Development and clinical evaluation of a rapid antibody lateral flow assay for the diagnosis of SARS-CoV-2 infection

Abstract Background The novel coronavirus disease 2019 (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has quickly spread worldwide since its outbreak in December 2019. One of the primary measures for controlling the spread of SARS-CoV-2 infection is an accurate assay for its diagnosis. SARS-CoV-2 real-time PCR kits suffer from some limitations, including false-negative results in the clinic. Therefore, there is an urgent need for the development of a rapid antibody test kit for COVID-19 diagnosis. Methods The nuclear capsid protein (N) and spike protein 1 (S1) fragments of SARS-CoV-2 were expressed in Escherichia coli, and rapid antibody-based tests for the diagnosis of SARS-CoV-2 infection were developed. To evaluate their clinical applications, the serum from COVID-19 patients, suspected COVID-19 patients, recovering COVID-19 patients, patients with general fever or pulmonary infection, doctors and nurses who worked at the fever clinic, and health professionals was analyzed by the rapid antibody test kits. The serum from patients infected with Mycoplasma pneumoniae and patients with respiratory tract infection was further analyzed to test its cross-reactivity with other respiratory pathogens. Results A 47 kDa N protein and 67 kDa S1 fragment of SARS-CoV-2 were successfully expressed, purified, and renatured. The rapid antibody test with recombinant N protein showed higher positive rate than the rapid IgM antibody test with recombinant S1 protein. Clinical evaluation showed that the rapid antibody test kit with recombinant N protein had 88.56 % analytical sensitivity and 97.42 % specificity for COVID-19 patients, 53.48 % positive rate for suspected COVID-19 patients, 57.14 % positive rate for recovering COVID-19 patients, and 0.5−0.8 % cross-reactivity with other respiratory pathogens. The analytical sensitivity of the kit did not significantly differ in COVID-19 patients with different disease courses (p < 0.01). Conclusions The rapid antibody test kit with recombinant N protein has high specificity and analytical sensitivity, and can be used for the diagnosis of SARS-CoV-2 infection combined with RT-PCR.

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Development and Clinical Evaluation of a Rapid Antibody Lateral Flow Assay for the Diagnosis of SARS-CoV-2 Infection

10.21203/rs.3.rs-94508/v1 ◽

2020 ◽

Author(s):

Kesheng Li ◽

Chongxiang Tong ◽

Xiaoqin Ha ◽

Chaoning Zeng ◽

Xia Chen ◽

...

Keyword(s):

Clinical Evaluation ◽

False Negative ◽

Antibody Test ◽

Cross Reactivity ◽

Respiratory Pathogens ◽

N Protein ◽

Specificity And Sensitivity ◽

Test Kit ◽

Positive Rate ◽

Rapid Antibody

Abstract Background: The novel coronavirus disease 2019 (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has quickly spread worldwide since its outbreak in December 2019. One of the primary measures for controlling the spread of SARS-CoV-2 infection is an accurate assay for its diagnosis. SARS-CoV-2 real-time PCR kits suffer from some limitations, including false-negative results in the clinic. Therefore, there is an urgent need for the development of a rapid antibody test kit for COVID-19 diagnosis.Methods: The nuclear capsid protein (N) and spike protein 1 (S1) fragments of SARS-CoV-2 were expressed in Escherichia coli, and rapid antibody-based tests for the diagnosis of SARS-CoV-2 infection were developed and compared. To evaluate their clinical applications, the serum from COVID-19 patients, suspected COVID-19 patients, recovering COVID-19 patients, patients with general fever or pulmonary infection, doctors and nurses who worked at the fever clinic, and health professionals was analyzed by the rapid antibody test kits. The serum from patients infected with Mycoplasma pneumoniae and patients with respiratory tract infection was further analyzed to test its cross-reactivity with other respiratory pathogens.Results: A 47 kDa N protein and 67 kDa S1 fragment of SARS-CoV-2 were successfully expressed, purified, and renatured. The rapid antibody test with recombinant N protein showed higher sensitivity and specificity than the rapid IgM antibody test with recombinant S1 protein. Clinical evaluation showed that the rapid antibody test kit with recombinant N protein had 88.56% sensitivity and 97.42% specificity for COVID-19 patients, 53.48% positive rate for suspected COVID-19 patients, 57.14% positive rate for recovering COVID-19 patients, and 3.20%-3.27% cross-reactivity with other respiratory pathogens. The sensitivity of the kit did not significantly differ in COVID-19 patients with different disease courses (p < 0.01).Conclusion: The rapid antibody test kit with recombinant N protein has high specificity and sensitivity, and can be used for the diagnosis of SARS-CoV-2 infection combined with RT-PCR.

