{'fraction_non_alphanumeric': 0.034857358775741545, 'fraction_numerical': 0.020120914415265444, 'mean_word_length': 5.198477751756441, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Notable and Emerging Variants of SARS-CoV-2 Virus: A Quick Glance

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the etiological agent of coronavirus disease-2019 (COVID-19), is a highly contagious pathogenic coronavirus to emerge and spread in human populations. Although substantial exertions have been laid to avert spread of COVID-19 by therapeutic and preventive countermeasures, but emergence of SARS-CoV-2 variants as a result of mutations make the infection more ominous. New viral confers a higher nasopharyngeal viral load, increased viral transmissibility, higher infectiousness, immune escape, increased resistance to monoclonal/polyclonal antibodies from convalescence sera/vaccine, and an enhanced virulence. Thus, it is pertinent to monitor evolving mutations and genetic diversity of SARS-CoV-2 as it is decisive for understanding the viral variants. In this review we provide an overview of colloquial nomenclature and the genetic characteristics of different SARS-CoV-2 variants in the context of mutational changes of the circulating strains, transmissibility potential, virulence and infectivity.


Introduction
Coronavirus disease-2019 (COVID-19), a highly contagious disease, caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), is a global mounting public health problem, that often leads to a series of pathologies which are detrimental to human health condition with unprecedented consequences. The first case of this viral infection was testified in Wuhan, the capital city of the Hubei province of China in December 2019, then SARS-CoV-2 rapidly disseminated all over the world and circumstances became worse within a short period of time, compelling the World Health Organization (WHO) to declare it as pandemic on 11th March 2020; as most of the countries experienced massive spikes in cases of COVID-19 [1]. However, numbers of infected cases, acute events and mortality rates allied to COVID-19 displayed vast variation from country to country or in different populace. As per records of WHO by 16th of May 2021, 162,177,376 infection cases have been reported with a global death of 3,364,178 [2].
The reservoir of coronaviruses are bats, but it is believed to be transmitted to the human via an intermediate animal host [3]. SARS CoV-2 is an enveloped virion that measure approximately 120 nm in diameter, belonging to subfamily Orthocoronavirinae, in the family Coronaviridae, order Nidovirales, and realm Riboviria; containing positivesense-single strand of RNA with a genome size of 29.99 kb encoding for multiple nonstructural and structural proteins [4,5]. SARS-CoV-2 virus enters into type II pneumocytes of lung through Angiotensin Converting Enzyme 2 (ACE2) receptor [6], and then it starts replicating and migrating down to the airways and enters alveolar epithelial cells in the lungs. The swift replication of SARS-CoV-2 in the lungs might induces a robust immune response and pathogenesis manifests as mild viral pneumonia to severe acute respiratory distress syndrome [7,8]. Initial cytokine storm is responsible for occurrence of macrophage activation syndrome and acute respiratory distress syndrome/ respiratory failure. Simultaneously, SARS-CoV-2 spreads to the other organ(s) and infects cells which express ACE2 receptor, resulting in a in multi-organ damage [7]. Cytokine storm is followed by phase of immune dysregulation which is considered as a core culprit in the development of sepsis related complications [9].
Communal symptoms of SARS-CoV-2 infection are pyrexia, tussis, dyspnea, pharyngitis, myalgia, headache, olfactory and taste dysfunction (hyposmia/anosmia or ageusia). The widespread concern is the emergence of SARS-CoV-2 variants with concerning phenotypes as a result of mutation(s). SARS-CoV-2, despite being endowed with proofreading activity (a function attributed to nsp14) during viral replication [6], have a high mutation rate, and the absolute number of mutations increases with every round of infection either through genetic drift or through selection, and become fixed in different populations. Monitoring of these evolving mutations and genetic diversity of SARS-CoV-2 is decisive for understanding the viral variants and assuring the performance of new diagnostic tests and countermeasures (vaccines and therapies) against COVID-19. Thus, this study aims to perform a descriptive review of the basic virology of variants of SARS-CoV-2 i.e. we will highlight the current understanding; provide an overview of the nomenclature and genetic characteristics of viral variants in the context of mutational changes of the circulating strains, transmissibility potential, virulence and infectivity (Table 1).
The SARS-CoV-2 genome also contains leader and transcription-regulatory sequence (TRS) [5]. The leader sequence (* 70 bases) is present at 5 0 end, out of which 7-10 bases are transcription-regulatory sequences referred as TRS-L. Similarly, adjacent to each ORF, TRS-B motifs are present; TRS-L and TRS-B are responsible for the discontinuous synthesis of intermediate negative strands of sgRNA [6]. Of the four structural genes, S gene is significantly diverged from the corresponding sequence in SARS CoV, while other shares more than 90% homology [4][5][6]. The SARS-CoV-2 S is a trimeric glycoprotein, consists of 1273 amino acids. It contains two subunits viz S1 and S2 [6]. S1 is responsible for the viral entry by attaching to host cell's ACE2 receptor through the S1 receptor-binding domain (RBD), while the S2 subunit allows virus-cell fusion of viral and cellular membranes. This process requires S protein priming by host proteases such as TMPRSS2 in cleavage sites S1/S2, a polybasic (furin) cleavage motif, at the S1/S2 boundary, and the S20 site [5,6]. Additional distinctive genomic features of SARS-CoV-2 is, accessory gene orf8 and a presence of four amino acid residues (PRRA) at the junction of subunits S1 and S2. This insertion creates a polybasic cleavage site (RRAR), which allows effective cleavage by furin and other proteases [6].

