostics Elecsys Anti-SARS-CoV-2 total antibody assay appears superior in performance to the Abbott diagnostics SARS-CoV-2 IgG assay in accurately detecting participants with a history of confirmed COVID-19 disease at 6 months follow-up. This finding should be born in mind in the planning of future seroprevalence studies, especially when considering the use of anti-nucleocapsid assays.

Features of the humoral response to immunization "Gam-COVID-Vac" and in patients with COVID-19

The paper present the results of a survey of people who have undergone immunization with a combined vector vaccine for the prevention of coronavirus infection COVID-19 «Sputnik V - Gam-COVID-Vac», as well as COVID-19 recovalents. Using a quantitative enzyme-linked immunosorbent assay, the levels of specific IgG were determined in persons who had had different degrees of severity before vaccination, in persons who were immuno-negative before immunization, as well as in convalescents who had undergone coronavirus infection of varying severity. The immunological targeting of antibodies against various SARS-CoV-2 proteins is considered.

[Seroprevalence of SARS-CoV-2 among children and adolescents in Germany-an overview]

Hintergrund

SARS-CoV-2-Antikörperstudien ergänzen und erweitern die Erkenntnisse aus der Meldestatistik laborbestätigter COVID-19-Fälle um Informationen zu unentdeckten Fällen.

Ziel der Arbeit

Der vorliegende Beitrag fasst bisherige Ergebnisse zur SARS-CoV-2-Prävalenz aus seroepidemiologischen Studien in Deutschland zusammen, die sich auf Kinder und Jugendliche konzentrieren, und ergänzt die bereits vorliegende Übersicht zur Seroprävalenz bei Erwachsenen und speziell bei Blutspendenden in Deutschland.

Material und Methoden

Die Ergebnisse der Übersichtsarbeit beruhen auf einer fortlaufenden systematischen Recherche in Studienregistern, Literaturdatenbanken, von Preprint-Veröffentlichungen und Medienberichten seroepidemiologischer Studien in Deutschland sowie deren Ergebnissen.

Ergebnisse

Mit Stand 17.09.2021 sind uns 16 deutsche seroepidemiologische Studien, die sich auf Kinder und Jugendliche konzentrieren, bekannt geworden. Für 9 dieser Studien liegen Ergebnisse vor. Für fast alle untersuchten Settings lag die SARS-CoV-2-Seroprävalenz für Kinder im Kita- und Grundschulalter in der ersten COVID-19-Welle deutlich unter 1 % und für Jugendliche unter 2 %. Im Verlauf der Pandemie wurden höhere Seroprävalenzen von bis zu 8 % für Kinder im Grundschulalter ermittelt.

Diskussion

Ergebnisse von SARS-CoV-2-Antikörperstudien bei Kindern und Jugendlichen in Deutschland liegen bislang erst in geringem Umfang und basierend auf lokal-regionalen, nichtrepräsentativen Stichproben vor. In künftigen Studien gilt es, einerseits abzuschätzen, welcher Anteil der Kinder und Jugendlichen bereits eine Infektion hatte oder geimpft ist. Zum anderen gilt es, die Verbreitung körperlicher und psychischer Beeinträchtigungen im Nachgang einer Infektion zu untersuchen.

From Infection to Immunity: Understanding the Response to SARS-CoV2 Through In-Silico Modeling

Background

Immune system conditions of the patient is a key factor in COVID-19 infection survival. A growing number of studies have focused on immunological determinants to develop better biomarkers for therapies.

Aim

Studies of the insurgence of immunity is at the core of both SARS-CoV-2 vaccine development and therapies. This paper attempts to describe the insurgence (and the span) of immunity in COVID-19 at the population level by developing an in-silico model. We simulate the immune response to SARS-CoV-2 and analyze the impact of infecting viral load, affinity to the ACE2 receptor, and age in an artificially infected population on the course of the disease.

Methods

We use a stochastic agent-based immune simulation platform to construct a virtual cohort of infected individuals with age-dependent varying degrees of immune competence. We use a parameter set to reproduce known inter-patient variability and general epidemiological statistics.

