Evidence and speculations: vaccines and therapeutic options for COVID-19 pandemic

ABSTRACT

A novel coronavirus (2019-nCov) emerged in China, at the end of December 2019 which posed an International Public Health Emergency, and later declared as a global pandemic by the World Health Organization (WHO). The International Committee on Taxonomy of Viruses (ICTV) named it SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus-2), while the disease was named COVID-19 (Coronavirus Disease- 2019). Many questions related to the exact mode of transmission, animal origins, and antiviral therapeutics are not clear yet. Nevertheless, it is required to urgently launch a new protocol to evaluate the side effects of unapproved vaccines and antiviral therapeutics to accelerate the clinical application of new drugs. In this review, we highlight the most salient characteristics and recent findings of COVID-19 disease, molecular virology, interspecies mechanisms, and health consequences related to this disease.

KEYWORDS:

• SARS-CoV-2
• covid-19
• genome
• therapeutic options
• vaccine development
• human health

Introduction

Until 2002, human coronaviruses were not widely known to be pathogenic to humans, however, some strains only caused mild infections in immunocompetent individuals¹. The devastating epidemic of Severe Acute Respiratory Syndrome (SARS) caused by SARS-CoV in 2002–2003,^1,2 and Middle East Respiratory Syndrome (MERS) caused by MERS-CoV in 2012,³ circulated around the world, caused hundreds of mortalities, and posed billions of dollars of economic burden worldwide.⁴ We could learn from the MERS-CoV epidemic that new and lethal CoVs can emerge anywhere and can cause epidemics or pandemics.^4,5 However, the precautionary measures were not taken seriously, and recently (December, 2019), the world encountered another devastating outbreak of Coronavirus Disease-19 (COVID-19)⁶

Since the emergence of COVID-19 infection, several cases of atypical pneumonia of unexplained etiology were reported in Wuhan, Hubei province, China at the beginning of January 2020^7. It was first isolated from the viral metagenomics of bronchoalveolar-lavage (BAL) samples of patients presenting pneumonia-like symptoms in Wuhan.⁸ Later on, January 30, 2020, novel coronavirus (2019- nCoV) was positioned under the Public Health International Emergency category by the World Health Organization (WHO) due to its rapid spread and high mortality and morbidity rates.⁶ The International Committee on Taxonomy of Viruses (ICTV) named this virus as SARS-CoV-2 based on its similarity with SARS-CoV.⁶ It was not known until Dec 31st, 2019 when the WHO was informed and cases were finally reported and entered into the Chinese National Health Database. Although the outbreak in China appears to be in control, this highly contagious disease is now circulating across the globe, with a daily increase in the number of deaths and infected cases and the number of affected countries. COVID-19 disease has already created havoc situation by infecting millions of individuals and causing death of hundreds of thousands of people worldwide.

The addition of information on a daily basis, due to the COVID-19 pandemic, to compile the knowledge and provide facts and evidence of the information speculated earlier is highly important. The present and most probably future emergence of such zoonotic pathogens, their economic impact, and lack of impelling therapeutic strategies have now made it clear that our preparedness to cope or treat the CoV pandemic is very limited. Nevertheless, immediate sequencing of the SARS-CoV-2 genome, development of predictive models, and phylogenetic tracing analysis allowed a better understanding of the virus biology, its rapid monitoring, and response compared to earlier Human CoVs epidemics.¹ This review article highlights the current status, recent findings, and scientific evidence, especially epidemiological features, entry mechanism, pathophysiology, antiviral therapies, health consequences, and research progress of continuously evolving COVID-19.

Emergence of COVID-19

Both the SARS-CoV-2 and bat CoV closely resemble each other. The bat is considered as the primary source, while the intermediate host that may exist between bats and humans is still unclear^9. Recent studies have shown that for SARS-CoV-2-like CoVs Pangolin can be the natural reservoir, and might be the possible intermediate host between bats and humans. The Pangolin-CoV shares 90.5% and 91.1% similarity to Bat CoV (RaTG13) and SARS-CoV-2, respectively.¹⁰ Whereas some of the vital Receptor binding motif (RBM) residues of SARS-CoV are adapted to civet and human Angiotensin-Converting Enzyme 2 (ACE2), and cause partial viral adaptations to two different host species that significantly raised cross-species viral transmission and replication between the two host species. It can be considered that either palm civets did not participate as intermediate hosts for SARS-CoV-2, or they have passed this novel CoV to humans very quickly before adapting to civet ACE2. The phylogenetic analysis of SARS-CoV-2 (bat origin) has indicated a high affinity toward ACE2 derived from diverse animal species, except for rats and mice. These animal species may be possible intermediate hosts or used as animal models for SARS-CoV-2 infections.¹¹ In 2005, the SARS-CoV was isolated in wild palm civets near Wuhan. Knowing the fact that SARS-CoV RBD has already well evolved to civet ACE2 (except for residue 487), the wild animals who dwell near Wuhan should be screened out for SARS-CoV-2.¹²

