A variety of unique genetic markers that are associated with changes in receptor binding, reduced neutralization with previously generated antibodies. They are also correlated with reduced therapy effectiveness, possible diagnostic impacts, or expected increased transmissibility. Variants of concern might require suitable measures for public health, e.g., enhanced sequence surveillance, enhanced laboratory characterization, or epidemiological surveys to determine the ease with which the virus is transmitted to others. The seriousness of the disease, therapeutic effectiveness, and whether vaccines currently approved are protective are major concerns of public health [17, 19]. The epidemiological update by WHO, as of 31-May 2021, described seven variants of interest (VOIs). These VOIs are B.1.427/B.1.429, P.2, B.1.525, P.3, B.1.526, B.1.617.1 and, C.37.The current variants of interest which are circulating worldwide are shown in Table 1 and Fig. (2).

Variant shows an increase in transmissibility, more severe disease (e.g., increasing hospitalizations or death), a substantial decrease in antibody neutrality produced during pre-infection or vaccination, decreased efficacy or failure to identify diagnoses, etc. Any variant of concern might require appropriate public health action, such as notification of vaccine efficacy to the WHO under the International Health Regulations, reporting to CDC, local or regional efforts to control spreading, etc. Additional factors may also include in order to create new diagnostics measures or modify vaccinations or therapies based on the characteristics of this variant [19]. The first variant of concern, the B.1.1.7 (Alpha), was obtained in the United Kingdom on 18-December 2020, followed by B.1.351 (Beta) in South Africa. On 11-January 2021, the P.1 (Gamma) variant of concern was reported in Brazil. More recently, the new variant B.1.617.2 (Delta) was reported in India in early October 2020 and is considered VOC on 11-May 2021. All four said variants of concern (Alpha variant, Beta variant, Gamma variant, and delta variant) have amino acid substitution in the N-terminal domain (NTD) and the receptor-binding domain (RBD), whereas N501Y mutation was common in all the variants. The current variant of concern that is circulating worldwide is listed in (Table 2 and Fig. 1)

Ascorbic acid, commonly termed vitamin C, is a water-soluble vitamin that humans and other primates cannot synthesize. Vitamin C, due to its multiple roles in many metabolic pathways, contributes to immune-modulating effects. It is a potent antioxidant because it reduces damaging ROS. It can forage many reactive oxidants species, like superoxide radicals, peroxyl radicals (aqueous), etc. It can restore the active forms of the essential membrane antioxidants, including glutathione and vitamin E (tocopherols), hence acts like a co-antioxidant [26, 27]. It has been observed that high ascorbate levels have stimulatory effects on neutrophil mobility. The mechanism is linked with the inhibition of the MPO/H₂O₂/Halide system, preventing the neutrophil membrane's auto-oxidation. Studies have suggested that ascorbic acid results in an enormous proliferation of natural killer cells. Due to its immune-modulating functions, it was tested in patients and showed positive results since it reduced mortality and symptoms [28].

Vitamin D, which plays various roles in the body, acts on both innate and adaptive immunological responses. Vitamin D boosts the innate immune system by upregulation of antimicrobial peptides, including cathelicidin and defensins [29]. While vitamin A influences epithelial integrity, it plays a part in developing the epithelium as a barrier function; it also regulates the differentiation, maturation, and activity of macrophages and neutrophils to boost killing and phagocytosis [30]. The research indicates that zinc, vitamin D, and calcium are essential components of the immune system and perform synergistic functions at different phases of host defenses, like preserving the integrity of biological barriers and the functionality of cells that comprise the innate and adaptive systems [31]. Vitamin B6 and B12 directly impact leucocytes and CD4+ T cells. These cells play a critical role in establishing an adaptive immune response to a wide range of pathogens. Furthermore, folic acid promotes T-cell proliferation [32]. Each of these vitamin and mineral intakes might enable recovery from COVID-19 infection. Yet, there is still a scarcity of preclinical and clinical research on vitamins and minerals in the treatment of COVID-19 [33]. Large doses of all these nutrients demonstrated beneficial outcomes during COVID-19 infection, and due to their low level of risk, these are an excellent supplement to patient care [29].

