We have now been living with COronaVIrus Disease for the past one year.
COVID-19 emerged in December 2019, and in March of 2020 was declared a pandemic by the World Health Organization. The devastating effect of the causative SARS-CoV-2 virus has infected millions of humans across 212 countries and led to more than 1.4 million deaths globally as of 24 November 2020. There has been exponential growth in the number of scientific publications related to COVID-19. The pandemic has significantly affected biomedical researchers, by first halting research and then resulting in the concentration of scientific resources toward better understanding the SARS-CoV-2 virus and developing vaccines and therapeutics (Palayew et al., 2020; Zamora-Ledezma et al., 2020) . The advent of genomics technologies and computational approaches has accelerated scientific breakthroughs in the past year. The genome sequence of the virus appeared online on January 10, and within weeks, the structures of several viral proteins were determined. Within months, clinical trials of vaccines and therapeutics began, and positive reports on vaccines are currently appearing. Moreover, an array of drugs approved for other viral infections are being studied for COVID-19 treatment in hundreds of clinical trials worldwide. Here, we review current knowledge related to SARS-CoV-2 gained in the past year, including its progression, pathology, prevention, and therapeutics. We discuss what is currently known about the virus and how far medicine has progressed in the fight against COVID-19.
Coronaviruses (CoVs) are a large group of viruses that infect the upper respiratory tract in humans and cause common cold and flu-like infections. Their name originates from the presence of club-shaped glycoprotein projections (called spikes) that arise from the surface of the viral envelope and impart a crown-like appearance to the viral particles, similarly to the Sun's corona ( Figure 1A ). The CoVs belong to the order Nidovirales of the subfamily Orthocoronaviridae in the family Coronoviridae. All CoVs have zoonotic origin, and cause respiratory and intestinal infections in several animals, including humans. On the basis of genomic organization and phylogenetic relationships, CoVs are classified into four genera: α-CoV, β-CoV, γ-CoV, and δ-CoV. The α-CoVs and β-CoVs infect various mammals (such as bats, cattle, domestic animals, livestock, and humans), whereas the γ-CoVs and δ-CoVs infect avians and sometimes mammals (Woo et al., 2012) .
CoVs were first identified as infectious bronchitis viruses in 1937, infecting avian species and devastating poultry stocks. The strain of CoV responsible for 15-30% of cases of the common cold in humans was identified in 1965 (Bhargava 2020). To date, seven strains of human CoV have been identified as belonging to α-CoVs or β-CoVs.
The four mild strains (HCoV-NL63, HCoV-229E, HCoV-OC43, and HKU1) induce mild respiratory disease of the upper respiratory tract in infants, older people, and immunocompromised individuals. In comparison, the three highly virulent pathogenic strains, severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome CoV (MERS-CoV), and SARS-CoV-2 cause acute respiratory distress syndrome, result in pulmonary failure and fatality in humans (Forni et al., 2016; Masters and Perlman, 2013; Su et al., 2016) . The 
Guangdong province in South China spread to approximately 28 countries and infected nearly 8000 individuals, causing symptoms of fever, headache, and respiratory problems, such as shortness of breath, taking 774 human lives (Bhargava 2020; Zhong et al., 2003) . Ten years later, in 2012, the MERS-CoV outbreak endemic in Middle Eastern countries, particularly Saudi Arabia, affected nearly 2500 people (Bhargava 2020; Wang et al., 2013; Zhong et al., 2003) . Both these β-CoVs are genetically different and have been reported to originate from bats (Cui et al., 2019) . These outbreaks indicate the potential for the emergence of super/lethal strains of human CoVs in the future with high transmission and mortality rates together with a long latency period between infection and detectable symptoms, thus, potentially resulting in devastating pandemics (Patrick) . The viral pneumonia-like disease was first reported at the end of November 2019 near the wet seafood market in Wuhan Province, China. The CoV first reported in Wuhan was described to be a novel virus belonging to the β-CoV category and was designated as 2019 novel coronavirus (2019-nCOV) by Chinese researchers (Shereen et al., 2020) . However, on 11 February 2020, the virus was renamed SARS-CoV-2, and the disease was named COVID-19 (Drosten et al., 2003; Fouchier et al., 2003; Zhong et al., 2003) .