Results

By assuming the viremia at day 30 of the infection to be the proxy for lethality, we reproduce in-silico several clinical observations and identify critical factors in the statistical evolution of the infection. In particular, we evidence the importance of the humoral response over the cytotoxic response and find that the antibody titers measured after day 25 from the infection are a prognostic factor for determining the clinical outcome of the infection. Our modeling framework uses COVID-19 infection to demonstrate the actionable effectiveness of modeling the immune response at individual and population levels. The model developed can explain and interpret observed patterns of infection and makes verifiable temporal predictions. Within the limitations imposed by the simulated environment, this work proposes quantitatively that the great variability observed in the patient outcomes in real life can be the mere result of subtle variability in the infecting viral load and immune competence in the population. In this work, we exemplify how computational modeling of immune response provides an important view to discuss hypothesis and design new experiments, in particular paving the way to further investigations about the duration of vaccine-elicited immunity especially in the view of the blundering effect of immunosenescence.

SARS-CoV-2 Serologic Assays Dependent on Dual-Antigen Binding Demonstrate Diverging Kinetics Relative to Other Antibody Detection Methods

Longitudinal studies assessing durability of the anti-severe acute respiratory syndrome coronavirus 2 (anti-SARS-CoV-2) humoral immune response have generated conflicting results. This has been proposed to be due to differences in patient populations, the lack of standardized methodologies, and the use of assays that measure distinct aspects of the humoral response. SARS-CoV-2 antibodies were serially measured in sera from a cohort of 44 well-characterized convalescent plasma donors over 120 days post-COVID-19 symptom onset, utilizing eight assays, which varied according to antigen source, the detected antibody isotype, and the activity measured (i.e., binding, blocking, or neutralizing). While the majority of assays demonstrated a gradual decline in antibody titers over the course of 120 days, the two electrochemiluminescence immunoassay Roche assays (Roche Diagnostics Elecsys anti-SARS-CoV-2 [qualitative, nucleocapsid based] and Roche Diagnostics Elecsys anti-SARS-CoV-2 S [semiquantitative, spike based]), which utilize dual-antigen binding for antibody detection, demonstrated stable and/or increasing antibody titers over the study period. This study is among the first to assess longitudinal, rather than cross-sectional, SARS-CoV-2 antibody profiles among convalescent COVID-19 patients, primarily using commercially available serologic assays with Food and Drug Administration emergency use authorization. We show that SARS-CoV-2 antibody detection is dependent on the serologic method used, which has implications for future assay utilization and clinical value.

The Duration, Dynamics, and Determinants of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Antibody Responses in Individual Healthcare Workers

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immunoglobulin G (IgG) antibody measurements can be used to estimate the proportion of a population exposed or infected and may be informative about the risk of future infection. Previous estimates of the duration of antibody responses vary.

METHODS

We present 6 months of data from a longitudinal seroprevalence study of 3276 UK healthcare workers (HCWs). Serial measurements of SARS-CoV-2 anti-nucleocapsid and anti-spike IgG were obtained. Interval censored survival analysis was used to investigate the duration of detectable responses. Additionally, Bayesian mixed linear models were used to investigate anti-nucleocapsid waning.

RESULTS

Anti-spike IgG levels remained stably detected after a positive result, for example, in 94% (95% credibility interval [CrI] 91-96%) of HCWs at 180 days. Anti-nucleocapsid IgG levels rose to a peak at 24 (95% CrI 19-31) days post first polymerase chain reaction (PCR)-positive test, before beginning to fall. Considering 452 anti-nucleocapsid seropositive HCWs over a median of 121 days from their maximum positive IgG titer, the mean estimated antibody half-life was 85 (95% CrI 81-90) days. Higher maximum observed anti-nucleocapsid titers were associated with longer estimated antibody half-lives. Increasing age, Asian ethnicity, and prior self-reported symptoms were independently associated with higher maximum anti-nucleocapsid levels and increasing age and a positive PCR test undertaken for symptoms with longer anti-nucleocapsid half-lives.

CONCLUSIONS

SARS-CoV-2 anti-nucleocapsid antibodies wane within months and fall faster in younger adults and those without symptoms. However, anti-spike IgG remains stably detected. Ongoing longitudinal studies are required to track the long-term duration of antibody levels and their association with immunity to SARS-CoV-2 reinfection.

