**1. Introduction**

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) a member of the Coronaviridae family emerged in late 2019 in Wuhan, China, and has caused a global pandemic of acute respiratory disease in all ages of the population, ranging from mild symptoms to mortality [1]. It belongs to the family of coronaviruses (CoVs), sharing

79% of the genome sequence with SARS-CoV and 50% with Middle East respiratory syndrome coronavirus (MERS-CoV), while the rest of its structure is shared with other betacoronavirus [1]. In this enveloped positive sense, single-stranded ribonucleic acid (RNA) with crown-like S-shaped spiked proteins virus [2], there is an incubation period of 1–14 days with a median onset time of 8 days [1]. Transmission of SARS-CoV-2 is predominantly via droplets, aerosol, and airborne pathways [1]. It achieves invasion by using angiotensin-converting enzyme 2 (ACE2) receptors and human proteases as entry pathways, eventually fusing with the cell membranes in the lung [1]. Mild clinical presentations of this virus include fever, dry cough, and pneumonia [3]. Most patients with mild manifestations of this infection recover [3]. More severe cases cause other issues besides respiratory problems, such as injury to the myocardial cells and heart arrhythmias [3]. Other reported problems caused by this virus are in the central nervous system (CNS), gastrointestinal tract (GIT), musculoskeletal system, hypercoagulability leading to stroke, and organ failure [3]. In the most critical cases, end-organ failure and acute respiratory distress have led to death, especially in those with comorbidities such as hypertension and obesity [2, 3].

Coronaviruses continuously evolve due to mutations that occur during the replication of their genome [4]. Variants that emerged throughout the pandemic differ from each other due to one or more mutations, such as the number and location of substitutions in the spike (S) protein that makes each unique [4]. The United States SARS-CoV-2 Interagency Group (SIG) defines four classes of SARS-CoV-2 variants which are: variants being monitored (VBM), variants of interest (VOI), variants of concern (VOC), and variants of high consequence (VOHC) [4]. Thirteen variations in the S protein of coronavirus disease 2019 (COVID-19) have been detected by late November 2020 [3]. On November 30, 2021, SIG classified Omicron as a VOC, replacing the Delta variant [4]. Currently, there is no VOI and the VBM are Alpha, Beta, Gamma, Epsilon, Eta, Lota, Kappa, 1.616.3, Mu, and Zeta [4].

Of the two VOCs, Delta and Omicron, the Delta variant has been shown to cause a more severe illness in the unvaccinated as compared to the vaccinated [5]. November 2021, the Omicron variant was first discovered in Botswana [5]. Omicron was designated as a VOC by the World Health Organization (WHO), which stated that early research suggests that it carries a higher risk of reinfection than other variants [5]. Currently, in the United States, the Omicron variant is the most common [5]. In December of 2020, the Delta variant was first found in India but has since spread across 178 countries [5]. Changes to the S protein may render the Delta variant up to 50% more transmissible than other prior COVID-19 variants, according to research [5].

Significant advances have been made toward "real-time" generation and sharing of SARS-CoV-2 data throughout the pandemic [6]. As a result, a computational tool, Phylogenetic Assignment of Named Global Outbreak Lineages (Pangolin) was developed to assign the most likely lineage to a genome sequence for managing and interpreting the rapid generation and sharing of data worldwide, at either a national or regional level [6]. Hence this nomenclature was developed to name and track global transmission lineages of SARS-CoV-2 [6]. Nextstrain and GISAID focus on the prevalence and persistence of a variant by 'clades' [6]. Thus, the WHO has established a structure for nomenclature using GISAID, Nextstrain, and Pangolin (Pango) nomenclature, so that the scientific community may be able to name and track the variants of SARS-CoV-2 [7].

According to the WHO situation report as of March 22, 2022, there has been a 7% rise in COVID-19 positive cases for the week of March 14–20, 2022, versus the week before [8]. However, there has been a 23% decrease in mortality in comparison to the *Perspective Chapter: SARS-CoV-2 Variants – Two Years Post-Onset of the Pandemic DOI: http://dx.doi.org/10.5772/intechopen.105913*

week before [8]. About 12 million new cases and slightly below 33 thousand deaths have been reported in this period among the six WHO regions [8]. Approximately 468,000,000 COVID-19 positive cases and slightly over 6,000,000 deaths have been disclosed universally as of March 22, 2022 [8].

COVID-19 has become a continuously evolving disease due to rapid changes in the viral variants and despite mitigation strategies such as facial masks, social distancing, hand hygiene, vaccine therapies, and other therapies [2]. One of the earliest VOCs was the Alpha variant (B.1.1.7), which had a high transmissibility rate [2]. Recently, Omicron (B.1.1.529) appears to be at least two times more transmissible than Delta, with Delta variations being 50–70% more transmissible than earlier variants such as Alpha [2]. VOCs remain prevalent, particularly among unimmunized persons [2]. VBM were more problematic earlier in the pandemic and were more notable for high transmission and increased virulence [2]. These include B.1.1.7 (Alpha), B.1.351 (Beta), and P.1 (Gamma), and of lesser concern, Epsilon (B.1.427 and B.1.429), Eta (B.1.525), Iota (B.1.526), Kappa (B.1.617.1), 1.617.3, Mu (B.1.621, B.1.621.1), and Zeta (P.2) [2].

The two other categories described by the WHO include VOI, which are variants that are widely circulating within a population or have the potential to have an impact on a population, and VOHC, which are mutations that elude vaccines and current therapies that are in place. Currently, there are no circulating VOI and VOHC [2]. Given the continuous evolution of SARS-CoV-2, the impact of variants on public health may be reclassified based on their attributes and prevalence. VOCs and VOIs may differ from those of other reporting agencies because of the impact the variants may cause by location. The purpose of this paper is to discuss the genetic lineages of SARS-CoV-2 that have emerged as variants and circulated globally during the 2 years since the onset of the COVID-19 pandemic.