All human CoVs have zoonotic origin and are capable of transmission among mammalian hosts; however, most CoVs originate in bats and are transmitted to humans through domestic animals (Forni et al., 2016; Su et al., 2016) . Thus, bats are considered the natural host and primary reservoir of human CoVs (Cui et al., 2019) (Shi and Hu, 2008; Zheng et al., 2004) . MERS-CoV had camels as the primary zoonotic host and was reported in bats of the genera Pipistrellus and Perimyotis, thus, suggesting that bats are the key reservoir and transmission agent (Cui et al., 2019; Huynh et al., 2012) . The S-gene encoding spike protein is considered the hotspot of genetic recombination among different bat CoV strains (Hon et al., 2008; Huynh et al., 2012; Wu et al., 2016) . Because of the genetic variability in different CoV strains and the frequent genetic recombination between strains, the emergence of novel variants of CoVs was predicted before 2002 (Nagy and Simon, 1997; Rowe et al., 1997) . The progenitors of SARS-CoV evolved through recombination within Rhinolophus bats and then was transmitted to farm civets (Paguma larvata) through fecal-oral transmission; infected civets then transmitted it to market civets, in which the viruses underwent substantial mutation before spreading to the human population in Guangdong, China (Cui et al., 2019) . SARS-CoV-2 is the seventh member of the human CoVs (Corman et al., 2018; Wu et al., 2020b; Zhou et al., 2020) . SARS-CoV-2 was transmitted to humans from infected animals in the market and then spread rapidly throughout the world via human-to-human contact involving exposure to respiratory droplets or aerosols through nosocomial transmission. Studies have indicated that bats are the natural reservoir of SARS-CoV-2, as with other human CoVs (Banerjee et al., 2019; Hampton, 2005; Li et al., 2005a; Wu et al., 2020b; . The first genome sequence of SARS-CoV-2 was compared with the bat CoV RaTG13 and human SARS-CoV, and the SARS-CoV-2 genome has been found to show a nucleotide sequence identity of 96.2% with the bat CoV RaTG13 and 79.5% with human SARS-CoV (Naqvi et al., 2020; Zhou et al., 2020) , thus, indicating a close relationship between bat and human CoVs, and suggesting that both strains might have evolved from a common ancestor strain that diversified through genetic recombination, mutation, and natural selection during transmission. SARS-CoV-2 is thought to have originated in bats via genetic recombination of existing bat CoV strains and to have been transmitted from bats to humans either directly or through unknown intermediate hosts, similarly to the roles of civets and camels in SARS-CoV and MERS-CoV, respectively . Recent data suggest that CoVs capable of infecting humans emerged in bats around 40 years ago and have been circulating undetected since then, thus suggesting bats as the primary reservoir of the SARS-CoV-2 lineage (Boni et al., 2020) . Detailed analysis of the protein sequence data has also indicated the potential of pangolins, turtles, and snakes acting as intermediate hosts of SARS-CoV-2, owing to the presence of ACE2 receptors in the CoVs reported from these animals (Guan et al., 2020) . A recent study has used protein mapping to characterize the protein interaction networks of the three CoVs (SARS-CoV-1, MERS-CoV, and SARS-CoV-2) in humans, thus revealing important molecular mechanisms and potential therapeutic interventions common to the three CoVs (Gordon et al., 2020a) .
SARS-CoV-2 is an unsegmented single-stranded positive-sense RNA virus.
Structurally, it is a spherical or pleomorphic enveloped virus. The genome size of SARS-CoV-2 is ~29 Kb RNA, which is between the genome sizes of SARS-CoV (~28 Kb) and MERS-CoV (~30 Kb) (De Wit et al., 2016; Wu et al., 2020a; Wu et al., 2020b; Zehra et al., 2020; Zhou et al., 2020) . The genome is organized as a 5´-leader-UTRsreplicase-S-E-M-N-3´UTR-poly (A) tail sequence and is characterized by the presence of a variable number (6-12) of open reading frames (ORFs) between conserved genes (ORF1ab, S, E, M, and N) and nine sub-genomic mRNAs, nine transcription regulatory elements and two terminal untranslated regions (UTRs) ( Figure 1B ) .
The 5´ UTR and 3´ UTR are important for RNA-RNA interaction and binding of viral and cellular proteins. The sizes of UTRs differ among SARS-CoV-1, SARS-CoV-2, and MERS-CoV. Two-thirds of the viral genome located at the 5´ end constitutes the first ORF (ORF1a/b); ORF1a and ORF1b contain frameshifts encoding two long polypeptides (pp1a and pp1ab), which after priming and processing produce 16 nonstructural proteins (nsp1-16), which are necessary for genome maintenance. These polypeptides are processed by viral encoded (chymotrypsin-like-3cLpro-/main protease-Mpro-nsp5/Papain like-nsp3) proteases. One-third of the genome near the 3´ end The N protein, in association with genomic RNA, forms the nucleocapsid, which maintains the genome structure inside the envelope and plays significant roles in viral assembly, budding, and the host cellular response to viral infection. The details on the various SARS-CoV-2 proteins and their roles are shown in Table 1 .