Remdesivir (GS-5734) is an experimental nucleoside analog approved by US-FDA (Food and drug administration, USA). It is a broad-spectrum antiviral monophosphoramidate prodrug which is an adenine derivative formulated by Gilead Sciences [54]. Remdesivir (RDV) has the potential to inhibit the replication of coronavirus pathogens, like MERS-CoV and SARS-CoV [54]. Both these coronaviruses are structurally similar to SARS-CoV-2, which led to the usage of RDV in treatment [55, 56]. RNA Dependent RNA Polymerase (RdRp) is a major enzyme involved in genomic replication. RdRp is usually found conserve in the members of viruses in a family, making it an ideal target for antiviral therapy. Once the virus invades the host cell, it uses its viral genomic RNA as a template for the synthesis of RdRp, which is required for protein synthesis of the host cell. RdRp further completes transcription of sub-genomic RNA, viral genomic RNA, and structural protein-related mRNA [57] (Fig. 2). Such a key role of the polymerase makes it an ideal target for broad-spectrum antiviral drugs. RDV enters the host cell in the form of a prodrug and gets converted to RDV-Monophosphate. Its active form, RDV-triphosphate, is similar to ATP (adenosine triphosphate) and competitively binds to the RdRp of viruses with similar efficacy. Under the US FDA’s compassionate use guidelines, this drug was initially allowed to be administered in a US patient and showed successful treatment [58]. Using the data obtained from ACTT (Adaptive COVID-19 Treatment Trial), EUA (Emergency Use Authorization) has been given to Remdesivir [59]. Currently, 200 mg of Remdesivir can be given intravenously on day 1, followed by 100 mg daily for nine days. RDV can decrease viral load in the lungs and alleviate clinical symptoms and also respiratory function. These findings support the use of this medication to treat severe human coronavirus infections [60]. RDV outperformed other antivirals in reducing SARS-CoV-2 infection (EC50 = 0.77 M) such as ribavirin (EC50 = 109.5 M), penciclovir (EC50 = 95.96 M), nitazoxanide (EC50 = 2.12 M), chloroquine (EC50 = 1.13 M), hydroxychloroquine (EC50 = 0.77 M), and favipiravir (EC50 = 61.9 M) [61].

Fig. (4). Action mechanism of Baricitinib, Imatinib, Ruxolitinib, and Hydroxychloroquine. Baricitinib is an anti-inflammatory JAK inhibitor that suppresses the cytokine storm that occurs due to high cytokine release. The antiviral effect of baricitinib is seen due to its affinity to AP-2 protein-AAKI, followed by inhibition of SARS-CoV-2 endocytosis. Imatinib attenuates pro-inflammatory molecules like IL-6, NF-kB, and TNF-alpha, which are key molecules involved in the progression of the cytokine storms. Ruxolitinib acts as a Jaki and inhibits JAK. Hydroxychloroquine inhibits terminal glycosylation of the ACE2 receptor that reduces the binding efficiency between ACE2 on the host cells and the SARS-CoV/SARS-CoV-2 spike protein. (Created with biorender.com).

A few of the side effects of the drugs are diarrhea, abnormal liver function, rashes, low blood pressure, nausea, and vomiting [56]. The authors suggested that studies on the use of RDV in the therapies of COVID-19 are constrained by the limited size of the group, the short period of follow-up, possibly incomplete data given the nature of the program, a lack of details on patients treated at baseline, and the lack of a randomized control group [62-63]. A meta-analysis comprising three randomized controlled trials with a total of 1691 patients showed that remdesivir enhanced the probabilities of hospital release; however, remdesivir therapy did not lower the likelihood of death in COVID-19 patients. The findings show that remdesivir is beneficial at slowing illness development and hastening patient recovery, especially when given early in the illness stage. However, on the other hand, it may be less beneficial in improving recovery periods in severely sick patients [62].