Genomic analysis

The SARS-CoV-2 genome sequence available in GenBank on 10^th January 2020, by the Shanghai Public Health Clinical Center & School of Public Health, Fudan University, and in a subsequent publication.¹³ The SARS-CoV-2 is an enveloped, spherical in shape, single-stranded, positive-sense RNA virus. It is the seventh and new member of the genus Betacoronavirus.^9,13 The complete genome of SARS-CoV-2 has 29870-bp (excluding the poly-A tail), which contains 15 genes and encodes for 9860 amino acids (Figure 1). The S gene that codes for the spike glycoprotein located on the surface of the viral envelope is responsible for binding to the host’s cell via ACE2.^12,14 Five identified typical Open reading Frame (ORFs) on the same coding strand, including ORF1ab polyprotein (7096-aa), membrane protein (222-aa), an envelope protein (75- aa), nucleocapsid protein (419-aa), and spike glycoprotein (1273-aa).¹³ It shares less than 50% similarity to HCoV-NL63, −229E, -HKU1, -OC43, and MERS-CoV.⁹ Due to difference of merely five nucleotides, it is suggested that SARS-CoV-2 has emerged from SARS-CoV.⁶

Figure 1. The genome organization of SARS-CoV-2 to cause COVID-19 disease in humans

The CoVs error-prone RNA-dependent RNA polymerases (RdRP) help them in adaptive evolution, allows mutations, subspecies diversity, and recombination events to occur frequently. Mutation in the SARS-CoV genome during the 2002–2004 epidemic increased its affinity for host cellular receptors and enhanced its replication in human host cells, cumulating in higher virulence.¹⁵ The cryo-electron microscopy, transmission electron microscopy, and Scanning electron microscopy images have shown that changes have occurred in spike glycoprotein of SARS-CoV-2.⁶ The genetic mutations can potentially affect the spread and severity of the SARS-CoV-2.¹⁶ In SARS-CoV, even a single N501T mutation corresponding to the S487T mutation can enhance the binding affinity of SARS-CoV-2 RBD to human ACE2. Recently, population genetics-based analysis has indicated that different viral strains differ at orf1ab and ORF8 regions, while new variations of the spike protein in SARS-CoV-2 are linked to natural selections and mutations. Therefore, further studies are necessary to evaluate the impacts of these mutations on transmission, symptoms development, and recovery from COVID-19 disease.¹⁷ Any mutation events in SARS-CoV-2 patients occurred at the 501 positions (also the 494 positions) should be monitored properly.¹¹ From December-2019 to April 5^th-2020, a total of 95 complete genome sequences of SARS-CoV-2 are available in GenBank, National Microbiology Data Center (NMDC), and NGDC Genome Warehouse. In total 116 mutations were found after a genomic analysis, out of which 3 most common mutations are present in 28144 T > C in ORF8 gene, 8782 C > T in ORF1ab gene, and 29095 C > T in the N gene.¹⁶