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Evaluation of a Rapid Immunochromatographic test kit to the Gold Standard Fluorescent Antibody test for diagnosis of rabies in animals in Bhutan

10.21203/rs.2.19414/v3 ◽

2020 ◽

Author(s):

Tenzin Tenzin ◽

Kelzang Lhamo ◽

Purna B Rai ◽

Dawa Tshering ◽

Pema Jamtsho ◽

...

Keyword(s):

Predictive Value ◽

Fluorescent Antibody ◽

False Negative ◽

Rapid Test ◽

Antibody Test ◽

Reference Standard ◽

Predictive Values ◽

Percent Agreement ◽

Test Kit ◽

Resource Poor

Abstract Background: Rabies kills approximately 59,000 people in the world each year worldwide. Rapid and accurate diagnosis of rabies is important for instituting rapid containment measures and for advising the exposed people for postexposure treatment. The application of a rapid diagnostic tests in the field can greatly enhance disease surveillance and diagnostic activities, especially in resource poor settings. In this study, a total of 179 brain tissue samples collected from different rabies suspect animal species (113 dogs, 50 cattle, 10 cats, 3 goats, 2 horses, and 1 bear) were selected and tested using both rapid immunochromatographic kit and the reference standard fluorescent antibody test (FAT). We evaluated the sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of a rapid antigen detection test kit produced by BioNote, Inc. (Hwaseong-si, Korea) relative to a FAT for its fit-for-purpose for confirmation of clinical cases of rabies for early response and enhancing rabies surveillance. Results: Among 179 samples examined in this study, there was a concordance in results by the rapid test and FAT in 115 positive samples and 54 negative samples. Test results were discordant in 10 samples which were positive by FAT, but negative (false negative) by rapid kit. The rapid test kit showed a sensitivity of 92% (95% CI: 85.9 – 95.6) and specificity of 100% (95% CI: 93.4 – 100) using FAT as the reference standard. The positive and negative predictive values were found to be 100% (95% CI:96.7 – 100) and 84.4% (95% CI: 73.6 – 91.3), respectively. Overall, there was 94.4% (95% CI: 90 – 96.9) test agreement between rapid test and FAT (Kappa value = 0.874) with a positive percent agreement and negative percent agreement of 92 and 100%, respectively. Conclusions: Our finding demonstrated that the rapid test kit (BioNote) can be used for rabies surveillance and confirming clinical case of rabies in animals for making rapid decisions particularly controlling rabies outbreaks in resource poor settings.

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Validation of rapid antibody (IgG-IgM) test kit for SARS COV-2 infection in Qatar

Journal of Public Health Research ◽

10.4081/jphr.2021.2421 ◽

2021 ◽

Author(s):

Jesha Mundodan ◽

Samina Hasnain ◽

Hayat Khogali ◽

Soha Shawqi Al Bayat ◽

Dina Ali ◽

...

Keyword(s):