The global spread of SARS-CoV-2 has resulted in tens of thousands of mutations in the native strain within a short time. To rapidly share and disseminate the information related to virus genomes, mutations, and evolution GISAID took the initiative involving a public-private-partnership (Elbe and Buckland-Merrett, 2017 ). Subsequently, a CoV-GLUE database was developed to interpret and analyze the SARS-CoV-2 virus genome sequences, with a focus on amino acid sequence variation (http://covglue.cvr.gla.ac.uk/#/replacement) (Singer et al., 2020) . The D614G mutation in the viral S-protein is now the most prevalent globally ( Figure 2 ). However, it may be noted that the D614 prevalent epidemics are still prevalent in many locations when G614 first began to appear. The D614G mutant virus is not associated with increased mortality or clinical severity but is more transmissible among hosts and associated with a higher viral load (Korber et al., 2020; Volz et al., 2020) . It has now been shown that the substitution D614G enhances viral replication in the respiratory tract and increases neutralization susceptibility (Plante et al., 2020) . At present, the SARS-CoV-2 virus has a low mutation rate; however, as the pandemic progresses, it can acquire mutations with fitness advantages and immunological and drug resistance (Callaway, 2020b; Padhi et al., 2020c; Padhi and Tripathi, 2020) .
The S-protein of CoVs forms a trimeric clover-shaped structure that binds a range of hosts and initiates pathogenesis ( Figure 3A ). The S-protein is highly glycosylated and is made up of two subunits, the S1 head and the S2 filament, which protrudes as a clubshaped projection from the viral envelope (Towler et al., 2004) . The receptor-binding domain (RBD) of the trimeric S-protein binds the human angiotensin-converting enzyme 2 (ACE2) receptor, thus, initiating conformational changes that drive membrane fusion ( Figure 3B ) (Tortorici and Veesler, 2019 ). The RBD is responsible for determining the cellular tropism and host range of human CoVs, whereas the S2 subunit, with the specific tandem domain heptads HR1 and HR2, mediates the fusion of the viral envelope with the host cell membrane Yu et al., 2020) . The S1 subunit is differentiated into an N-terminal and a C-terminal domain designated S 1 -NTP and S 1 -CTP, respectively. The RBD is present in the C-terminal domain of the S-protein and mediates specific binding of the S 1 subunit to the ACE2 receptor. In contrast, the Nterminal domain plays a role in transmission between hosts and modulates the host cell endoplasmic reticulum, thus, inhibiting the interferon responses in the host cell (Babcock et al., 2004; Li et al., 2003; Li et al., 2005b; Qu et al., 2005) .
The phylogenetic and genomic similarities among various CoVs, specifically in the gene encoding RBD, render CoVs more amenable to human transmission . Strains of CoV differ in binding affinity for human ACE2, thus, resulting in variation in their infection ability, transmission rate, and pathogenicity. In the past few months, SARS-CoV-2-RBD has undergone several mutations conferring stability to the viral particle (Andersen et al., 2020; Benvenuto et al., 2020; Zhou et al., 2020) . The binding affinity of the S-protein of SARS-CoV-2 for the human receptor is much higher (~4.7 nM) than that of SARS-CoV (~31 nM) and MERS-CoV (~16.7 nM) (Wang et al., 2020c) . A detailed understanding of the mechanism and specificity of the RBD-ACE2 interaction may aid in predicting the generation of potential strains with greater transmission and pathogenicity, and in monitoring future disease outbreaks. In addition, the information should aid in disease management and potentially in designing specific RBD-based therapeutics (vaccines, antibodies, or drugs). Knowledge of the direction of evolution and prediction of the residues whose mutation increases the affinity of RBD/ACE2 interactions may aid in the identification of favorable and unfavorable target residues in the design of new therapeutic strategies (Padhi et al., 2020b) .
The optimization of the binding affinity of the SARS-CoV-2 S-protein for the human ACE2 receptor may also be attributed to the presence of functional polybasic cleavage sites for proteases, such as furin. These cleavage sites, also known as S1-S2 cleavage sites, occur at the interface of the S1 and S2 subunits through the insertion of 12 nucleotides, thus resulting in the acquisition of O-linked glycans at the region flanking the cleavage site (Walls et al., 2020) . The RBD is the most variable part of the genome of CoVs. A seven-residue stretch critical for delimiting the host according to the specific affinity toward the ACE2 receptor differs in various CoVs, and natural selection has promoted optimal binding affinity during the course of host shifting Wrapp et al., 2020; Zhou et al., 2020) . The furin cleavage site at the junction of the S1 and S2 subunits allows for furin-mediated cleavage of the two subunits of Sprotein, and enables the RBD to specifically bind the ACE2 receptor and facilitate the fusion of host cells and subsequent entry of the virus into host respiratory tract cells (Follis et al., 2006; Jia et al., 2005; Zhou et al., 2020) . Moreover, O-linked glycans have a mucin-like domain that hides the antigenic determinant sites of the S-protein of SARS-CoV-2, thus, resulting in immunoevasion (Bagdonaite and Wandall, 2018).