Neutralizing Antibodies in COVID-19 Patients and Vaccine Recipients after Two Doses of BNT162b2

The evaluation of the neutralizing capacity of anti-SARS-CoV-2 antibodies is important because they represent real protective immunity. In this study we aimed to measure and compare the neutralizing antibodies (NAbs) in COVID-19 patients and in vaccinated individuals. One-hundred and fifty long-term samples from 75 COVID-19 patients were analyzed with a surrogate virus neutralization test (sVNT) and compared to six different SARS-CoV-2 serology assays. The agreement between the sVNT and pseudovirus VNT (pVNT) results was found to be excellent (i.e., 97.2%). The NAb response was also assessed in 90 individuals who had received the complete dose regimen of BNT162b2. In COVID-19 patients, a stronger response was observed in moderate–severe versus mild patients (p-value = 0.0006). A slow decay in NAbs was noted in samples for up to 300 days after diagnosis, especially in moderate–severe patients (r = −0.35, p-value = 0.03). In the vaccinated population, 83.3% of COVID-19-naive individuals had positive NAbs 14 days after the first dose and all were positive 7 days after the second dose, i.e., at day 28. In previously infected individuals, all were already positive for NAbs at day 14. At each time point, a stronger response was observed for previously infected individuals (p-value < 0.05). The NAb response remained stable for up to 56 days in all participants. Vaccinated participants had significantly higher NAb titers compared to COVID patients. In previously infected vaccine recipients, one dose might be sufficient to generate sufficient neutralizing antibodies.

Longitudinal variation in SARS-CoV-2 antibody levels and emergence of viral variants: implications for the ability of serological assays to predict immunity

BACKGROUND

Serological assays are being deployed to monitor antibody responses in SARS-CoV-2 convalescents and vaccine recipients. There is a need to determine whether such assays can predict immunity, as antibody levels wane and viral variants emerge.

METHODS

We measured antibodies in a cohort of SARS-CoV-2 infected patients using several high-throughput serological tests and functional neutralization assays. The effects of time and spike protein sequence variation on the performance and predictive value of the various assays was assessed.

FINDINGS

Neutralizing antibody titers decreased over the first few months post-infection but stabilized thereafter, at about 30% of the level observed shortly after infection. Serological assays commonly used to measure antibodies against SARS-CoV-2 displayed a range of sensitivities that declined to varying extents over time. Quantitative measurements generated by serological assays based on the spike protein were better at predicting neutralizing antibody titers than assays based on nucleocapsid, but performance was variable and manufacturer positivity thresholds were not able to predict the presence or absence of detectable neutralizing activity. Even though there was some deterioration in correlation between serological measurements and functional neutralization activity, some assays maintained an ability to predict neutralizing titers, even against variants of concern.

INTERPRETATION

The ability of high throughput serological assays to predict neutralizing antibody titers is likely crucial for evaluation of immunity at the population scale. These data will facilitate the selection of the most suitable assays as surrogates of functional neutralizing activity and suggest that such measurements may have utility in clinical practice.

A SARS-CoV-2 Label-Free Surrogate Virus Neutralization Test and a Longitudinal Study of Antibody Characteristics in COVID-19 Patients

Methods designed to measure severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) humoral response include virus neutralization tests to determine antibody neutralization activity. For ease of use and universal applicability, surrogate virus neutralization tests (sVNTs) based on antibody-mediated blockage of molecular interactions have been proposed. A surrogate virus neutralization test was established on a label-free immunoassay platform (LF-sVNT). The LF-sVNT analyzes the binding ability of SARS-CoV-2 spike protein receptor-binding domain (RBD) to angiotensin-converting enzyme 2 (ACE2) after neutralizing RBD with antibodies in serum. The LF-sVNT neutralizing antibody titers (50% inhibitory concentration [IC50]) were determined from serum samples (n = 246) from coronavirus disease 2019 (COVID-19) patients (n = 113), as well as the IgG concentrations and the IgG avidity indices. Although there was variability in the kinetics of the IgG concentrations and neutralizing antibody titers between individuals, there was an initial rise, plateau, and then in some cases a gradual decline at later time points after 40 days after symptom onset. The IgG avidity indices, in the same cases, plateaued after an initial rise and did not show a decline. The LF-sVNT can be a valuable tool in research and clinical laboratories for the assessment of the presence of neutralizing antibodies to COVID-19. This study is the first to provide longitudinal neutralizing antibody titers beyond 200 days post-symptom onset. Despite the decline of IgG concentration and neutralizing antibody titer, IgG avidity index increases, reaches a plateau, and then remains constant up to 8 months postinfection. The decline of antibody neutralization activity can be attributed to the reduction in antibody quantity rather than the deterioration of antibody quality, as measured by antibody avidity.