Predictive Value ◽

Local Population ◽

Rapid Test ◽

Antibody Test ◽

Molecular Testing ◽

Rt Pcr ◽

Test Kit ◽

Asymptomatic Individuals ◽

Sensitivity Specificity ◽

Rapid Antibody

Background: In response to the growing coronavirus disease 2019 (COVID-19) pandemic and the shortage of laboratory based molecular testing capacity and reagents, multiple diagnostic test manufacturers have developed rapid and easy to use devices to facilitate testing outside laboratory settings. These kits are either based on detection of proteins from SARS-CoV-2 virus or detection of antigen or human antibodies generated in response to the infection. However, it is important to understand their performance characteristics and they must be validated in the local population setting.Design and Methods: The objective is to assess the validity of the rapid test for IgG and IgM immunoglobulins compared to the current gold standard reverse transcription polymerase chain reaction (RT-PCR) test. A total of 16951 asymptomatic individuals were tested by the Ministry of Public Health track-and-trace team using both rapid immunodiagnostic test and RT-PCR as part of screening across various random settings with potential risk of community interaction prior to gradual lifting of restrictions in Qatar. Rapid test was considered to be posiive if both IgG and IgM are positive, while only IgG/IgM positive was considered as rapid test negative. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated.Results: The sensitivity of rapid test kit was found to be 0.9%, whereas the specificity was found to be 97.8%. the PPV was found to be 0.3% whereas the NPV was found to be 99.4%.Conclusion: Based on the outcome and results of the study, it appears that the sensitivity and PPV of the rapid antibody test are low. As such, this test is not recommended for use to assist in taking clinic-based decisions or decisions related to quarantine/isolation.

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Potential applications of the rapid COVID-19 antibody test kit screening in comparison to the RT-PCR in patients and personnel at the Department of Obstetrics and Gynecology.

10.1101/2021.07.16.21259725 ◽

2021 ◽

Author(s):

Amarin Narkwichean ◽

Wittaya Jomoui ◽

Wipada Laosooksathit ◽

Tanawin Nopsopon ◽

Krit Pongpirul

Keyword(s):

False Positive Rate ◽

Antibody Test ◽

Medical Personnel ◽

Obstetrics And Gynecology ◽

Rt Pcr ◽

Risk Area ◽

Asymptomatic Patients ◽

Test Kit ◽

Potential Applications ◽

Rapid Antibody

Objective To explore potential applications of the rapid antibody test for COVID-19 screening, in comparison to RT-PCR, for emergency obstetric and gynecological procedures, and medical personnel in the Department of Obstetrics and Gynecology. Methods A cross-sectional study was conducted in expected 290 participants: 230 patients and 60 medical staff, during the four-month national COVID-19 outbreak period (Aug-Sep 2020, and Dec 2020-Jan 2021). All participants underwent both rapid antibody tests and RT-PCR (at admission for patients). Results A total of 270 participants completed the study. Fever and URI symptoms were present in 6/210 patients (2.8%) while one patient (0.5%) had a history of traveling to a high-risk area. However, only two (1%) asymptomatic patients had positive IgM results. Concerning the medical personnel, 10% fell into the patient under investigation (PUI) category. 4/60 (6.7%) IgM positive was observed in the staff cohort in which 3/4 came from non-PUI participants. Neither participant had RT-PCR positive demonstrating a 1.9% total false positive rate. Conclusion Rapid point-of-care antibody test can be used to screen either a pregnant coming for delivery, a patient who requires urgent/emergency operative procedures, or medical personnel, at least in the defined lower-prevalence COVID-19 situation.

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Evaluation of a Rapid Immunochromatographic test kit to the Gold Standard Fluorescent Antibody test for diagnosis of rabies in animals in Bhutan

10.21203/rs.2.19414/v1 ◽

2019 ◽

Author(s):

Tenzin Tenzin ◽

Kelzang Lhamo ◽

Purna B Rai ◽

Dawa Tshering ◽

Pema Gyamtsho ◽

...

Keyword(s):