The specific RBD-ACE2 interaction facilitates the attachment and subsequent entry of viral particles into host cells. The entry of the CoVs is caused by furin-mediated cleavage and triggered by cellular proteases, such as transmembrane protease serine 2 (TMPRSS2) and cathepsins present on the host cell surface (Bertram et al., 2011; Glowacka et al., 2011) . A Gln residue in the RBD (Gln479 in SARS-CoV and Gln394 in SARS-CoV-2) interacts with the Lys31 residue of the human ACE2 receptor, thus, forming a strong interaction between the RBD and ACE2 Wu et al., 2012) . After the formation of the RBD-ACE2 ligand-receptor complex, the S2 protein activates the fusion of the viral envelope with the host cell membrane. HR1 and HR2 interact, forming a six-helical bundle that juxtaposes the viral S2 protein with the Cterminus of the ectodomain of the ACE2 receptor Yu et al., 2020) .
Hypertension drugs (such as losartan) that act as ACE2 inhibitors or angiotensin receptor blockers can competitively inhibit the RBD-ACE2 interaction and prevent the entry of the virus into the host cell; therefore, these drugs may be used as therapeutic agents against SARS-CoV-2 (Gurwitz, 2020) . After membrane fusion, the virus uses the clathrin-dependent or independent endocytic pathway and autophagy via lysosomes to enter host cells (Pelkmans and Helenius, 2003; Sieczkarski and Whittaker, 2002) . The endocytic pathway used by SARS-COV-2 is pH-dependent. Lysosomes are characterized by acidic pH, and acid hydrolases play a crucial role in the degradation of the viral envelope inside the autophagosome-lysosome complex, thus, releasing the viral genome for replication, transcription, and translation. Because the degradation of the autophagosome-lysosome complex is pH-dependent, lysosomotropic drugs such as chloroquine and hydroxychloroquine (which neutralizes the pH inside the lysosomal cavity by blocking proton pumps), chlorpromazine (an inhibitor of clathrin-dependent endocytosis), and inhibitors of cathepsins (endosomal/lysosomal cysteine proteases that act in protein degradation) could aid in controlling the entry of viral particles and subsequent infection (Al Bari, 2017; Degtyarev et al., 2008; Inoue et al., 2007; Schrezenmeier and Dorner, 2020; Wang et al., 2008; Yang and Shen, 2020) .
The acidic pH of the autophagosomes aids in the dissolution of the viral envelope and the release of the viral genomic RNA inside the host cell. Because the viral RNA is positive-sense, it can directly enter the replication and transcription cycle; these processes occur at the cytoplasmic membrane and involve coordinated synthesis of continuous and discontinuous RNA. Two-thirds of the 5´-end of the SARS-CoV-2 RNA,
constituting the first open reading frame (ORF 1a/b), has a frameshift between ORF 1a and ORF1a/b that guides the production of two large polypeptides, pp1a and pp1b (de Wilde et al., 2018; Guo et al., 2020) . These polypeptides are then processed by the virally encoded chymotrypsin-like-3CLpro, main protease-Mpro-nsp5, and papain-like-nsp3 proteases into 16 nonstructural proteins (nsps) that constitute the viral replicasetranscriptase complex. This complex consists of helicase (nsp13), RNA dependent RNA polymerase (RdRp, also known as nsp12), and other nsps that are packed in membrane vesicles (Sawicki and Sawicki, 2005) . After ORF1, a series of multiple ORFs are preceded by a short repeated sequence called the transcription regulatory sequence (TRS). Replication and transcription are performed by the replicasetranscriptase complex, which organizes itself in double-membrane vesicles (ER).
ORFs that serve as a template for the production of RNA. The replicase-transcriptase complex, similarly to those in other positive-sense RNA viruses, has RNA-dependent RNA polymerase, RNA helicase, and protease activities. The transcription catalyzed by RdRP toward the 3´-end of genome results in the production of a nested series of many sub-genomic RNAs with leader sequences at the 5´-end. This region produces the structural proteins S, E, M, and N, which are needed for virion assembly; other accessory proteins required for maintenance and replication of the viral genome; and several additional proteins that aid in blocking the host innate immune response (Hussain et al., 2005; Perrier et al., 2019) .
The replicase component of the replicase/transcriptase complex system encoded by ORF 1a recognizes the single-stranded positive-sense genomic RNA and copies it into a complementary strand (genomic minus-strand), which is used as a template to produce multiple copies of the positive-sense genomic RNA through continuous RNA synthesis. The transcriptase produces a nested series of subgenomic messenger RNAs via discontinuous transcription, which are translated into structural and non-structural viral proteins. The viral proteins are assembled at the surfaces of the ER membrane along with the genomic RNA, thus, constituting a mature virion that is enclosed in the vesicles at the ERGIC face of the Golgi complex. The vesicles enclosing viral particles are fused with the internal plasma membrane, bud out by exocytosis, and release vesicle-coated virus particles from infected cells (Perrier et al., 2019) . The released viral particles trigger the host immune system and result in the production of aggressive inflammatory responses, thus releasing an excessive amount of proinflammatory cytokines in a process called cytokine storm (Braciale and Hahn, 2013; Ragab et al., 2020) . The overactivated and abnormal host immune response during SARS-CoV-2 pathogenesis results in the host attacking its own cells and tissues rather than targeting the viral particles (Asrani and Hassan, 2020). The production of proinflammatory cytokines and the resulting excessive inflammatory reactions lead to acute respiratory distress syndrome (ARDS), which can result in lung injury, decreased oxygen saturation, and widespread tissue damage, thus, causing multiorgan failure, unfavorable prognosis and instantaneous death in patients with COVID-19 Ragab et al., 2020; Shimizu, 2019; Thompson et al., 2011) . The increased levels of cytokines facilitate the influx of immune cells, process such as macrophages, neutrophils, and T-cells, from the circulation toward the site of infection; this process has a destructive effect on human tissues by destabilizing endothelial cell to cell interactions and causing lung injury and severe ARDS, thereby leading to low oxygen saturation levels, which are a significant cause of mortality in patients with COVID-19 (Shimizu, 2019) . The patients show lymphopenia with or without leucopenia; the lymphocyte counts decline, thus resulting in high neutrophil to lymphocyte ratios, which are typically associated with the severity of the disease (Singhal, 2020) .