The protective effect of SARS-CoV-2 antibodies in Scottish healthcare workers

Background

Healthcare workers (HCWs) are believed to be at increased risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. It is not known to what extent the natural production of antibodies to SARS-CoV-2 is protective against re-infection.

Methods

A prospective observational study of HCWs in Scotland (UK) from May to September 2020 was performed. The Siemens SARS-CoV-2 total antibody assay was used to establish seroprevalence in this cohort. Controls, matched for age and sex to the general local population, were studied for comparison. New infections (up to 2 December 2020) post antibody testing were recorded to determine whether the presence of SARS-CoV-2 antibodies protects against re-infection.

Results

A total of 2063 health and social care workers were recruited for this study. At enrolment, 300 HCWs had a positive antibody test (14.5%). 11 out of 231 control sera tested positive (4.8%). HCWs therefore had an increased likelihood of a positive test (OR 3.4, 95% CI 1.85–6.16; p<0.0001). Dentists were most likely to test positive. 97.3% of patients who had previously tested positive for SARS-CoV-2 by reverse transcriptase (RT)-PCR had positive antibodies. 18.7% had an asymptomatic infection. There were 38 new infections with SARS-CoV-2 in HCWs who were previously antibody negative, and one symptomatic RT-PCR-positive re-infection. The presence of antibodies was therefore associated with an 85% reduced risk of re-infection with SARS-CoV-2 (hazard ratio 0.15, 95% CI 0.06–0.35; p=0.026).

Conclusion

HCWs were three times more likely to test positive for SARS-CoV-2 than the general population. Almost all infected individuals developed an antibody response, which was 85% effective in protecting against re-infection with SARS-CoV-2.

In this study, healthcare workers were three times more likely to test positive for #SARSCoV2 than the general population. Almost all infected individuals developed an antibody response, and this was 85% effective in protecting against re-infection.https://bit.ly/3mLPUmk

Seroprevalence of SARS-CoV-2 IgG antibodies and risk factors in health care workers at an academic medical center in Boston, Massachusetts

Healthcare workers (HCWs) are at an increased risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel virus that causes Coronavirus Disease (COVID-19). We aim to assess the seroprevalence of SARS-CoV-2 IgG among healthcare workers and compare risk-factors between seropositive and seronegative HCWs. In this observational study, serum samples were collected from HCWs between July 13th to 26th, 2020 at Boston Medical Center (BMC). Samples were subsequently tested for SARS-CoV-2 IgG antibody using the Abbott SARS-CoV-2 IgG assay. Participants also answered a questionnaire capturing data on demographics, history of COVID-19 symptoms, occupation, infection prevention and control measures. Overall, 95 of 1743 (5.5%) participants tested positive for SARS-CoV-2 IgG. Of these, 1.8% of the participants had mild or no COVID-19 symptoms and did not require a diagnostic test. Seropositivity was not associated with gender, occupation, hand hygiene and personal protective equipment (PPE) practices amongst HCWs. However, lack of physical distancing among health care workers in work areas and break room was associated with seropositivity (p = 0.05, p = 0.003, respectively). The majority of the HCWs are negative for SARS-CoV-2 IgG. This data highlights the need to promote infection prevention measures, and the importance of distance amongst co-workers to help mitigate infection rates.

SARS-CoV-2 infection rates of antibody-positive compared with antibody-negative health-care workers in England: a large, multicentre, prospective cohort study (SIREN)

BACKGROUND

Increased understanding of whether individuals who have recovered from COVID-19 are protected from future SARS-CoV-2 infection is an urgent requirement. We aimed to investigate whether antibodies against SARS-CoV-2 were associated with a decreased risk of symptomatic and asymptomatic reinfection.