Toyama Chemical Co., Ltd. developed favipiravir by exploring a chemical library for antiviral activity towards the influenza virus [64]. Favipiravir is a pyrazine carboxamide derivative that selectively inhibits the virus's RdRp and prevents replication (Fig. 2). The phosphoribosylated form of favipiravir, favipiravir-RTP (favipiravir-ribofuranosyl-50-triphosphate), inhibits the RNA polymerase enzyme. HGPRT (human hypoxanthine-guanine phosphoribosyl-transferase) enzyme has been observed to play a vital role in converting favipiravir to its active phosphoribosylated form [65]. The catalytic domain of RNA polymerase recognizes this dynamic form of favipiravir and favipiravir blocks its catalytic activity.

No toxic effect of this drug is seen in mammalian cells and it does not affect their nucleic acid synthesis [65]. This drug is completed its phase-III clinical trial in Japan and is undergoing a phase-III clinical trial in the USA. In March 2020, Favipiravir was approved for the treatment of COVID-19 patients in China [56]. According to Fujii and colleagues, favipiravir therapy is related to the early reduction of fever in COVID-19. The timely start of favipiravir medication may reduce the length of COVID-19 treatment [66]. Udwadia et al. concluded that early oral favipiravir treatment might shorten the length of clinical manifestations in people with moderate COVID-19, as seen by a considerably shorter time of clinical cure [67-68]. Consistent with the previous findings, Hassanipour et al. found in their meta-analysis a drop in all-cause fatality in individuals who took Favipiravir compared to those who received control medications. They also noted that Favipiravir treatment had minimal acceptable side effects such as nausea, vomiting, diarrhea, and increased serum transaminases [67]. However, several studies have demonstrated that favipiravir exposures decline over time, rendering dosage difficult [69].

Ruxolitinib, also known as Rux or by its trade name Jakavi, is a highly effective and potent selective inhibitor of Janus Kinase (JAK) 1 and 2. It also inhibits the action of JAK-3 and Tyrosine Kinase-2 with comparatively lower potency [70] (Fig. 4). Approved by the European Union (EU), this drug is utilized in the treatment of diseases like PV (Post-Polycythemia Vera) and PMF (Primary Myelofibrosis) [71]. Rux shows high efficacy due to its broad anti-inflammatory properties against MPN (Myeloproliferative neoplasm), which plays a crucial role in causing cytokine storm due to inflammation-causing cytokines and interleukins [72]. In COVID-19, severely affected patients develop ARDS (acute respiratory distress syndrome), which ultimately causes multiple organ failures linked with the release of pro-inflammatory cytokines. The condition is addressed as a cytokine storm. Upon administering a cumulative dosage of Rux of 135mg per day for nine days, a reduction was seen in CIS score in COVID-19 patients. The motive was to administer Rux for a short-term duration which could help balance the cytokine storm without completely suppressing the immune system, which may cause reactivation of the virus. The end goal of this treatment is to stop or prevent further damage to the organs in severely affected COVID-19 patients. A pilot study concluded that in an initial group of patients impacted by COVID-19, though not in extreme respiratory distress needing invasive ventilation, the use of Rux as a compassionate treatment reduces the hyper inflammatory state, leading to more efficient ventilation and faster recovery [73].

Imatinib, a tyrosine kinase inhibitor drug, is a US-FDA-approved [74]. It is used to treat a tumor in CML (chronic myelogenous leukemia) and gastrointestinal stromal tumors. This drug functions by targeting BCR-ABL fusion protein [75]. It also inhibits C-KIT, platelet-derived growth factor receptor, and ABL-1/2. Imatinib has immunomodulatory properties and attenuates pro-inflammatory molecules, like IL-6, NF-kB, and TNF-alpha, which are key molecules involved in the progression of cytokine storms. The primary mechanism of action for imatinib can be seen in Fig. (4). It reduces pulmonary edema, prevents any histological damage, and improves endothelial barrier dysfunction [76]. Due to these characteristics, imatinib was considered as a potential drug that could help alleviate ARDS and prevent multiple organ failure in severely affected COVID-19 patients [77, 78]. It has also shown antiviral activity against MERS and is expected to treat SARS-CoV-2. In SARS-CoV-2, the virus is thought to undergo cell fusion with the help of ABL-2. Imatinib functions by blocking ABL-2 and prevents the spread of infection. This treatment is believed to be very effective when administered early in the stages of the disease. A recent trial published in The Lancet Respiratory Medicine found that imatinib does not shorten the time to cessation of supplementary oxygen and mechanical ventilation in hospitalized COVID-19 patients. However, the decline in fatality suggests that imatinib may provide a therapeutic effect in individuals with COVID-19, justifying additional research [79]. WHO will put three known drugs to the test, one of which is imatinib. It may tackle both the coronavirus and inflammation, prevent viral penetration into human cells, and decrease the release of pro-inflammatory molecules known as cytokines [80].

