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Introduction: The outbreak of Coronavirus Disease 2019 (COVID-19) affected a large number of people worldwide within a short period of time. The mortality rate was high, and there was no specific medicine available to cure it. Therefore, the situation demanded the rapid development of a vaccine. When the vaccines were introduced, there was limited knowledge about their efficacy, side-effects, and duration of protection. Hence, a detailed study was conducted in these areas. Aim: To study the immunological responses following Covishield vaccination and determine the duration of protection offered by the vaccine. Materials and Methods: The present study is a prospective observational study conducted at Sree Balaji Medical College and Hospital, Chennai, Tamil Nadu, India from March 2021 to May 2023, involving the staff members of the hospital and college who were above 18-year-old. Individuals with immunodeficiency, those on immunosuppressive medication, or anyone with proof of COVID-19 were excluded. The total sample size was 56. Blood samples were collected before administering the vaccine, at 0, 3, and 12 months, and tested for Complete Blood Count (CBC), COVID-19-specific Immunoglobulin G (IgG), Cluster of Differentiation (CD) 45, CD3, CD4, CD8, etc. The findings were statistically analysed using Statistical Packages of Social Sciences (SPSS) software version 22.0 and STATA software version 10. Results: A total of 154 volunteers initially provided the first blood samples. However, only 129 of them provided blood samples 2nd time and received two doses of the vaccine. Out of these, only 56 participants completed the fourth time blood test. The results consistently demonstrated a rise in IgG levels over time, with consistently higher levels observed in females. Participants above 45 years exhibited higher IgG levels. Individuals with co-morbidities also showed an increase in IgG levels. The research revealed that individuals who were initially IgG negative experienced a greater fold increase in IgG levels after the first dose of vaccination. The incidence and duration of side-effects post-vaccination reduced with each successive vaccine dose. Conclusion: All the volunteers developed adequate IgG antibodies. Two doses of Covishield effectively resulted in lasting immunity in 94.64% of cases, and three doses achieved 100% immunity. Females and participants above 45 years consistently exhibited higher antibody levels. Individuals with co-morbidities also developed antibodies, albeit at slightly lower levels. The side-effects were mild and short-lived. No long-term after effects were detected even after two years and two months following vaccination.

Introduction: Three years into the Coronavirus Disease-2019 (COVID-19) pandemic, questions linger regarding long-term vaccine efficacy, potential side effects, and the risk of future viral waves. Despite vaccinations, no existing vaccine offers complete protection, contributing to ongoing fears and vaccine hesitancy. Asymptomatic carriers and unattained herd immunity add layers of complexity. This study seeks to examine the status of immunity in vaccinated and unvaccinated individuals amidst the shifting landscape of different Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) variants. Aim: To determine the percentage of unvaccinated individuals who have developed COVID-19 specific antibodies and to compare the factors influencing immunity in both unvaccinated and vaccinated individuals. Materials and Methods: This prospective cohort study was conducted at Sree Balaji Medical College and Hospital, Chennai, Tamil Nadu, India, from March 2021 to May 2023. Participants, aged 18-82 of both sexes, were divided into two groups. Group-1 comprised our college and hospital staff who were vaccinated, and Group-2 consisted of members from the local community in the Chromepet, Chennai area who remained unvaccinated. Blood samples were collected from both groups to assess immunity status. Statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS, version 22.0). The tests used included the Chi-square test, p-value, mean, and standard deviation. Results: Blood group “B” was more commonly observed in Group-2. The prevalence of co-morbidities was higher in Group-2. Immunological markers CD4 and CD8 were below normal in some individuals in Group-2. By April 2022, 53 (95%) out of 56 persons in Group-1 and by December 2022, 24 (96%) out of 25 persons in Group-2 tested positive for COVID-specific IgG antibodies. By May 2023, 100% of the volunteers in both groups were found to be positive. Conclusion: This study suggests that natural immunity may be effective in protecting against COVID-19. Whether vaccinated or not, by the end of the two-year study, all individuals in the study group had developed COVID antibodies.

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 virus-destroying cells post-vaccination, specifically focusing on their ability to produce the vital antiviral cytokine Interferon gamma (IFNγ) 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 γ 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