METHODS

A large, multicentre, prospective cohort study was done, with participants recruited from publicly funded hospitals in all regions of England. All health-care workers, support staff, and administrative staff working at hospitals who could remain engaged in follow-up for 12 months were eligible to join The SARS-CoV-2 Immunity and Reinfection Evaluation study. Participants were excluded if they had no PCR tests after enrolment, enrolled after Dec 31, 2020, or had insufficient PCR and antibody data for cohort assignment. Participants attended regular SARS-CoV-2 PCR and antibody testing (every 2-4 weeks) and completed questionnaires every 2 weeks on symptoms and exposures. At enrolment, participants were assigned to either the positive cohort (antibody positive, or previous positive PCR or antibody test) or negative cohort (antibody negative, no previous positive PCR or antibody test). The primary outcome was a reinfection in the positive cohort or a primary infection in the negative cohort, determined by PCR tests. Potential reinfections were clinically reviewed and classified according to case definitions (confirmed, probable, or possible) and symptom-status, depending on the hierarchy of evidence. Primary infections in the negative cohort were defined as a first positive PCR test and seroconversions were excluded when not associated with a positive PCR test. A proportional hazards frailty model using a Poisson distribution was used to estimate incidence rate ratios (IRR) to compare infection rates in the two cohorts.

FINDINGS

From June 18, 2020, to Dec 31, 2020, 30 625 participants were enrolled into the study. 51 participants withdrew from the study, 4913 were excluded, and 25 661 participants (with linked data on antibody and PCR testing) were included in the analysis. Data were extracted from all sources on Feb 5, 2021, and include data up to and including Jan 11, 2021. 155 infections were detected in the baseline positive cohort of 8278 participants, collectively contributing 2 047 113 person-days of follow-up. This compares with 1704 new PCR positive infections in the negative cohort of 17 383 participants, contributing 2 971 436 person-days of follow-up. The incidence density was 7·6 reinfections per 100 000 person-days in the positive cohort, compared with 57·3 primary infections per 100 000 person-days in the negative cohort, between June, 2020, and January, 2021. The adjusted IRR was 0·159 for all reinfections (95% CI 0·13-0·19) compared with PCR-confirmed primary infections. The median interval between primary infection and reinfection was more than 200 days.

INTERPRETATION

A previous history of SARS-CoV-2 infection was associated with an 84% lower risk of infection, with median protective effect observed 7 months following primary infection. This time period is the minimum probable effect because seroconversions were not included. This study shows that previous infection with SARS-CoV-2 induces effective immunity to future infections in most individuals.

FUNDING

Department of Health and Social Care of the UK Government, Public Health England, The National Institute for Health Research, with contributions from the Scottish, Welsh and Northern Irish governments.

Characteristics of Anti-SARS-CoV-2 Antibodies in Recovered COVID-19 Subjects

Coronavirus Disease 2019 (COVID-19) is a global pandemic caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). While detection of SARS-CoV-2 by polymerase chain reaction with reverse transcription (RT-PCR) is currently used to diagnose acute COVID-19 infection, serological assays are needed to study the humoral immune response to SARS-CoV-2. Anti-SARS-CoV-2 immunoglobulin (Ig)G/A/M antibodies against spike (S) protein and its receptor-binding domain (RBD) were characterized in recovered subjects who were RT-PCR-positive (n = 153) and RT-PCR-negative (n = 55) using an enzyme-linked immunosorbent assay (ELISA). These antibodies were also further assessed for their ability to neutralize live SARS-CoV-2 virus. Anti-SARS-CoV-2 antibodies were detected in 90.9% of resolved subjects up to 180 days post-symptom onset. Anti-S protein and anti-RBD IgG titers correlated (r = 0.5157 and r = 0.6010, respectively) with viral neutralization. Of the RT-PCR-positive subjects, 22 (14.3%) did not have anti-SARS-CoV-2 antibodies; and of those, 17 had RT-PCR cycle threshold (Ct) values > 27. These high Ct values raise the possibility that these indeterminate results are from individuals who were not infected or had mild infection that failed to elicit an antibody response. This study highlights the importance of serological surveys to determine population-level immunity based on infection numbers as determined by RT-PCR.