The inoculum of COVID-19 is nosocomial infection, which spreads primarily through respiratory droplets, secretions, and direct contact with the upper respiratory tract (Li et al., 2020b) . SARS-CoV-2 isolated from fecal and blood samples from patients with pneumonia suggests the possibility of multiple transmission routes of the virus . The abundance of ACE2 receptor proteins on the epithelia of lung alveoli and enterocytes of the small intestine supports the existence of multiple transmission routes (Hamming et al., 2004; Zhang et al., 2020) . Epidemiological reports have revealed an incubation period of 1-14 days with a peak at 3-7 days; during the latency period, the virus becomes severely contagious . Patients show symptoms of influenza, sore throat, fever, cough, fatigue, and shortness of breath; in some cases, patients develop gastrointestinal problems, such as diarrhea and vomiting, whereas in severe cases, patients develop ARDS, thus resulting in multiorgan failure and death Huang et al., 2020) . Older people and those with comorbidities, such as diabetes, hypertension, pulmonary disease, asthma, bronchitis, and cardiovascular disorders, are prone to developing ARDS and death . Most people have good prognosis, but those older in age and/or with critical chronic disorders are adversely affected by the disease. Recently, a connection between the virus and neurological problems has been reported (Marshall, 2020; Paterson et al., 2020) . Some people who become ill with COVID-19 develop neurological symptoms including confusion, disorientation, agitation, and even psychosis; however, the underlying mechanisms are unclear (Paterson et al., 2020; Varatharaj et al., 2020) .
Growing evidence suggests that about one in five people infected with COVID-19 experience no symptoms. Such asymptomatic individuals can also transmit the virus as symptomatic people, albeit at a far lesser rate (Nogrady, 2020). Though estimating their contribution to outbreaks is challenging, it is crucial for managing the disease spread (Bi et al., 2020).
Immunization through vaccination is a preferred protection method in public health.
Vaccines are developed to enhance the passive immunity in individuals who do not exhibit the symptoms of the disease against the targeted pathogen, to prevent the occurrence of the disease after exposure to the specific pathogen (Han, 2015) . The development of an effective vaccine is a complex process that requires a long time for successful optimization, production, and clinical trials to establish the vaccine's purity, capability, and efficacy in vaccinated individuals (Black, 2015) . The most challenging issue in developing an anti-SARS-CoV-2 vaccine is achieving efficacy and clinical safety. An effective vaccine promotes the development of the innate immune system with specific instructions for recognizing the antigenic determinant of the causative pathogen (Calina et al., 2020) . In classical viral vaccines, the antigenic determinant epitope or attenuated viral particles are used as vaccine antigens to activate the host immune system to produce appropriate antibodies against the antigenic vaccine, thus, aiding in recognition of the specific viral pathogen upon future exposure. However, vaccine development is a time-consuming and expensive process with high failure rates. To produce a successful commercial vaccine, multiple candidates must be tested, and several years of high-quality research are typically required. Another challenge in the vaccine development process is the rapid development of mutations in RNA viruses in the genes encoding surface glycoproteins, which trigger the antigenic immune response and lessen the effectiveness of vaccines (Kramps and Probst, 2013) .
