Introduction
The COVID-19 pandemic has continued for more than two years infecting half of the world population either by natural infection or by vaccination. More than 760 million people have been infected and about 6.9 million deaths have occurred owing to SARS-CoV-2 infection as of April 2023.1 The available vaccines in India at the time of study were the Astra Zeneca (ChAdOx1 nCoV-19) vaccine (Covishield, Oxford/Astra Zeneca COVID-19 AZD1222, Serum Institute of India), a non-replicating viral vector vaccine (NRVV) and Bharat biotech BBV152 (Covaxin [BBV152]) containing inactivated whole virus; both of which required two doses for effective action. Covishield vaccine uses modified viral vector platform of chimpanzee adenovirus (ChAdOx1) that permits it to transfer spike protein of COVID-19 virus into human cells. Study based on the in-vitro live-virus neutralization and T-cell immune responses to the spike protein, showed that the efficacy of two doses of Covishield vaccine against moderate-to-severe COVID-19 was ~ 81.5%.3 Multiple studies have quoted that after natural SARS-CoV-2 infection or vaccination, vigorous T-cells response is responsible for production of anti-spike Neutralizing antibodies against multiple viral epitopes like spike Anti-receptor binding domain (RBD) & N-Terminal domain (NTD).4-6 The mRNA and adjuvant vaccines promote intracellular production of spike protein which is presented by antigen presenting cells to naïve T-cells (both CD8+ and CD4+ T cells) causing activation and differentiation of T-cells into effector cells and different subsets of memory T-cells. These memory cells once invoked either by natural infection or by vaccination, can persists even after decades. The interplay of this cellular and humoral response is essential for effective immunity.7-9
Vaccination against COVID-19 has reduced the severity of the disease, yet we are still facing waves of COVID-19 infection like the recent surge in China and India.10,11 Demand for booster dose is brought up with each wave of COVID-19, especially in vulnerable population like elderly, cancer patients and immunocompromised population.12 Studies have monitored the titer of neutralizing antibody at different time interval to check for adequate humoral immune response in healthy individuals.13-15Few studies have mentioned waning effect of humoral response at six months of vaccination by measuring IgG neutralizing antibodies (NAb) levels and proposed that reactivity & durability of cellular response ( CD8+ cells) can prevent severe disease against viral variant even when they escape from Nab. Equally, T-cell response at different time period has also been studied in healthy individuals. 4,16,17But there is paucity of data in immunocompromised individuals for sustained immune response especially with regards to T-cell-based immunity. Cirrhosis is associated with impairment of innate and adaptive immune system leading to acquired immunodeficiency. Both B and T-cell mediated immunity is hampered in form of B and T-cell depletion and dysfunction because of multifactorial causes like impaired production, proliferation and increased apoptosis.18,19 Hence, we selected patients of cirrhosis as immunocompromised population in our study. We studied different subset of T-cell memory cells along with antibody titer at baseline and at one year duration in vaccinated healthy individuals and patients with liver cirrhosis at one year to observe any difference in immune response.