Transmission

The previous controversy existed over the chances of COVID-19 infection transmission during the incubation period and before the onset of symptoms. The epidemiological studies have now confirmed the transmission of infection during the incubation period.¹⁸ The 4 days estimated median serial interval of SARS-CoV-2 is close or shorter than its median incubation period, which illustrates that chances of secondary transmission exist even prior to the onset of illness.¹⁹ Other routes of viral transmission include direct transmission through coughing, sneezing, virus-laden aerosol inhalation, infected person`s saliva,⁹ and if body fluids are exposed to mucous membranes of mouth, eyes, or nose, close contact, and fomites.¹⁵ During the previous outbreak of SARS-CoV, viral load was detected in the cerebrospinal fluid of acutely infected patients. Thus, the brain is also vulnerable to viral entry as ACE2 receptors are found over neurons and glial cells. According to a recent report, 78 out of 214 patients of COVID-19 have shown neurological manifestations, claiming that novel SARS-CoV-2 can damage the brain. However, the role of SARS-CoV-2 against the neurotropic potential is yet to be established.²⁰ Viral RNA has been found in the plasma of 15% of the most severely affected patients.²¹ The viral nucleic acids have been found in anal swabs and fecal samples, suggesting that possibility of fecal-oral transmission in COVID-19 infection should not be ruled out.¹³ The virus is also reported in the sewage system and could reflect disease burden in the community and guide the policymakers to decide for public health measures.²² The estimated reproduction number (R₀) value for COVID-19 disease is 3.6–4.0, which shows that 72–75% of transmissions must be avoided in order to deal with the infection increasing trend.¹⁵

Binding receptors and entry mechanism

Coronaviruses proteins expression in the host cell leads the coronavirus-host interplay either by changing host gene expression or by effecting host’s antiviral defense mechanisms.²³ The CoV spike protein mediates its entry into host cells by attaching itself with the host receptor and then fuse the viral membrane with the host cell membrane.¹¹ Changes occurring at spikes on receptor binding ligands cause cross-species transmission and zoonotic spillover.⁶ The spike proteolytic activation controls the release of the fusion peptide into host cellular membranes. Thus, several host proteases have been modified to proteolytically activate the spike protein including, trypsin cell surface transmembrane protease/serine (TMPRSS) proteases, endosomal cathepsins and furin.²⁴ However, due to large size genome, complicated genomic expression, the knowledge about mechanism and replication cycle of CoVs is still in an early stage compared to some other positive RNA viruses.²³

Different coronaviruses bind to different types of host receptors, such as ACE2 and CD26 for SARS-CoV and MERS-CoV, respectively.⁶ In the case of SARS-CoV-2, its RBD is structurally similar to SARS-CoV,²⁵ however, its binding affinity for ACE2 is 10–20 folds higher as compared to SARS-CoV.²⁰ The structure of SARS-CoV RBD shows that it has a core structure with RBM, which binds to the protruded claw-like structure of ACE2. Certain amino acids (at 442, 472, 479, 480, and 487 positions) have shown to modify viral attachment with human ACE2 while others at the same amino acid positions can increase viral attachment with civet ACE2. Notably, human receptor ACE2 favoring residues combined into one RBD could bind to human ACE2 with greater affinity and mediate viral entry very efficiently into human host cells.¹¹ The clear mechanism of SARS-CoV-2 attachment and infection to the human cell is illustrated in Figure 2.

Figure 2. The mechanism associated with the infectiousness of SARS-CoV-2 in the human body. After attachment to the ACE2 receptor of human cells, SARS-CoV-2 enters the cell and releases its nucleic material (RNA). Using human cell machinery, the viral genome undergoes transcription and translation, followed by RNA replication and packaging. During packaging, spike protein and other enveloping materials bind to the outer surface of newly synthesized virions. Once absolute nascent virions are formed, they are released from the cell and the cycle is repeated in the new healthy cells

Incubation period and symptomatic conditions

Initially, the incubation period for COVID-19 infected individuals was estimated to range from 1 to 14 days (minimum 3–10 days),¹⁵ and at least 14 days quarantine was recommended for the infected patients.²⁶ Later, it was estimated that the incubation period may range from 0 to 24 days.²⁷ The most common clinical presentation observed at the onset of COVID-19 disease includes headache (6.5%), dry cough (45.8%), fever (82.1%), dyspnea (6.9%), and fatigue (26.3%).²⁸ Whereas shortness of breath, sneezing, rhinorrhea, diarrhea, muscle ache, sore throat, vomiting, nausea, and acute respiratory distress syndrome are reported in other cases. Critically ill patients also suffered from septic shock and respiratory and vital organ failure.²⁹ According to CDC China, the majority of asymptomatic patients (80.9%) presented with mild pneumonia heavily released viruses at the initial phase of infection, which made viral spread containment very challenging.²⁷