Moreover, the CoVs appear to have a specific mechanism of being enclosed in vesicles in the host cells that lack the receptors. This process aids in recognition of the invading virus; in the absence of the receptors, the viruses are not recognized by the antibodies developed post-vaccination (Kikkert, 2020) . If the vaccine development process proceeds smoothly from conception to market, an effective and successful vaccine can be ready in approximately 12-18 months (Amanat and Krammer, 2020). Vaccines will be essential to decreasing morbidity and mortality if the SARS-CoV-2 establishes itself in the population. Covisheild is a DNA-based viral vector vaccine that uses a weakened chimpanzee adenovirus, which causes the common cold in chimpanzees. The virus has been modified so that it does not multiply in humans. The DNA encoding the SARS-CoV-2 S-protein was fused to the viral vector and introduced as recombinant DNA. When the vaccine enters a cell, S-protein is produced and induces an immune response against SARS-CoV-2. The third is Pfizer's BNT162 program evaluating four mRNA-based vaccines, all of which are combined with a lipid nanoparticle formulation. Two vaccine candidates include a nucleoside modified mRNA (BNT162b1 and BNT162b2), one includes mRNA of optimized SARS-CoV-2 RBD (BNT162b1), and one uses mRNA of SARS-CoV-2 full-length S-protein (BNT162b2). Other than these nanoparticle-based vaccines, ankara vector-based vaccines, adjuvanted vaccines, inactivated vaccines, fusion-protein based vaccines, recombinant protein, and live-attenuated vaccines are also being developed; however, all these vaccines appear to be many months away from reaching the market Prompetchara et al., 2020; Roper and Rehm, 2009; Thanh Le et al., 2020) . Various candidate vaccines in phase II and III clinical trials are shown in Table 3 .
In mid-November 2020, three vaccine developers reported data suggesting that their vaccines are highly effective. On 9 November, Pfizer and BioNTech released the first compelling evidence that their BNT162b2 vaccine is 90% effective in phase III trials (with more than 38,000 participants) (Callaway, 2020d) . On 18 November, they reported that the vaccine's efficacy was more than 94% in high-risk groups and was consistent across racial groups with no serious side effects (Thomas, 2020) . However, their vaccine presents major challenges in distribution, because it must be stored at -80 C, and two doses are required for effective protection. On 11 November, the controversial Russian Gamaleya Research Institute's Sputnik V vaccine was reported to be 92% effective in preventing COVID-19 in an interim efficacy analysis report (with more than 16,000 participants) (Callaway, 2020c; Ramasamy et al., 2020) . On 16 November, Moderna reported 94.5% efficacy of the mRNA-1273 vaccine in phase III trials (with more than 30,000 participants) (Callaway, 2020a) . Notably, Moderna's vaccine remains stable in conventional refrigerators for 1 month and in ordinary freezers for 6 months.
Both Pfizer/BioNTech and Moderna are planning to apply for an emergency use authorization of their vaccine from the U.S. Food and Drug Administration (FDA) in the coming days. Once approved, these vaccines would be the first mRNA vaccines to get approval for public use. However, recent reports on reinfections have prompted questions about the long-term immunity to SARS-CoV-2 and the prospects for a vaccine (Tillett et al., 2020; .
In the absence of any clinically proven treatment strategy, the management and treatment of COVID-19 is mostly supportive, with the only aim being mortality reduction.
Good hygiene, social distancing, and quarantine practices are recommended worldwide to reduce the transmission of the virus. Several repurposed drugs are being used, including antiviral and antimalarial drugs, as tactical treatments aiming to limit the adverse effects of viral pathogenesis. Antiviral drugs target specific proteins essential for the viral life cycle and disrupt various stages of viral growth. The antiviral drugs used against common RNA viruses include reverse transcriptase inhibitors (nucleoside reverse transcriptase inhibitor and nonnucleoside reverse transcriptase inhibitors) RdRp inhibitors, and protease inhibitors (target TMPRSS2 and other viral proteases that are involved in the processing of large polypeptide coded by ORF1) (Rajarshi et al., 2020) .
However, no evidence-based and clinically demonstrated strategy has been shown for the treatment of COVID-19. Most drugs currently in clinical trials are repurposed drugs designed for the treatment of other diseases (Dong et al., 2020; Gordon et al., 2020b; Guy et al., 2020; Tu et al., 2020) . Several clinical trials are also underway to evaluate the suitability and efficacy of repurposed antiviral drugs, such as remdesivir, ribavirin, galidesivir, favipiravir, darunavir, oseltamivir, and arbidol, for treating COVID-19 (Calina et al., 2020; Dong et al., 2020; Tu et al., 2020) . Several reports have indicated the development of resistance against antiviral therapies, owing to the rapid rate of mutation leading to the emergence of resistance in the viral strain. In such cases, a combination of these drugs, called the highly active antiviral therapy (HAAT) approach, has synergistic or additive effects of antiviral drugs, thus, delaying the progression of disease and increasing the survival rate of the patients. Various candidate drugs in phase II and III clinical trials are shown in Table 4 . The few drugs currently being used for the treatment and management of COVID-19 are discussed below and their structures are shown in Figure 4 .
In an international coalition, the WHO recently conducted a multi-center, openlabel global trial, called the SOLIDARITY trial, to investigate the potential of four repurposed drugs-remdesivir, hydroxychloroquine, lopinavir/ritonavir, and interferon-1a-for the treatment of COVID-19. The trial enrolled approximately 12,000 patients in 500 hospital sites in more than 30 countries. The interim results of the SOLIDARITY trial showed that the drugs had little or no effect on the mortality of hospitalized patients with COVID-19 . However, because the data were not scrutinized and reviewed, several questions on the report remain. The trial design also prioritized broad access, thereby resulting in significant heterogeneity in trial adoption, implementation, controls, and patient populations. Thus, scientists believe that whether any conclusive findings can be drawn from the trial results is unclear. The data also suggested that antiviral monotherapy for moderately to severely ill patients with COVID-19 might not be sufficient (Cao and Hayden, 2020) .