SARS-CoV-2 antibody magnitude and detectability are driven by disease severity, timing, and assay

Serosurveillance studies are critical for estimating SARS-CoV-2 transmission and immunity, but interpretation of results is currently limited by poorly defined variability in the performance of antibody assays to detect seroreactivity over time in individuals with different clinical presentations. We measured longitudinal antibody responses to SARS-CoV-2 in plasma samples from a diverse cohort of 128 individuals over 160 days using 14 binding and neutralization assays. For all assays, we found a consistent and strong effect of disease severity on antibody magnitude, with fever, cough, hospitalization, and oxygen requirement explaining much of this variation. We found that binding assays measuring responses to spike protein had consistently higher correlation with neutralization than those measuring responses to nucleocapsid, regardless of assay format and sample timing. However, assays varied substantially with respect to sensitivity during early convalescence and in time to seroreversion. Variations in sensitivity and durability were particularly dramatic for individuals with mild infection, who had consistently lower antibody titers and represent the majority of the infected population, with sensitivities often differing substantially from reported test characteristics (e.g., amongst commercial assays, sensitivity at 6 months ranged from 33% for ARCHITECT IgG to 98% for VITROS Total Ig). Thus, the ability to detect previous infection by SARS-CoV-2 is highly dependent on the severity of the initial infection, timing relative to infection, and the assay used. These findings have important implications for the design and interpretation of SARS-CoV-2 serosurveillance studies.

Evolution of antibody immunity to SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected 78 million individuals and is responsible for over 1.7 million deaths to date. Infection is associated with the development of variable levels of antibodies with neutralizing activity, which can protect against infection in animal models^1,2. Antibody levels decrease with time, but, to our knowledge, the nature and quality of the memory B cells that would be required to produce antibodies upon reinfection has not been examined. Here we report on the humoral memory response in a cohort of 87 individuals assessed at 1.3 and 6.2 months after infection with SARS-CoV-2. We find that titres of IgM and IgG antibodies against the receptor-binding domain (RBD) of the spike protein of SARS-CoV-2 decrease significantly over this time period, with IgA being less affected. Concurrently, neutralizing activity in plasma decreases by fivefold in pseudotype virus assays. By contrast, the number of RBD-specific memory B cells remains unchanged at 6.2 months after infection. Memory B cells display clonal turnover after 6.2 months, and the antibodies that they express have greater somatic hypermutation, resistance to RBD mutations and increased potency, indicative of continued evolution of the humoral response. Immunofluorescence and PCR analyses of intestinal biopsies obtained from asymptomatic individuals at 4 months after the onset of coronavirus disease 2019 (COVID-19) revealed the persistence of SARS-CoV-2 nucleic acids and immunoreactivity in the small bowel of 7 out of 14 individuals. We conclude that the memory B cell response to SARS-CoV-2 evolves between 1.3 and 6.2 months after infection in a manner that is consistent with antigen persistence.

Donor Anti-Spike Immunity is Related to Recipient Recovery and Can Predict the Efficacy of Convalescent Plasma Units

BACKGROUND

The novel coronavirus, SARS-CoV2 that causes COVID-19 has resulted in the death of more than 2.31 million people within the last year and yet no cure exists. Whereas passive immunization with COVID-19 convalescent plasma (CCP) provides a safe and viable option, selection of optimal units for therapy and lack of clear therapeutic benefit from transfusion remain as barriers to the use of CCP.

STUDY DESIGN AND METHODS

To identify plasma that is expected to benefit recipients, we measured anti-SARS-CoV2 antibody levels using clinically available serological assays and correlated with the neutralizing activity of CCP from donors. Neutralizing titer of plasma samples was measured by assaying infectivity of SARS-CoV-2 spike protein pseudotyped retrovirus particles in the presence of dilutions of plasma samples. We also used this assay to identify evidence of passive transfusion of neutralizing activity in CCP recipients.

RESULTS

Viral neutralization and anti-spike protein antibodies in 109 samples from 87 plasma donors were highly varied but modestly correlated with each other. Recipients who died of COVID-19 were found to have been transfused with units with lower anti-spike antibody levels and neutralizing activity. Passive transfer of neutralization activity was documented in 62% of antibody naive plasma recipients.