COVID-19 and human health

The outcomes for SARS-CoV-2 seemed to be very high as compared to SARS-CoV and MERS-CoV.³⁰ However, the placentas delivered from infected pregnant women resulted in negative for SARS-CoV-2 detection tests, with no morphological changes in the placenta.³¹ Another case of COVID-19 infected pregnant woman delivered at the 35th week of pregnancy in a negative-pressure operating room did not show any mother-to-child transmission.³² Psychological stress and anxiety are the major outcomes of viral outbreaks.³³ Mental illnesses have been reported to alter immunity and increase disease vulnerability,³⁴ thus, it can be related to high morbidity and mortality cases overall. Nevertheless, restricted exposure to the outside environment, long-term quarantine, lost income, no access to masks and preventive measures, fear of contracting the infection, and lack of facilities have fueled the severity of mental illness.³⁵ As China is the first country to encounter the COVID-19 infection, studies show that individuals faced disturbed sleep-wake behaviors due to constant exposure to electronic devices and minimal physical activities. Especially, children can easily be affected by altering circadian rhythms and are prone to develop mental illnesses.³⁶ Recently, a wide range of online surveys related to mental health is conducted by researchers in China, including medical staff fighting against COVID-19 infection, students, and the general population. The online psychological counseling services are also available for them.³⁷ COVID-19 pandemic has also adversely affected individuals with clinical histories, such as diabetes, Cardiac problems, or blood pressure issues.

Therapeutics and drugs

Currently, no specific therapeutic agents and licensed vaccines are available for preventing and controlling the CoV infection. Drugs used during SARS-CoV and MERS-CoV epidemics such as, Remdesivir, Baricitinib, Hydroxychloroquine, and Favipiravir have been found effective against COVID-19, while several other potential options are required to be tested.³⁸ Many other anti-viral drugs including, Nafamostat, Ribavirin, Ritonavir, and Arbidol; broad-spectrum antibiotics and interferons-α nebulization have also exhibited intermediate results against COVID-19.^39,40 Despite the increasing trend of COVID-19 infection, no drug is significantly validated by conducting large-scale clinical studies that can effectively reduce the SARS-CoV-2 viral load.⁴¹ An antiviral agent remdesivir, which effectively inhibits RNA-dependent RNA polymerase (RdRp) activity, evades viral exoribonuclease proofreading, and prevents viral replication both in vitro and in SARS-CoV mouse model has shown promising results against COVID-19 infection.⁴² In a clinical trial conducted to analyze the efficacy of remdesivir, a loading dose of 200 mg on day 1, followed by 100 mg once daily for the next 9 days was administered intravenously to hospitalized COVID-19 infected adult patients having lower respiratory tract infection. The results have suggested that patients receiving remdesivir recovered around 3 days earlier as compared to the placebo group, resulting in early discharge from the hospital.⁴³ Although remdesivir is a potential therapeutic option, further studies are required to analyze its uncertain negative side effects including, hepatic toxicity, vomiting, nausea, and rectal hemorrhage among others.³⁹ Favipiravir (FPV), another RdRp inhibitor drug, when used against RNA viruses it transforms into an active phosphoribosylated form that acts as a substrate of viral RNA polymerase. FPV is approved to use against SARS-CoV-2 infection in China after successful clinical studies against COVID-19 infected Chinese patients.⁴¹ A randomized experimental trial was conducted to examine the therapeutic efficacy of FPV plus interferon- α (IFN-α) versus Ritonavir (RTV)/Lopinavir (LPV) against confirmed COVID-19 infected patients. One group of patients received an oral dose of FPV (1:1600 mg twice on day-1, and 14: 600 mg twice on day-2) plus IFN-α. Whereas the second group of patients received a dosage of RTV/LPV (14: 400 mg/100 mg twice per day) plus IFN-α. The results have demonstrated that FPV showed faster viral clearance and significant improvement in chest imaging, in addition to less harmful effects on patients internal health.⁴⁴

