Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), positive-strand RNA coronavirus of Coronaviridae family, is responsible for causing devastating COVID-19 pandemic with millions of deaths worldwide (1). The genome consists of non-structural and structural proteins including envelope (E), membrane (M), and nucleocapsid (N) and spike (S). During SARS-CoV-2 course of infection, the spike glycoproteins interact with angiotensin converting enzyme 2 (ACE2) across the epithelial Alveolar Type 2 (AT2) progenitor cells and regulate several biochemical mechanisms or pathways to favor viral pathogenesis (2).
To date, majority of COVID-19 vaccine strategies aim to induce neutralizing antibodies against spike region, with ultimate goal of blocking SARS-CoV-2 early-stage infection (3). According to World Health Organization, 63 candidate vaccines are in human clinical trials, and > 172 vaccine candidates are at preclinical stages (4). Among SARS-CoV-2 vaccines at developmental stage, only Gam-COVID-Vac or Sputnik V utilized heterologous (adenovirus 5 and adenovirus 26) prime-boost recombinant adenovirus approach given 21 days apart (to overcome any pre-existing adenovirus immunity) responsibly provoked humoral and cellular immune response among 98% and 100% volunteers, respectively (5, 6).
Post-intramuscular injection of Sputnik V, the replication deficient Ad26 and Ad5 penetrate into host cells and deliver recombinant DNA into the nucleus to stimulate transcription via synthesizing mRNAs translating spike proteins which migrate towards the cell surface. The vaccinated cells may break spike proteins into fragments. Afterwards, protruding spikes, recognized by the immune system induces strong immune responses to kill the vaccinated cell. However, the cell debris containing spike proteins or fragments are taken up by antigen-presenting cell (APC), which presents fragments of the spike protein on cell surface. Upon encounter with helper T cells, the B cells are activated and start proliferation to generate antibodies against spike protein. The APC can also activate killer T lymphocytes to destroy SARS-CoV-2 infected cells displaying spike protein fragments on surfaces (7).
A Phase III clinical trial conducted by Gamaleya Research Institute Russia on 19,866 volunteers who received Sputnik V revealed strong efficacy, immunogenicity and safety results. The efficacy results were 91.6% with no serious adverse events (8, 9). Among vaccinated volunteers with history of wild type SARS-CoV-2 infection, the virus neutralizing antibodies level were 1.3–1.5 times higher (6). The levels of neutralizing antibodies are significantly important in context of SARS-CoV-2 vaccines. Reliable quantification of the antibody responses is critical for estimating the time of protection or possible vaccine related failures.
Depending upon SARS-CoV-2 global burden of continuously increasing COVID-19, there is a dire need for mass vaccination worldwide. In Pakistan due to several healthcare challenges, identification of neutralizing antibodies against SARS-CoV-2 might be crucial in formulating new vaccination strategies to curtail unprecedented COVID-19 epidemics. To date, 0.8 million people have been infected with COVID-19 in Pakistan and caused deaths in 16,999 individuals. As of 21 April 2021, 1,548,714 vaccine doses have been administered among general public (10). We aimed to evaluate SARS-CoV-2 spike antibody levels among Sputnik V vaccinated group of individuals in Pakistani population.