CONCLUSIONS

Since viral neutralization is the goal of CCP transfusion, our observations not only support the use of anti-spike SARS-CoV2 serology tests to identify beneficial CCP units, but also support the therapeutic value of convalescent plasma with high titers of anti-spike antibodies.

Multicenter Evaluation of the Clinical Performance and the Neutralizing Antibody Activity Prediction Properties of 10 High-Throughput Serological Assays Used in Clinical Laboratories

As the coronavirus disease 2019 (COVID-19) pandemic second wave is emerging, it is of the upmost importance to screen the population immunity in order to keep track of infected individuals. Consequently, immunoassays for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with high specificity and positive predictive values are needed to obtain an accurate epidemiological picture. As more data accumulate about the immune responses and the kinetics of neutralizing-antibody (nAb) production in SARS-CoV-2-infected individuals, new applications are forecast for serological assays such as nAb activity prediction in convalescent-phase plasma from recovered patients. This multicenter study, involving six hospital centers, determined the baseline clinical performances, reproducibility, and nAb level correlations of 10 commercially available immunoassays. In addition, three lateral-flow chromatography assays were evaluated, as these devices can be used in logistically challenged areas. All assays were evaluated using the same patient panels in duplicate, thus enabling accurate comparison of the tests. Seven immunoassays examined in this study were shown to have excellent specificity (98 to 100%) and good to excellent positive predictive values (82 to 100%) when used in a low (5%)-seroprevalence setting. We observed sensitivities as low as 74% and as high as 95% at ≥15 days after symptom onset. The determination of optimized cutoff values through receiver operating characteristic (ROC) curve analyses had a significant impact on the diagnostic resolution of several enzyme immunoassays by increasing the sensitivity significantly without a large trade-off in specificity. We found that spike-based immunoassays seem to be better correlates of nAb activity. Finally, the results reported here will add to the general knowledge of the interlaboratory reproducibility of clinical performance parameters of immunoassays and provide new evidence about nAb activity prediction.

Inference of SARS-CoV-2 spike-binding neutralizing antibody titers in sera from hospitalized COVID-19 patients by using commercial enzyme and chemiluminescent immunoassays

Whether antibody levels measured by commercially available enzyme or chemiluminescent immunoassays targeting the SARS-CoV-2 spike (S) protein can act as a proxy for serum neutralizing activity remains to be established for many of these assays. We evaluated the degree of correlation between neutralizing antibodies (NtAb) binding the SARS-CoV-2 spike (S) protein and SARS-CoV-2-S-IgG levels measured by four commercial immunoassays in sera drawn from hospitalized COVID-19 patients. Ninety sera from 51 hospitalized COVID-19 patients were tested by a pseudotyped virus neutralization assay, the LIAISON SARS-CoV-2 S1/S2 IgG, the Euroimmun SARS-CoV-2 IgG ELISA, the MAGLUMI 2019-nCoV IgG, and the COVID-19 ELISA IgG assays. Overall, the results obtained with the COVID-19 ELISA IgG test showed the highest agreement with the NtAb assay (κ, 0.85; 95% CI, 0.63–1). The most sensitive tests were the pseudotyped virus NtAb assay and the COVID-19 ELISA IgG assay (92.2% for both). Overall, the degree correlation between antibody titers resulting in 50% virus neutralization (NtAb₅₀) in the pseudotyped virus assay and SARS-CoV-2 IgG levels was strong for the Euroimmun SARS-CoV-2 IgG ELISA (rho = 0.73) and moderate for the remaining assays (rho = 0.48 to 0.59). The kinetic profile of serum NtAb₅₀ titers could not be reliably predicted by any of the SARS-CoV-2 IgG immunoassays. The suitability of SARS-CoV-2-S-IgG commercial immunoassays for inferring neutralizing activity of sera from hospitalized COVID-19 patients varies widely across tests and is influenced by the time of sera collection after the onset of symptoms.