Gold Standard ◽

Fluorescent Antibody ◽

False Negative ◽

Rapid Test ◽

Antibody Test ◽

Perfect Agreement ◽

Tissue Samples ◽

Test Kit ◽

Resource Poor ◽

Test Result

Abstract Background: Rabies kills approximately 59,000 people in the world each year worldwide. Rapid and accurate diagnosis of rabies is important for instituting rapid containment measures and for advising the exposed people for postexposure treatment. The application of a rapid diagnostic tests in the field can greatly enhance disease surveillance activities, especially in resource poor settings.Methods: From 2012 to 2017, a total of 179 brain tissue samples collected from different animal species (113 dogs, 50 cattle, 10 cats, 3 goats, 2 horses, and 1 bear) suspected of having died due to rabies were selected and tested using the rapid immunochromatographic kit from BioNote© company and compared to the Gold Standard Fluorescent Antibody test (FAT) for diagnosis of rabies.Results: Among 179 samples examined in this study, there was concordance in results by the rapid test and FAT in 115 positive samples and 54 negative samples. Test result were discordant in 10 samples which were positive by FAT, but negative (false negative) by rapid kit. The rapid test kit showed a sensitivity of 92% (95% CI: 85.9 – 95.6) and specificity of 100% (95% CI: 93.4 – 100) using FAT as the gold standard. The positive and negative predicative values were found to be 100% (95% CI:96.7 – 100) and 84% (95% CI: 73.6 – 91.3), respectively. Overall there was 94.41% (95% CI: 90 – 96.9) test agreement (almost perfect agreement) between rapid test and FAT (Kappa value = 0.874).Conclusions: Our results demonstrate the potential value of the rapid test kit for countries with limited diagnostic resources, including Bhutan. The rapid kit’s inability to correctly detect 10 FAT-positive samples (10 out of 179 (5.6%) were false negatives) in our study could have been due to the low viral load in the samples (< 102.0LD50/0.03ml) which could not be detected by the rapid kit as compared with the FAT. The human factor related to the varying experiences of the technicians who performed the test in the field also may have influenced the test result. The rapid test kit is inexpensive, rapid and easy to use in the field or in laboratory setting without the need for special training and can support to enhance rabies surveillance in resource poor countries.

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Going beyond clinical routine in SARS-CoV-2 antibody testing - A multiplex corona virus antibody test for the evaluation of cross-reactivity to endemic coronavirus antigens

10.1101/2020.07.17.20156000 ◽

2020 ◽

Cited By ~ 3

Author(s):

Matthias Becker ◽

Monika Strengert ◽

Daniel Junker ◽

Tobias Kerrinnes ◽

Philipp D. Kaiser ◽

...

Keyword(s):

Vaccine Development ◽

Antibody Test ◽

Cross Reactivity ◽

Antibody Testing ◽

N Protein ◽

Humoral Immune ◽

In Vitro Diagnostic ◽

Human Coronaviruses ◽

Multiplexed Immunoassay

Given the importance of the humoral immune response to SARS-CoV-2 as a global benchmark for immunity, a detailed analysis is needed to (i) monitor seroconversion in the general population, (ii) understand manifestation and progression of the disease, and (iii) predict the outcome of vaccine development. Currently available serological assays utilize single analyte technologies such as ELISA to measure antibodies against SARS-CoV-2 antigens including spike (S) or nucleocapsid (N) protein. To measure individual antibody (IgG and IgA) responses against SARS-CoV-2 and the endemic human coronaviruses (hCoVs) NL63, 229E, OC43, and HKU1, we developed a multiplexed immunoassay (CoVi-plex), for which we included S and N proteins of these coronaviruses in an expanded antigen panel. Compared to commercial in vitro diagnostic (IVD) tests our CoVi-plex achieved the highest sensitivity and specificity when analyzing 310 SARS-CoV-2 infected and 866 uninfected individuals. Simultaneously we see high IgG responses against hCoVs throughout all samples, whereas no consistent cross reactive IgG response patterns can be defined. In summary, our CoVi-plex is highly suited to monitor vaccination studies and will facilitate epidemiologic screenings for the humoral immunity toward pandemic as well as endemic coronaviruses.

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EVALUATION OF PERFORMANCE OF NINE SEROLOGICALASSAYS FOR THE DETECTION OF SARS-COV-2 ANTIBODIES.

10.36106/ijsr/1335520 ◽

2021 ◽

pp. 60-62

Author(s):

Tagajdid Mohamed Rida ◽

Konzi Clémence ◽

El Kochri Safae ◽

Elannaz Hicham ◽

Abi Rachid ◽

...

Keyword(s):

