T-cell Response after COVID-19 Vaccination: A Cross-sectional Study

Introduction: Even if antibody titers wane over time, T -cells demonstrate longevity and remain unaffected by viral variants. This study was done with the rationale of examining the virusdestroying cells post -vaccination, specifically focusing on their ability to produce the vital antiviral cytokine Interferon gamma (IFN gamma) upon potential reinfection and investigates the impact of the Covishield vaccine on T -cell response, which is a largely unexplored domain due to the novelty of the disease. Aim: To study T -cell response by doing IFN gamma release assay by employing Enzyme Linked Immunosorbent Spot (ELISpot) method. Materials and Methods: This was a cross-sectional study conducted from March 2021 to May 2023 at Sree Balaji Medical College and Hospital in Chennai, Tamil Nadu, India. The study included participants from all categories who were staff members of the College and Hospital. It is part of an extensive study done by the authors on immunological changes following Covishield vaccination. As part of this study, T -cell response was also separately studied by collecting an additional 2 mL of blood from 90 individuals one year after initiating the vaccination, specifically after administering the Covishield vaccine at 0 and 3 months. For T -cell response analysis, 2 mL of blood was collected and processed to separate Peripheral Blood Mononuclear Cells (PBMC). The Interferon Gamma Release assay (IGRA) was performed using ELISpot method, utilising a 96 -well plate. The spots appearing in the sample wells {Spot Forming Cells (SFC) = T -cells} were quantified using an automated ELISpot reader. Sample wells demonstrating more than 12 spots were considered positive. The results were analysed using various statistical tests, including Chi-square test, One-way Analysis of Variance (ANOVA), Kruskal-Wallis Test, Karl Pearson correlation coefficient, t -test, and Mann -Whitney U -test. Results: The number of spots in the wells containing blood samples from volunteers ranged from a minimum of 2 to a maximum of 631. Importantly, all participants had detectable spots in their sample wells. Out of 90 participants, 84 (93.4%) had more than 12 SFC, while 6 (6.6%) had less than 12 SFCs. High Immunoglobulin G (IgG) levels were positively correlated with good T -cell responses (SFC). Participants under 60 years of age and females exhibited superior responses. Individuals with co -morbidities had lower levels of T -cell response compared to the healthy/normal participants. Conclusion: The volunteers in this study exhibited robust humoral and cellular immunity, with females showing a significantly better response. The T -cell response remained strong even nine months after the second dose of the vaccine.