Baricitinib is an orally taken JAK inhibitor that is JAK1 and JAK2 specific [81]. It is reported to suppress the cytokine storm due to high cytokine release. Hence, it can be regarded as an anti-inflammatory. Initially, this drug was licensed to treat RA (rheumatoid arthritis). The antiviral effect of baricitinib is observed due to its affinity to AP-2 associated protein-AAKI. AAKI plays a significant role in SARS-CoV-2 endocytosis, inhibited by baricitinib [82] (Fig. 4). A trial with a limited number of patients was conducted, which was funded by Azienda-USL, Toscana Centro Committee for off-label drug use. Total 4mg of baricitinib/day was orally administered for 14 days along with Lopinavir/Ritonavir. No severe AE was observed in the patients after baricitinib treatment. No indications of bacterial, opportunistic infection, trombone-felitis, or hematologic toxicity were observed. In the case of COVID-19 disease, baricitinib has been identified as a promising anti-inflammatory drug. According to a recent study in Bangladesh, 8 mg daily oral intake of baricitinib for 14 days resulted in earlier stabilization of respiratory function, less need for ICU and intubation assistance, a lower 30-day death rate, and a lower 60-day re-hospitalization level compared to a dose of 4 mg/day [83]. When corticosteroids are not an option, the National Institutes of Health (NIH) panel advises using baricitinib in conjunction with remdesivir for treating COVID-19 patients that are hospitalized, non-intubated, and need oxygen support. The panel's revised guidelines for using baricitinib based on COV-BARRIER results, a multinational, randomized, placebo-controlled trial [81].Overall, the findings associated with an in vitro study results show that baricitinib acts when a significant cytokine release occurs, implying that therapeutically, this treatment may be beneficial before the development of an aggravated immune response. Baricitinib-treated COVID-19 patients consistently had lower pro-inflammatory cytokines (IL-6, TNF-, and IL-1) than baseline [84].

Ivermectin is a US FDA-approved broad-spectrum anti-parasitic drug. In vitro experiments with ivermectin have shown antiviral activity against a wide range of viruses [85]. Initially, ivermectin was reported to inhibit the import of host and viral proteins across the nuclear membrane [86]. It inhibits the interaction between human immunodeficiency virus I (HIV-1) and importin (IMP) α/β1 heterodimer. It is responsible for the nuclear import of integrase protein, as shown in Fig. (3). Later on, its ability to stop the replication of HIV was confirmed. In terms of SARS-CoV-2 infection, ivermectin is expected to target IMP α/β1 heterodimer by destabilizing it. It also prevents its binding to the viral protein, inhibits the entry of the viral protein into the nucleus, and lowers its ability to shut down antiviral response.

Critically ill SARS-CoV-2 patients are recommended a 150 μg/Kg dosage, which showed reduced mortality rate and resource requirement in the treatment. Few pharmacokinetics problems associated with ivermectin are cytotoxicity and low solubility. To overcome this, many liposomal systems and ivermectin nano-carriers are used in cell lines. They lowered cytotoxicity concerning the free ivermectin molecule [87]. The golden hamster model findings showed that IVM exerts anti-inflammatory activity in trial COVID-19 infection [88]. Preliminary research has found a propensity to reduce viral loads in the ivermectin cohort, as well as an ability to limit IgG titers, which may represent milder illness and clinical improvement in COVID-19 core symptoms linked with tissue injuries such as anosmia/hyposmia and coughing. These findings are consistent with evidence from studies in other countries that indicate quicker viral clearance in treated subjects [89]. However, the medicinal safety of IVM usage in kids, the aged, and pregnant women is currently relatively restricted. Further studies are urgently needed to optimize dose according to patient condition, age group, and risk assessment, particularly considering ivermectin's propensity for neurological, hepatotoxicity, and drug resistance/tolerance [90].