Clinical Performance ◽

Current Knowledge ◽

False Negative ◽

Antibody Test ◽

Antibody Detection ◽

Rt Pcr ◽

Igg Antibody ◽

Serological Tests ◽

Blood Samples ◽

Test Kit

Introduction: Currently, polymerase chain reaction (PCR) based viral RNAdetection is the standard for COVID-19 diagnosis [2]. Though, RNA testing based on throat or nasopharyngeal swabs has shown a number of false-negative results. Antibody detection tests have been developed to detect specic antibodies, IgM and IgG, to SRAS-CoV-2 virus. The clinical relevance of these tests is still under evaluation and is highly related to their clinical performance. Our objective is to assess analytical performances of nine SARS-CoV-2 antibodies immunoassays. Materiel and Method: We collected 80 blood samples from PCR-conrmed COVID-19 patients diagnosed in our Virology department (20 samples collected at day 10 after the onset of symptoms, 60 collected after day 14 following the onset of symptoms) and 20 blood samples from patients SARS-CoV-2 RT-PCR negative. All sera were tested with nine SARS-CoV-2 antibodies immunoassays ARCHITECT SARS-CoV-2 IgG® (Abbott), COVID-19 VIRCLIA® IgG MONOTEST (Vircell), COVID-19 VIRCLIA® IgM+IgA MONOTEST (Vircell), COVID-19 ELISA IgG® (Vircell), COVID-19 ELISA IgM+IgA® (Vircell), Elecsys® Anti-SARS-CoV-2 (Roche), FREND® COVID-19 IgG/IgM Duo (NanoEntek), COVID-PRESTO® (AAZ) and COVID-19 (SARS-CoV-2) IgM/IgG Antibody Test Kit® (Labnovation Technologies). Results: Sensitivity of tests increases once the seroconversion to anti-SARS-CoV-2 IgG positive in most individuals occurs toward the end of week 2 post-infection. COVID-19 PRESTO had the best accuracy in our study showing 100% sensitivity after day 14 following the onset of symptoms. All of the tests had a specicity of 100%. Conclusion: Serological tests are sensitive for the latest stages of COVID-19 infection. Recommendations on using SRAS-COV-2 antibody detection tests are continuously improving based on current knowledge of host antibody responses during infection. They are of great value in cases presenting COVID-19 symptoms with negative RT-PCR.

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Development of USI-Kit for Evaluation of Iodine Content in Iodized Salt

Walailak Journal of Science and Technology (WJST) ◽

10.48048/wjst.2020.10726 ◽

2020 ◽

Vol 17 (10) ◽

pp. 1149-1156

Author(s):

Sakaewan OUNJAIJEAN ◽

Kongsak BOONYAPRANAI ◽

Kanokwan KULPRACHAKARN ◽

Kittipan RERKASEM

Keyword(s):

Spectrophotometric Method ◽

False Positive ◽

False Positive Rate ◽

False Negative ◽

False Negative Rate ◽

Iodine Content ◽

Kappa Coefficient ◽

Test Kit ◽

Positive Rate ◽

Sensitivity Specificity

Iodine deficiency has been considered as a serious public health problem for the past decades. Universal salt iodization program is introduced and implemented to address such problem. To encourage this program in an effective and sustainable way, it is essential to regularly monitor whether salt is adequately iodized at various points along the supply chain. The traditional iodometric titration method has problems related to accessibility, cost, and time. Colorimetric test kits have been used extensively to measure coverage of iodized salt in household surveys due to its expediency and affordability. In Thailand, “I-KIT” is the most widely used. The visualization of intensive color, however, is inconvenient for untrained-user in determining the adequacy of iodine content. Thus, an improvement to make testing more precise and affordable is still required. In this respect, a new test kit namely USI-Kit was developed to assess iodine quality and semi-quantity in edible salt. The kit was tested to evaluate its performance, by comparing the result with the I-KIT and with the spectrophotometric method. Compared with I-Kit, the USI-Kit exerted the relative accuracy, sensitivity, specificity, false positive rate, false negative rate and Kappa coefficient value of 74.0, 76.3, 72.6, 27.4, 23.7 and 0.47, respectively. Compared to the spectrophotometric method, USI-Kit exerted the relative accuracy, sensitivity, specificity, false positive rate, false negative rate and Kappa coefficient value of 85.4, 80.1, 89.3, 10.7, 19.9 and 0.70, respectively. The finding suggested that a newly developed iodine test kit holds promise to be used in field inspection of iodine content in salt.

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Clinical Performance Evaluation of a SARS-CoV-2 Rapid Antibody Test for Determining Past Exposure to SARS-CoV-2

10.1101/2020.09.01.20180687 ◽

2020 ◽

Author(s):

Peter Findeisen ◽

Hugo Stiegler ◽

Eloisa Lopez-Calle ◽

Tanja Schneider ◽

Eva Urlaub ◽

...

