Characterisation Covid-19 IgG Antibody Kinetics Post Infection and Vaccination using the Boditech iCHROMA™ Fluorescence Immunoassay (FIA) Method

Abstract

Background: Coronavirus disease 2019 (COVID-19) is an emerging threat affecting millions of people worldwide. This study aims to assess the serologic profiles and time kinetics of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in patients with COVID-19 using the Boditech iCHROMA fluorescence immunoassay (FIA) method. Methods: Samples were collected from 43 subjects and analyzed for IgG antibodies against SARS-CoV-2 using the Boditech iCHROMA FIA. The iCHROMA processes the signal using a cut off index of 0.9 – 1.1, results <0.9 are interpreted as negative, results between 0.9 and 1.1 are interpreted as indeterminate and results >1.1 are interpreted as positive. Longitudinal samples were collected on days between day 1- day 100 post onset of symptoms in 1 subject, single samples collected on days 40-90 post infection in 30 subjects, and single samples collected at 3 time points (pre vaccination, 8-18 days post vaccination and 20-28 days post vaccination) collected from 12 subjects non-infected and previously infected. Results: The Covid-19 IgG concentration was detected (positive and cut off index above 1.1) on day 11 and remained consistently elevated for another 100 days of measurement in the sera of the single infected subject. The IgG cut off indices between day 20 and 100 ranged between 26.8 and 46.7, with a mean of 36.5. In the sera of the 30 patients infected with Covid-19 collected 40 – 90 days post infection, IgG antibodies were detected in all the samples. The IgG cut off indices ranged between 14.0 and 32.60, with a mean of 20.65. Of the twelve subjects who were vaccinated, eight had not been previously infected, while 4 had been previously infected. All 4 (100%) previously infected subjects seroconverted by the first time point (8-18 days post vaccination), whilst only 1/8 (12.5%) of the non-infected subjects seroconverted by the first time point (8-18 days post vaccination). By the second time point (20 - 28 days post vaccination), 2/12 subjects (17%) had failed to seroconvert, whilst 10/12 (83%) of the subjects had seroconverted. In addition, a previously infected subject whose IgG antibodies had dropped to a very low-level, after receiving the first dose of vaccination his IgG antibodies had gone back to his previous antibody levels at the first time point (8-18 days post vaccination). Conclusion: The Boditech iCHROMA FIA method for antibody testing is useful in detecting SARS-CoV-2 in a variety of situations at different time points but has its limitations with regards to determining the difference between previously infected and vaccinated individuals.