Ritonavir (RTV) and Lopinavir (LPV) are protease inhibitors (PIs) that target 3 C-like protease and papain-like protease of CoVs.⁴¹ The papain-like protease (PLpro) and 3 C-like protease (3CLpro) are coronaviral proteases essential for its replication that can be used as antiviral drug targets.⁴ In a clinical trial (Chinese Clinical Trial Register number, ChiCTR2000029308) conducted on the LPV/RTV treatment, 400/100 mg dose was administered orally twice per day to clinically confirmed COVID-19 patients for 14 days. The patients induced adverse gastrointestinal effects while, no decrease in viral load was observed; however, a slight decline in the death rate of severely infected SARS-CoV-2 patients was noticed in the LPV/RTV group as compared to the standard care group.⁴⁵ Thus, It is suggested that a higher concentration than the serum level might be required to inhibit the replication of pulmonary SARS-CoV-2 when treating with LPV/RTV alone.⁴⁶ Combination treatment of LPV/RTV plus ribavirin has also demonstrated effective results against SARS-CoV patients, and in vitro.⁴¹ The combinational therapy of LPV + RTV, and LPV + RTV ritonavir + IFN-β, have also displayed improved clinical symptoms and reduced viral loads.¹ During the COVID-19 epidemic in China, the use of traditional Chinese medicines (TCM) together with antivirals or antibiotics, has significantly blocked the SARS-CoV-2 replication and clinically recovered infected patients.³⁹ The most commonly used TCM are Glycyrrhizae Radix Et Rhizoma (Gancao), Saposhnikoviae Radix (Fangfeng), Fructus forsythia (Lianqiao), Astragali Radix (Huangqi), Atractylodis Macrocephalae Rhizoma (Baizhu) and Lonicerae Japonicae Flos.⁴⁷

Immunotherapeutics

Passive immunization of infected patients by administering plasma from seroconverted patients who have fully recovered from pathogenic infection has a very long history.⁴⁸ Convalescent plasma or Serum therapy has been reportedly used against several viral infections as a final resort to improve infected patients’ survival rates. In serum therapy, antibodies in the blood plasma of recovered COVID-19 patients, ‘human convalescent serum,’ are injected to combat infection in severely infected patients.⁴⁹ Shen et al. (2020) reported that convalescent plasma obtained from SARS-CoV-2 recovered patients when administered together with methylprednisolone and antiviral agents, improved the condition of patients with respiratory distress syndrome (ARDS) and laboratory-confirmed COVID-19 infection. Overall, the improvement observed in patients’ health included successful weaning from mechanical ventilation, rise in PaO2/FiO2, body temperature normalization, the decline in Sequential Organ Failure Assessment score, resolution of ARDS, decrease in viral loads and increase in neutralizing antibody titers.⁵⁰ To evaluate convalescent plasma efficacy against COVID-19 infection, FDA has encouraged randomized clinical trials and emphasized an investigation of novel drug applications as well.⁴⁸ However, highly specific substitutes of serum plasma, broadly active neutralizing monoclonal antibodies (MAbs) have been recently developed against various viruses.⁵¹

Currently, MAbs are widely considered as a treatment option against COVID-19 disease.⁴⁸ It is considered that the administration of MAbs directly to the infected individuals may play a crucial role in controlling CoV, as SARS recovered patients display highly potent NAb responses.⁵¹ The S-protein-specific neutralizing antibodies (NAbs) S309, isolated from a patient’s memory B cells who recovered from SARS-CoV in 2003, neutralized both SARS-CoV-1 and SARS-CoV −2 by ligating the RBD. The binding assays and Cryo-electron microscopy images illustrated that the conserved epitope of S309 consisted of glycans, and instead of MAb specificity for the RBD, it has no interference with ACE2 binding.⁵² The cross-neutralization affinity of SARS-CoV RBD-specific neutralizing MAbs highly depends on the resemblance of their RBDs. For instance, RBD-specific Abs of SARS-CoV can cross-neutralize bat-SARS-like-CoV strain ‘WIV1 RBD’ (8 amino acid differences to SARS-CoV), but not to ‘SHC014ʹ strain (24 amino acid differences). Such a cross-neutralizing affinity of RBD-specific mAbs of SARS-CoV should be further analyzed for SARS-CoV-2. For this purpose, analysis between SARS-CoV-2 and SARS-CoV RBD is required to identify the RBD-specific MAbs to evaluate further by clinical trials. Regeneron is currently working on identifying specific MAbs effective against SARS-CoV-2. The efficacy of combination therapy of MAbs and remdesivir could be further evaluated as an ideal therapy for COVID-19.⁵¹