Mutation: Variants and Strain
Mutation is a definite natural change that occurs in the genomic sequence of SARS-CoV-2 virus during process of viral replication [10,11]. Natural selection processes will decide the fate of newly arising mutation. Mutations that are deleterious to the virus will be purged from the population, and few are essentially maintained in the population; they facilitate viral replication, transmissibility or immune escape i.e. if mutation favors the viral fitness, then it increases in frequency and evolute as a new variant [12]. However, frequency of mutation also depends on chance events. Hence, interplay between natural selection and chance events decide the evolution of new variants in host cells, populations and across the world [13]. If a genome differs from the reference sequence by one or more than one mutation then it is refereed as variant [14]. Specific variant which has different phenotypic characteristics like, virulence or transmissibility is called as a strain [15].

Nomenclature of SARS-CoV-2 Variants
Nomenclature helps to understand genomic epidemiology, surveillance and control of the infections [16]. Genetic diverseness of virus is primarily classified into specific large group called 'clades'. On a phylogent tree, each specific 'clade' refers to a monophyletic group which is having a common ancestor. Thus, 'clades' classify genetic diverseness and pathogen phylogeny into mutually exclusive and equally divergent set of groups [17]. Notable Variants of SARS-CoV-2 1. Spike D614G variant: Substitution of aspartic acid to glycine at 614 amino acid position in spike protein of SARS-CoV-2 virus has been found in January and February 2020 and this mutation governs a vital conformational alteration in the spike protein between the S1 and S2 domains, that favors the binding to the ACE2 receptor and thus increases the probability of infection. Over a few months after the perception of ancestral strain, spike D614G variant became globally dominant, presumably because of enhanced ability to bind with human ACE2 receptor. D614G mutation also exhibit enhanced replication activity in both human (nasal and bronchial airway epithelial cell) and in animal models with a rapid transmissibility. Patients infected with the G variant shows a higher nasopharyngeal viral load than the wild-type strain, but there seems to be no difference in disease severity. Loss of smell is found to be the predominant symptom with G variant infectivity as this variant has boosted ability to bind with ACE2 receptor in olfactory epithelium. Sera of animal which were infected with this strain contains higher neutralizing antibody compared to original strain, suggestive of no additional effect of this variant on the efficacy of vaccine or of diagnostic and therapeutic measures [22][23][24][25] A. N501Y: The N501Y (substitution of Asparagine to Tyrosine at 501 amino acid position) mutation is of major concern of B.

Role of Genome Sequencing in Surveillance
Genomics evaluate the particular virology in real time.
First library of consistently sized fragments is generated by cleaving multiple genomes at randomly. Then unordered sequenced segments are generated by automated computation. Finally segments of overlapping sequence are constructed to know genome consensus [52]. Figure 1 depicts the steps involved in the genomic sequencing of SRAS-CoV-2 [53]. Genome sequencing helps to, identify the origin of infection; comprehend the evolution of pathogen, Fig. 1 Steps in genomic sequencing of virus  confirm the circulating variants and to develop the therapeutics and vaccines. Next generation sequencing (NGS) has sensitivity of 98.4% and specificity of 97.2% which is distantly improved equated to conventional RT-PCR [53]. Evaluation of samples in NGS will be imperative to guide the policy makers for the fight against SARS-CoV-2 in future. Figure 2 depicts proposed flow of work to identify and genomic counseling of new SARS-CoV-2 variant in future.

Conclusion
Emergent SARS-CoV-2 variants (evolving largely due to mutations in S protein, NTD and RBD), have in common an increased viral transmissibility, higher infectiousness, immune escape, increased resistance to monoclonal/polyclonal antibodies from convalescence sera/vaccine, and an enhanced virulence; altogether enabling a higher rate of severe disease. Thus, national authorities should continue the genomic surveillance, which might aid disease control, management and prevention efforts.