Humoral Responses and Serological Assays in SARS-CoV-2 Infections

In December 2019, the novel betacoronavirus Severe Acute Respiratory Disease Coronavirus 2 (SARS-CoV-2) was first detected in Wuhan, China. SARS-CoV-2 has since become a pandemic virus resulting in hundreds of thousands of deaths and deep socioeconomic implications worldwide. In recent months, efforts have been directed towards detecting, tracking, and better understanding human humoral responses to SARS-CoV-2 infection. It has become critical to develop robust and reliable serological assays to characterize the abundance, neutralization efficiency, and duration of antibodies in virus-exposed individuals. Here we review the latest knowledge on humoral immune responses to SARS-CoV-2 infection, along with the benefits and limitations of currently available commercial and laboratory-based serological assays. We also highlight important serological considerations, such as antibody expression levels, stability and neutralization dynamics, as well as cross-reactivity and possible immunological back-boosting by seasonal coronaviruses. The ability to accurately detect, measure and characterize the various antibodies specific to SARS-CoV-2 is necessary for vaccine development, manage risk and exposure for healthcare and at-risk workers, and for monitoring reinfections with genetic variants and new strains of the virus. Having a thorough understanding of the benefits and cautions of standardized serological testing at a community level remains critically important in the design and implementation of future vaccination campaigns, epidemiological models of immunity, and public health measures that rely heavily on up-to-date knowledge of transmission dynamics.

Longitudinal observation and decline of neutralizing antibody responses in the three months following SARS-CoV-2 infection in humans

Antibody responses to SARS-CoV-2 can be detected in most infected individuals 10-15 d after the onset of COVID-19 symptoms. However, due to the recent emergence of SARS-CoV-2 in the human population, it is not known how long antibody responses will be maintained or whether they will provide protection from reinfection. Using sequential serum samples collected up to 94 d post onset of symptoms (POS) from 65 individuals with real-time quantitative PCR-confirmed SARS-CoV-2 infection, we show seroconversion (immunoglobulin (Ig)M, IgA, IgG) in >95% of cases and neutralizing antibody responses when sampled beyond 8 d POS. We show that the kinetics of the neutralizing antibody response is typical of an acute viral infection, with declining neutralizing antibody titres observed after an initial peak, and that the magnitude of this peak is dependent on disease severity. Although some individuals with high peak infective dose (ID50 > 10,000) maintained neutralizing antibody titres >1,000 at >60 d POS, some with lower peak ID50 had neutralizing antibody titres approaching baseline within the follow-up period. A similar decline in neutralizing antibody titres was observed in a cohort of 31 seropositive healthcare workers. The present study has important implications when considering widespread serological testing and antibody protection against reinfection with SARS-CoV-2, and may suggest that vaccine boosters are required to provide long-lasting protection.

Persistence of Anti-SARS-CoV-2 Antibodies in Non-Hospitalized COVID-19 Convalescent Health Care Workers

Although antibody response to SARS-CoV-2 can be detected early during the infection, several outstanding questions remain to be addressed regarding the magnitude and persistence of antibody titer against different viral proteins and their correlation with the strength of the immune response. An ELISA assay has been developed by expressing and purifying the recombinant SARS-CoV-2 Spike Receptor Binding Domain (RBD), Soluble Ectodomain (Spike), and full length Nucleocapsid protein (N). Sera from healthcare workers affected by non-severe COVID-19 were longitudinally collected over four weeks, and compared to sera from patients hospitalized in Intensive Care Units (ICU) and SARS-CoV-2-negative subjects for the presence of IgM, IgG and IgA antibodies as well as soluble pro-inflammatory mediators in the sera. Non-hospitalized subjects showed lower antibody titers and blood pro-inflammatory cytokine profiles as compared to patients in Intensive Care Units (ICU), irrespective of the antibodies tested. Noteworthy, in non-severe COVID-19 infections, antibody titers against RBD and Spike, but not against the N protein, as well as pro-inflammatory cytokines decreased within a month after viral clearance. Thus, rapid decline in antibody titers and in pro-inflammatory cytokines may be a common feature of non-severe SARS-CoV-2 infection, suggesting that antibody-mediated protection against re-infection with SARS-CoV-2 is of short duration. These results suggest caution in using serological testing to estimate the prevalence of SARS-CoV-2 infection in the general population.