An antigen-presenting cell called dendritic cell (DC) induces a potential immune response via antigen presentation.⁵³ DCs resides in the periphery, and are more active at engulfing and presenting exogenous antigens in an immature state, but become mature when come across the DAMPs and PAMPs.⁵⁴ DC vaccination, an alternate form of immunotherapy is a simple injection with tolerating therapy. Most cases reported zero adverse effects of DC vaccination. In short, DC vaccination is a Targeted Therapy, which means the DC is trained against the patient’s infection or disease. For the investigation of a potential Immunotherapeutic against SARS-CoV-2, engineering technology has been used for the synthetic minigenes conserved domains of polyprotein protease and structural proteins of the virus. The SARS-CoV-2 invading via spike protein attachment to ACE2 receptor, and replication depends on all these viral proteins molecular mechanism. Shenzhen Geno-Immune Medical Institute (SGIMI) developed a lentivirus vector-based candidate vaccine called LV-SMENP-DC. The LV-SMENP DC vaccine was synthesized by dendritic cell modification with the SARS-CoV-2 immune-modulatory gene and minigene SMENP expressing lentivirus vectors (NHP/TYF). On March 24, 2020, the clinical trial phase 1 was conducted involving 100 patients and the expected date of completion of the study is December 31, 2024 (NCT04276896). In the current study, the efficacy and safety of the COVID-19 proposed LV-SMENP DC vaccine will be investigated along with activated antigen-specific cytotoxic T cell (TC cell) vaccine.^10,55,56 The objectives of this study are (i) to inject and infuse LV-SMENP DC and TC cell vaccines to SARS-CoV-2 infected patients and healthy volunteers to evaluate the safety, and (ii) to investigate the LV-SMENP DC and TC cells vaccine efficacy.⁵⁷

A recent study has identified that MAb (CR3022) can potentially bind with S-RBD of SARS-CoV-2, because, antibody’s epitope does not overlap with the divergent ACE2 RBM. CR3022 can be considered as the potential therapeutic candidate to treat COVID-19 infection either alone or in combination with other Nabs.² Moreover, the soluble version of ACE2, fused with the FC domain of an immunoglobulin (ACE2-Fc), may block the viral entry and provide a NAbs with high breath to avoid any viral escape. It will not only recruit the immune system to build lasting immunity but also balance the ACE2 level declined during lung infection and treat acute respiratory distress pathophysiology simultaneously. Such a treatment would help infected patients before the availability of commercially available protective vaccines.⁵⁸ Among various other sources of Abs, chicken egg yolk antibodies (IgY) can be considered as a substitute to mammalian Abs that have been studied against SARS-CoV. Chicken egg yolk antibodies (IgY) have higher binding affinity to certain antigens, the extraction and production cost is low and possesses greater pathogen-neutralizing activity in lungs as compared to mammalian Abs.⁵⁹ It is also suggested that phage display methodology can be effectively used against SARS-CoV-2 by using highly specific MAbs such as chicken egg yolk antibodies (IgY).⁶⁰

Chicken MAbs IgY has proved to be highly specific compared to polyclonal Abs while recognizing a unique epitope. Hence, SARS CoV-2 epitope identified for spike protein (S) could be a significant candidate for Abs-based vaccine, as immunotherapeutic drugs play a vital role in treating and combating infections both clinically and scientifically. Additionally, the application of chicken scFv IgY against SARS CoV-2 spike protein (S) by phage display method can be considered as an effective model for highly specific MAbs (IgY) for mass production and use in long term.⁵⁹ From a panel of several human MAbs, Zost et al. identified those which can target S glycoprotein, exhibit neutralizing activity, and capable of fully blocking S protein RBD from interaction with human ACE2. Two NMAbs COV2-2196 and COV2-2130 can potentially recognize non-overlapping sites, significantly bind to S and neutralize SARS-CoV-2 virus. The SARS-CoV-2 infected mouse models, when passively immunized with COV2-2130 or COV2-2196 MAbs alone or in combination, simultaneously protected mice from abrupt weight loss, reduced viral load and inflammation in lungs. Additionally, passive immunization of MAbs COV2-2196 or COV2-2381, as a monotherapy to rhesus macaques models infected with SARS-CoV-2 infection developed immunity against the virus. These results provide a structure-based platform for coherent vaccine design and the selection of effective immunotherapeutics.⁶¹

Large gaps exist in our understanding of epidemiological risk factors, immunopathology risk with SARS-CoV-2, and pathological immune mediators during CoVs infections. Carefully designed human infection models, by using less virulent CoVs, such as human CoV OC43, may provide a safe avenue to identify the probability of immune response enhancement in CoV infected models.⁶² Nevertheless, the development of clinical drugs for coronaviruses is challenging because the repeatedly emerging novel coronaviruses with diverse features require the specific drug for each newly emerged virus. Therefore, designing and development of novel broad-spectrum antiviral drugs that can potentially target all coronaviruses, in general, maybe the only treatment option against reemerging and circulating coronaviruses.