Remedesivir, the most promising antiviral drug being used for COVID-19 treatment, is a broad-spectrum antiviral drug, repurposed for single-stranded RNA viruses (such as SARS, MERS, and Ebola) (Mulangu et al., 2019) . It is an adenosine nucleoside analog prodrug that inhibits the activity of the RdRp enzyme by integrating into newly synthesized viral RNA strands, thus, delaying the chain termination of viral RNA and inhibiting the replication and synthesis of RNA (Saha et al., 2020; Wang et al., 2020a) .
Recent data from a randomized, open-label, phase III clinical trial, including 584 patients with moderate COVID-19 have indicated that a 5-day course of remdesivir has statistically significantly better results than standard care at 11 days of treatment. The clinical importance of this finding is uncertain (Spinner et al., 2020) . Recent data from a randomized, double-blind, placebo-controlled trial of intravenous remdesivir have indicated that this treatment is statistically superior to placebo in shortening the time to recovery in adults (Beigel et al., 2020) . Another nucleoside analog inhibitor, EIDD-2801, has been found to be effective against remdesivir-resistant SARS-CoV; EIDD-2801 decreases the replication and pathogenesis of CoVs similarly to remdesivir (Sheahan et al., 2020) .
Favipiravir is a prodrug with a similar mechanism of action to that of remdesivir. In cells, it is converted into an active phosphoribosylated form (favipiravir-RTP), which binds and inhibits viral RdRP. However, being a guanine analog, it structurally differs from remdesivir (Furuta et al., 2017) . It induces mutations in the viral RdRP complex, thus resulting in a large proportion of nonviable viruses within the total virus population. The drug was approved as the first anti-COVID 19 drug in China in March 2020, after a clinical trial showed promising data with very few adverse effects. The drug causes a rapid decrease in viral load within 4 days and demonstrates up to 88% clinical improvement in patients with mild to moderate COVID-19.
Ivermectin is an anti-parasitic and antiviral drug approved by the FDA for the treatment of parasitic worms, HIV, and dengue . It suppresses host cellular processes by inhibiting nuclear transport by importin α/β1, thus, leading to a decrease in viral replication (Caly et al., 2012) . Ivermectin also inhibits the replication of SARS-CoV-2 in vitro, and a single dose can result in a ~5000-fold reduction in viral RNA at 48 h (Caly et al., 2020) . The drug is also in clinical trials for treating COVID-19. A recent observational retrospective study of patients with COVID-19 has reported significantly lower mortality with ivermectin treatment (Rajter et al., 2020) .
Lopinavir and ritonavir is a combination drug used for the treatment of HIV. These compounds are aspartate protease inhibitors, and therefore, are presumed to inhibit the SARS-CoV-2 3CLpro enzyme (Furuta et al., 2017) . The drugs are effective against SARS-CoV and MERS-CoV in vitro and animal models; however, they have not found to be useful for treating severe COVID-19 in a randomized, controlled, open-label trial in hospitalized adults patients .
Hydroxychloroquine/chloroquine are widely known anti-malarial and anti-autoimmune drugs (Savarino et al., 2003) . Some data suggest that these drugs also inhibit the replication of CoVs (Vincent et al., 2005) and cause a rapid decrease in SARS-CoV-2 viral load. In the initial stages of the pandemic, the drugs were widely used worldwide, because clinical studies reported that the administration of hydroxychloroquine improves the conditions of patients with COVID-19. A further study has confirmed that the combination of hydroxychloroquine with azithromycin is more effective than hydroxychloroquine alone (Arshad et al., 2020) . Emerging preclinical and clinical data indicate that neither chloroquine nor hydroxychloroquine provides clinical benefit against COVID-19 (Fatima et al., 2020; Funnell et al., 2020) . The (Randomised Evaluation of COVid-19 thERapY) trial on hydroxychloroquine, which enrolled more than 15,000 patients from 175 NHS hospitals in the UK, has concluded that there is no beneficial effect of hydroxychloroquine in patients hospitalized with COVID-19 . In June of 2020, the FDA confirmed that hydroxychloroquine/chloroquine alone is not effective in treating COVID-19, and their risks may outweigh their potential benefits. Therefore, the FDA revoked the emergency use of hydroxychloroquine/chloroquine to treat COVID-19.