Keyword(s):

Common Cold ◽

Whole Blood ◽

High Performance ◽

Clinical Performance ◽

Method Comparison ◽

Antibody Test ◽

Cross Reactivity ◽

Percent Agreement ◽

Antibody Tests ◽

Rapid Antibody

The true prevalence and population seropositivity of SARS-CoV-2 infection remains unknown, due to the number of asymptomatic infections and limited access to high-performance antibody tests. To control the COVID-19 pandemic it is crucial to understand the true seroprevalence, but not every region has access to extensive centralized PCR and serology testing. Currently available rapid antibody tests lack the accuracy needed for recommendation by health authorities. To fill this gap, we analyzed and validated the clinical performance of a new point-of-care SARS-CoV-2 Rapid Antibody Assay, a chromatographic immunoassay for qualitative detection of IgM/IgG antibodies for use in near-patient settings. Analysis was performed using 42 Anti-SARS-Cov-2 positive (CoV+) and 92 Anti-SARS-Covid-2 negative (CoV-) leftover samples from before December 2019, using the Elecsys® Anti-SARS-CoV-2 as the reference assay. Analytical specificity was tested using leftover samples from individuals with symptoms of common cold collected before December 2019. The SARS-CoV-2 Rapid Antibody Test was 100.0% (95% CI 91.59-100.00) sensitive and 96.74% (95% CI 90.77-99.32) specific with an assay failure rate of 0.00%. No cross-reactivity was observed against the common cold panel. Method comparison was additionally conducted by two external laboratories, using 100 CoV+/275 CoV- samples, also comparing whole blood versus plasma matrix. The comparison demonstrated for plasma 96.00% positive/96.36% negative percent agreement with the Elecsys Anti-SARS-CoV-2 and overall 99.20% percent agreement between whole blood and EDTA plasma. The SARS-CoV-2 Rapid Antibody Test demonstrated similar clinical performance to the manufacturer's data and to a centralized automated immunoassay, with no cross-reactivity to common cold panels.

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Quantitative measurement of anti-SARS-CoV-2 antibodies: Analytical and clinical evaluation

Journal of Clinical Microbiology ◽

10.1128/jcm.03149-20 ◽

2021 ◽

Author(s):

Victoria Higgins ◽

Anselmo Fabros ◽

Vathany Kulasingam

Keyword(s):

Antibody Response ◽

Clinical Evaluation ◽

Predictive Value ◽

Cross Reactivity ◽

Analytical Sensitivity ◽

Past Infection ◽

Clinical Sensitivity ◽

Limit Of Quantitation ◽

Sensitivity Specificity ◽

Roche Diagnostics

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of Coronavirus Disease 2019 (COVID-19). While molecular-based testing is used to diagnose COVID-19, serologic testing of antibodies specific to SARS-CoV-2 is used to detect past infection. While most serologic assays are qualitative, a quantitative serologic assay was recently developed that measures antibodies against the S protein, the target of vaccines. Quantitative antibody determination may help determine antibody titer, facilitate longitudinal monitoring of the antibody response, including antibody response to vaccines. We evaluated the quantitative Roche Elecsys® Anti-SARS-CoV-2 S assay. Specimens from 167 PCR-positive patients and 103 control specimens were analyzed using the Elecsys® Anti-SARS-CoV-2 S assay on the cobas e411 (Roche Diagnostics). Analytical evaluation included assessing linearity, imprecision, and analytical sensitivity. Clinical evaluation included assessing clinical sensitivity, specificity, cross-reactivity, positive predictive value (PPV), negative predictive value (NPV), and serial sampling from the same patient. The Elecsys® Anti-SARS-CoV-2 S assay exhibited highest sensitivity of 84.0% 15-30 days post-PCR positivity, no cross-reactivity, specificity and PPV of 100%, and NPV between 98.3%-99.8% ≥14 days post-PCR positivity, depending on the seroprevalence estimate. Imprecision was <2% at 9.06 U/mL across 6 days, the negative QC was consistently negative (<0.40 U/mL), the manufacturer’s claimed limit of quantitation of 0.40 U/mL was verified, and linearity across the analytical measuring range was observed, except at the low end (<20 U/mL). Lastly antibody response showed high inter-individual variation in level and time of peak antibody titre and trends over time.

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