Azithromycin, a macrolide antibiotic, earlier exhibited promising effects on patients of COVID-19. Its anti-inflammatory, antiviral, immunomodulatory, and antimalarial characteristics made it especially well-suited not only to prevent this novel, highly contagious virus but also to alleviate the potentially fatal symptoms associated with its infection [91]. However, according to the Randomized Evaluation of COVID-19 Therapy (RECOVERY) study, azithromycin cannot be considered an appropriate treatment for COVID-19 hospitalized patients. Referral of azithromycin is not linked to a decline in deaths and length of hospital stay [92]. Sivapalan et al. reported that the combination of azithromycin and hydroxychloroquine did not improve the chances of survival [93]. Data from the UK-based, randomized trial (PRINCIPLE) indicate that, even after probable antiviral and anti-inflammatory characteristics, azithromycin is ineffective in treating COVID-19, even though it was previously used during the disease [94].

One type of vaccine is the whole virus vaccine that employs inactivated full virus or lives attenuated virus, representing a conventional strategy for viral vaccinations [112]. This platform is preferable due to its inherent immunogenicity and capacity to stimulate Toll-like receptors; however, safety concerns are still present in this type of vaccination strategy due to inefficient inactivation or attenuation of the viruses. Coronavirus vaccines are mainly an issue. Furthermore, an increased infectivity rate after immunization with this type of vaccine is observed in some findings [113].

This platform is considered safe, effective, and potent because it does not include whole virulent virus but consists only of antigens that might be engineered peptides or proteins, expressing only a part of the immunogen. Many structural proteins, including spike (S), envelope (E), and others, can generate immune response since they can generate NAbs by acting as an antigen, expressed by SARS-CoV-2.

This platform is based on immunizing with nucleic acid-DNA or mRNA [112]. This platform offers significant flexibility in regards to antigen manipulation and high-speed development possibility. MRNA-based vaccine, mRNA-1273 by Moderna, completed phase 3 of clinical trials and showed vaccine efficacy of 94% [114]. Inovio Pharma International Vaccine Institute came up with a DNA vaccine candidate called INO-4800, which demonstrates a perfect safety profile and its vaccination generates both humoral and cellular responses, encouraging its continued development to prevent infection, disease, and death in the global population. Because of its high safety profile, it could become a favored vaccine choice for high-risk groups such as the elderly and those living with co-morbid conditions [115, 116]. BNT162b2 mRNA Vaccine by Pfizer-BioNTech completed its phase III clinical trial and was approved for emergency use in many countries. It is an mRNA vaccine encapsulated by lipid nanoparticles that encode the full-length spike protein of SARS-CoV-2. It is shown that two doses of BNT162b2 were safer over two months, delivering 95% efficiency against symptomatic SARS-CoV-2 infection in individuals 16 years or older [117].

This platform includes the usage of live viruses genetically modified to hold different pathogenic qualities, evoke an immune response, and code additional proteins [118]. This platform effectively generates immune response since it infects cells and survives in the body for a great deal of time, and can directly respond to the antigen-presenting cells [119]. The gene that encodes the spike protein may convey to the host cell via viral vectors. It exploits the viral vector’s ability to infect and transfer mRNA into the host cell.

The Indian government has constituted the task force, National Expert Groupon vaccine Administration for Coronavirus Disease, to provide guidelines on vaccine administration in India [124]. Based on the NEGVAC, the vaccines for COVID-19 will be administered first to frontline workers, healthcare workers, and individuals above 45 years, followed by individuals younger than 45 years that correlated with comorbidities. The government of India has set up a committee consisting of a panel of experts from various specialties, including nephrology, cardiology, oncology, and pulmonology, to decide the clinical criteria, based on which individual with comorbidities be prioritized first for vaccination.

