Date of Award

11-11-2025

Degree Type

Thesis

Degree Name

MPhil in Biological and Biomedical Sciences

First Advisor

Prof. Zahra Hasan

Second Advisor

Dr. Kiran Iqbal

Third Advisor

Dr. Hammad Hassan

Department

Biological and Biomedical Sciences

Abstract

Background: Vaccine-induced immunity against Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) played an important role in determining the course of the COVID 19 pandemic. Understanding immunity in the population is essential for guiding future vaccination policies. In Pakistan, the early national immunization campaign relied heavily on Sinopharm (BBIBP-CorV) and Sinovac (CoronaVac), later boosted with mRNA booster vaccines (PfizerBioNTech and Moderna). There is less information available regarding the long-term humoral and cellular immune memory in the post-pandemic period, particularly in those who received inactivated vaccinees. This study aimed to evaluate the longevity and functional profile of humoral and cellular immunity in participants three years after they had received inactivated COVID-19 vaccine type. We compared responses in those who had or who had not received mRNA boosters.
Objective: To compare the magnitude and persistence of memory responses both, humoral and cellular immunity between individuals vaccinated with inactivated vaccines alone and those who received an additional mRNA booster dose, three years post vaccination.
Methods: A total of 41 healthy individuals vaccinated with inactivated COVID-19 vaccines in 2021 were enrolled and divided into two groups: non-boosted (NB) and mRNA-boosted (WB). Serum was analysed for spike RBD (Receptor Binding Domain) specific IgG antibodies using Enzyme-Linked Immunosorbent Assay (ELSIA). Peripheral Blood Mononuclear Cells (PBMCs) isolated from blood and were stimulated in vitro with SARS-CoV-2 spike (S1) peptide pools to assess T-cell responses via multicolour flow cytometry, including cytokine production (IL-2, IFNγ, TNF-α) and memory (Tcm, Tem, Temra) and naïve (Tn) subset characterization based on CD45RO/CD45RA expression. Correlation analyses were performed to evaluate the relationship between cytokine functionality and memory phenotype. Results: All participants demonstrated sustained RBD-specific IgG seropositivity, confirming persistent humoral immunity three years after vaccination, with no significant differences between NB and WB groups. Spike (S1) specific T-cell responses were detected in all individuals, characterized by CD4⁺ dominant cytokine production and the presence of polyfunctional T cells co-expressing IL-2, IFN-γ, and TNF-α. Memory subset analysis revealed enrichment of Tem and Tcm upon spike exposure, indicating long-term and functional immune response. Correlation 5 analysis showed strong positive associations between cytokine producing T cells and memory subsets, reflecting a coordinated and functionally stable cellular immune network.
Conclusion: This study demonstrates that three years after inactivated COVID-19 vaccination, individuals retain both humoral and cellular immune memory, with or without mRNA boosters. The presence of durable antibody titers, polyfunctional T cells, durable and effector memory subsets, represents the persistence of cellular immunity as a key driver of long-term protection. These findings provide essential insights into vaccine induced immune longevity and have direct implications for optimizing booster vaccination strategies in populations initially immunized with inactivated vaccines.

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