Vaccines

There is no currently approved vaccine available for COVID-19, though some pharmaceutical companies and research centers are working to rapidly develop a vaccine. Of the structural proteins of SARS-CoV-2, S protein is the foremost intermediator of viral entry into the cell upon attachment to ACE2 receptor and the leading factor to determine the virus pathogenicity, host range, and virus-neutralizing antibodies, which is why it is considered to be a key target element to design a vaccine against SARS-CoV-2.⁶³ S-protein has two subunits (i) N-terminal S1 (RBD) and (ii) C-terminal S2, because of the high conservation and non-mutational S2 part, become a critical target part for the design a potent vaccine.^64,65 On the bases of the above discussion, different vaccines have been developed (in clinical trials, see Table 1) against the S protein by genetic engineering of the RNA or DNA that is responsible for the synthesis of S protein copies.

Table 1. Different approaches applied by various pharmaceutical industries and research centers to develop efficient vaccine for COVID-19

Type/name of Vaccine	Manufacturer	Preparation strategy	References
SARS-CoV-2 vaccine	US National Institute of Allergy and Infectious Diseases	m-RNA based vaccine	^Citation66
	Stermirna Therapeutics		^Citation67
Nucleic Acid Based vaccine	CureVac		^Citation68
	Moderna		^Citation69
BNT162	Pfizer and BioNTech		^Citation70
INO-4800	Inovio Pharmaceuticals	Bacterial DNA encoding SARS-CoV-2 spike protein	^Citation71
Nasal Vaccine for COVID-19	University of waterloo		^Citation72
Opencorona	Cobra Biologics and Karolinska Institute		^Citation73
bacTRL-Spike	Symvivo Corporation		^Citation74
Inactivated viral vaccine	Chinese Center for Disease Control and Prevention	Viral strain	^Citation75
Corona vaccine	GeoVax and BravoVax	MVA-VLP vaccine platform and SARS-CoV-2 genetic sequences	^Citation76
SARS-CoV-2 vaccine	Clover Biopharmaceuticals	recombinant 2019-nCoV S protein subunit-trimer based vaccine	^Citation77
Whole virus vaccine	Johnson & Johnson	Adenovirus-vectored vaccine using AdVac® and PER.C6® technology	^Citation78
Whole virus vaccine	Codagenix and serum institute of India	Live attenuated vaccine	^Citation79
Subunits based vaccine	University of Queensland	Protein-based vaccine using Molecular Camp platform	^Citation80
	Vaxart	Vaccine using Oral recombinant protein	^Citation81
Subunit based Vaccine	New York blood center, Fudan University New York	RBD protein of coronavirus-based vaccine	^Citation82
Whole virus	University of Oxford	Protein modified whole virus	^Citation83
PiCoVacc	Sinovac Biotech	Inactivated SARS-CoV-2 virus	^Citation84
Whole Inactivated Virus	Wuhan Institute of Biological Products		^Citation68
Ad5-nCoV	CanSino Biologics	recombinant adenovirus type 5 vector (using spike protein)	^Citation85
Not Available	Sanofi and GSK	Recombinant protein-based vaccine (protein-based antigen together with an adjuvant)	^Citation86
V591	Merck	Using measles virus vector platform	^Citation87
Immunotherapeutics
LV-SMENP-DC	Shenzhen Geno-Immune Medical Institute	Dendritic cells modified with artificial antigens	^Citation55
Nanoparticle	Novavax	Recombinant protein nanotechnology	^Citation88
Antibody based vaccine	Vir Biotechnology	Monoclonal antibodies	^Citation88