The committee set up by the Indian government has recommended that individuals with kidney diseases, heart diseases that lead to hypertension, cancers, and sickle cell anemia should include in the priority list. The Indian government has decided to procure 600 million doses of the SARS-CoV-2 vaccines from vaccine manufacturers, like Pfizer, Moderna, etc. The government of India procured covishield, manufactured by Serum Institute of India, and covaxin manufactured by Bharat Biotech Ltd to administrated initially. Procuring of COVID-19 vaccine is the first requirement, but the distribution and vaccination to India's large population represent a considerable logistic challenge. The covishield manufactured by SII, a Pune-based company, was approved for limited (emergency) use by DGCI on January 2, 2021. Initially, the vaccine was approved in two doses 5-12 weeks apart, but after review by the expert committee, the period increases to 48-84 days.

On January 2, 2021, the DGCI's subject expert committee also approved Bharat Biotech's COVID-19 Vaccine “Covaxin” for emergency use in India. Covaxin has been recommending to be inoculated under clinical mode only. This vaccine was approved based on phase 3 immunogenicity data from 24K volunteers after the Ist dose of covaxin and 10K volunteers after the 2nd dose of covaxin. However, on March 3, 2021, the phase III clinical trial results of covaxin showed that it is 80.7% efficient in preventing SARS-CoV-2 infection. After review, the subject expert committee of DGCI gave authorization for emergency use and so that covaxin is no longer administered in clinical trial mode only.

Some vaccines are under development and, some are under production in other parts of the world that require storage conditions as low as -70^oC but the vaccines produced by India for distribution require a storage temperature of 2-8^oC only. The Indian government was constantly working on the logistics for the fast and effective distribution of the vaccines manufactured by Indian companies. The manufactures airlift the COVID-19 vaccines in the cold storage boxes with a digital temperature setup to four depots in Mumbai, Karnal, Kolkata, and Chennai. The government used the insulated van to transport COVID-19 vaccines to the designated places in the country.

The general concern with the vaccination process, also making it a significant challenge in corona viral infection, is that it might not induce long-term immunity so that reinfection might be possible. Vaccine success depends heavily on the vaccine target and platform. Hence choosing a suitable vaccine platform from all the available options, like nucleic acid, whole virus, subunit, or recombinant vector, is incredibly challenging [125, 126]. Finding undesired immune-potentiation that results in eosinophilic infiltration poses another challenge following test infection after immunization with vaccines [113]. The immune-escape variants have sparked questions about vaccine efficacy as the world ramps up SARS-CoV-2 immunization. COVID-19 vaccines have shown up to 95% effectiveness in reducing clinical events and up to 100% efficacy in preventing acute illness or hospital admission in environments with pre-existing variants. New variants that evade both normal and vaccine-induced immunity have raised questions about whether vaccines successfully prevent mild and severe COVID-19. A correlation of immunity against mild and severe SARS-CoV-2 infection would go a long way toward providing evidence for these decisions and overcoming the challenges that new variants are putting in the form of global SARS-CoV-2 regulation through widespread implementation of successful immunization [127].

Fig. (6). Development of a crRNA and its mode of action against known coronaviruses [130]. (Created with biorender.com).

The short-term durability is likely to be adequate for slowing the spread of SARS-CoV-2 and resuming normalcy. However, the global spread of SARS-CoV-2 and the emergence of S protein variants and other variants across the globe can restrict vaccine efficacy. SARS-CoV-2 eradication from the population may be difficult due to concerns of unvaccinated and immunocompromised individuals. Since different mRNAs containing mutant S proteins can be rapidly synthesized and included within the LNP (lipid nanoparticle delivery system) carrier, the mRNA vaccine formulation is suitable for repeated or modified vaccination. In comparison, the AdV (adenovirus vector system) vector formulation induces AdV-specific immunity, which may restrict the effectiveness of repeated boosters due to immune-mediated vector clearance [128]. Vaccine development is an expensive process and usually takes years for it to be licensed [129]. Conducting integrated clinical trials, licensing safe and reliable vaccines in a transparent and accountable way, assessing effectiveness throughout (and after) vaccine deployment, providing equitable vaccine access globally, and so on are some of the additional problems that must be addressed [111].