Arbidol (umifenovir), a broad-spectrum drug, has been found to be effective against several viral diseases, such as influenza, adenovirus, respiratory syncytial virus, and Ebola, and has been used for decades in China and Russia for treating respiratory viral infections (Blaising et al., 2014; Boriskin et al., 2008) . It is currently being repurposed for the treatment of COVID-19. Arbidol is approved in China for the clinical treatment of COVID-19. In the influenza virus, it binds the cell surface glycoprotein hemagglutinin and prevents the fusion of the viral membrane with the receptor (Kadam and Wilson, 2017) . Arbidol targets the S-protein/ACE2 interaction, thereby inhibiting viral entry by interfering with viral binding to host cells (Padhi et al., 2020a; Wang et al., 2020d) . It also inhibits postentry stages by blocking intracellular vesicle trafficking (Wang et al., 2020d) . Clinical trials comparing the efficacy of arbidol with favipiravir are also being conducted (Chen et al., 2020a) . Some patients have recovered successfully from COVID-19 after receiving a combination arbidol and lopinavir/ritonavir treatment Wang et al., 2020e) . Arbidol monotherapy has been found to be superior to lopinavir/ritonavir as well as favipiravir (Chen et al., 2020a) .
Corticosteroids, such as dexamethasone, are glucocorticoid receptor agonists and immunosuppressive agents. On the basis of data from the RECOVERY trial, the WHO REACT (Rapid Evidence Appraisal for COVID-19 Therapies) working group has reported that critically ill patients with COVID-19 are less likely to die when administered corticosteroids than people who do not receive this treatment . A meta-analysis that pooled data from seven randomized clinical trials conducted in 12 countries has indicated that the systemic administration of corticosteroids, such as dexamethasone, to critically ill patients with COVID-19 is associated with lower mortality, thus suggesting that steroids may be used as a part of the standard treatment for people with severe COVID-19 . However, the safety and efficacy of corticosteroids for the treatment of COVID-19 have not been rigorously studied.
Beyond the above-discussed drugs, several others are being tested for their ability to treat COVID-19, such as ribavirin, fedratinib, oseltamivir, darunavir, sofosbuvir, clevudine, brensocatib, truvada, galedesivir, nafamostat, lianhuaqingwen, and baricitinib (De Clercq, 2004; Elfiky, 2020; Koren et al., 2003; Li et al., 2020a; Runfeng et al., 2020; Sarzi-Puttini et al., 2020; Velavan and Meyer, 2020; Wang et al., 2020b; Wu and Yang, 2020) .
A potential alternative, but a reliable approach to ameliorate the adverse effects of COVID-19 aims to boost and strengthen the immune system, thus, allowing the adaptive immune component to be neutralized and combatting SARS-CoV-2. In this approach, several natural product-based formulations are used in the management of the disease, given that herbal medicines have the potential to treat COVID-19. Several herbal formulations have been recommended for the management of COVID-19, such as Indian giloy (Tinospora cordifolia) and ashwagandha; these have shown to be effective in the management COVID-19 in India. Giloy/amrita in the Indian system of medicine has substantial therapeutic properties and is recognized for its immunomodulatory properties and ability to boost the nonspecific immune system by acting on macrophages (Kapil and Sharma, 1997; Nair et al., 2006; Sharma et al., 2012) . Withania somnifera, commonly known as ashwagandha/Indian ginseng, has been recognized in Ayurveda for its excellent antioxidative, anti-inflammatory, and immunomodulatory activities (Bharti et al., 2016) . Withanone, the active ingredient of ashwagandha, has been shown to be a potent phytochemical for controlling SARS-CoV-2 entry into the host cell; withanone can dock effectively at the interface of the ACE2-RBD complex, thus, decreasing the RBD-ACE2 interaction (Balkrishna et al., 2020) . Withanone obtained from Withania somnifera and caffeic acid phenethyl ester obtained from bee propolis have the potential to effectively interact with the substratebinding pocket of viral main protease and block the RNA replication machinery of SARS-CoV-2. Moreover, these natural compounds also modulate the host surfacebound protease TMPRSS2, thereby inhibiting viral entry into host cells .
The COVID-19 pandemic has sparked a research revolution, as scientists are working at breakneck speed to understand the disease and find a cure. Scientists have demonstrated how rapidly they can adapt to re-emerging and emerging threats. Non-COVID-19 research has taken a back seat during the pandemic. The past year has seen rapid advances in understanding of the biology, pathogenesis, and clinical characteristics of the disease. There are still several unknowns about the mechanisms of SARS-CoV-2 biology and pathogenesis. Epidemiologists are predicting short-and long-term projections to prepare for and potentially mitigate the spread and impact of the disease. However, the pandemic has exposed our susceptibly to microbial pathogens and gaps in our therapeutic arsenal. COVID-19 is not the first pandemic that humanity has faced, and it will not be the last. However, it has taught us how unprepared the world was for such an outbreak, and provided us with lessons in preparing and responding to virus warnings in the future. We hope that the continued research will lead to breakthroughs and help us be better prepared for future outbreaks.
The authors declare that there are no funding sources to be acknowledged. forms a complex with NSP10; involved in the S-adenosyl-Lmethionine cap methylation of mRNA NSP11
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