Stermina therapeutics, GeoVax-BravoVax industry, Clover Biopharmaceuticals, Chinese Center for Disease Control and Prevention, US National Institute of Allergy and Infectious Diseases, Vir Biotechnology, Johnson & Johnson, Pfizer, Merck Sanofi, and Novavax and many other companies are making their efforts to develop a vaccine by using various techniques such as mRNA-based vaccine, DNA-based vaccine, inactivated virus vaccine, and protein subunit-trimer-based vaccine. Some of these vaccines are now progressed to phase 2 and 3 trails.^2,71,78 Recently, a vaccine named as ‘PiCoVacc’ developed by Sinovac Biotech using inactivated virus was effective against 11 various strains of SARS. The vaccine was tested on various animal models such as mice, rats, and non-human primates. The vaccine effectively blocked the replication of all 10 strains of SARS-CoV-2 in infected rats, mice, and picramates.^84,89 Shenzhen GenoImmune Medical Institute has also prepared a vaccine named LV-SMENP-DC. The vaccine is prepared from dendritic cells modified with artificial antigens that can produce antibodies against SARS-CoV-2, the vaccine is also in an animal trial version.⁵⁵ Further, the details of various vaccines against SARS-CoV-2, production strategies, and manufacturers are enlisted in Table 1.

Multiple platforms including RNA and DNA-based platforms, next-generation sequencing, autologous dendritic cell-based vaccine, inactive virus vaccine, replicating or non-replicating viral-vectored vaccines, and reverse genetics are being used to develop vaccines against the SARS-CoV-2.^90,91 During the pandemic, these potential platforms can cut the development time of more conventional vaccines. However, still, there are several challenges such as optimizing antigen design and investigating the potential duration of immunity.⁹⁰ According to WHO, more than 137 candidates undergoing preclinical phase and 23 in the early clinical phase.⁹² However, vaccine development is a lengthy and expensive process and takes many years to manufacture a licensed vaccine.⁹³ For example, the first vaccine against Ebola was approved 43 years after the virus was discovered. On average, it takes about 10 years to develop a vaccine. However, due to the significant global crisis, everyone is hoping that this time it will be different.⁹⁴ Some experts argued that 18 months for developing the first vaccine is an extremely aggressive schedule while others believe that it might be ready by the end of 2020.⁹⁴ An ideal vaccine against the SARS-CoV-2 would confer immunity for a minimum of 6 months, effective after one or two vaccinations, reduce onward transmission of the pathogen, and would protect target populations such as immunocompromised individuals, older adults, and those with comorbidities.⁹⁵ Even if the vaccine is approved, it will bring its difficulties such as production, delivery, and accessibility. Several obstacles like geographical, commercial, and nationalistic factors could stand in the way of maximizing vaccine utility and effectiveness in the field.⁹⁴

Conclusion and prospect

SARS-CoV-2 pandemic not only imposed physical illness and deaths around the globe, but also affected the mental health of the kids, adults, and disabled subjects who are locked down at homes, many of them are suffering from xenophobia and insomnia. Due to the lack of any effective and approved vaccine, health workers are treating patients with a higher and frequent dose of previously used broad-spectrum antibiotics and antivirals, though the patients have recovered from COVID-19, many of them are facing severe adverse effects. Remdesivir, Chloroquine, and related salts, and Arbidol have caused severe complications in patients, such as mental illness, cardiac, hepatic, and kidney-related issues. Several multinational pharmaceutical companies are in the race to develop and commercialize the COVID-19 vaccine as quickly as possible. Besides pharmaceutical industries, many research centers and departments of several universities are also working to develop an efficient vaccine by using various approaches. There are more than 40 strains of SARS-CoV-2 reported until now; therefore, the developed vaccine must be effective against all of these strains. Vaccines production should not be limited to few strains but to all occurring strains to eradicate SARS-CoV-2 from this world.

Funding

The authors acknowledge the postdoctoral Science Foundation grant number“2020M672291” (for S.K), operating grant support from the National Natural Science Foundation of China (grants no: 81870942, 81471174 and 81520108011), National Key Research and Development Program of China (grant no: 2018YFC1312200), and Innovation Scientists and Technicians Troop Constructions Projects of Henan Province of China (for MX).

Disclosure of potential conflicts of interest

The authors of this manuscript declare that there is no conflict of interest.

Additional information

Funding

China Postdoctoral Science Foundation Grant (2020M672291) for Suliman Khan.