{'fraction_non_alphanumeric': 0.03317535545023697, 'fraction_numerical': 0.009619724667118032, 'mean_word_length': 5.307651245551601, 'pattern_counts': {'":': 0, '<': 1, '<?xml version=': 0, '>': 0, 'https://': 1, 'lorem ipsum': 0, 'www.': 1, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
A comprehensive overview of vaccines developed for pandemic viral pathogens over the past two decades including those in clinical trials for the current novel SARS-CoV-2

The unprecedented coronavirus disease 2019 (COVID-19) is triggered by a novel strain of coronavirus namely, Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2). Researchers are working around the clock to control this pandemic and consequent waves of viral reproduction, through repurposing existing drugs as well as designing new vaccines. Several countries have hastened vaccine design and clinical trials to quickly address this outbreak. Currently, more than 250 aspirants against SARS-CoV-2 are in progress, including mRNA-replicating or non-replicating viral vectored-, DNA-, autologous dendritic cell-based-, and inactivated virus-vaccines. Vaccines work by prompting effector mechanisms such as cells/molecules, which target quickly replicating pathogens and neutralize their toxic constituents. Vaccine-stimulated immune effectors include adjuvant, affinity, avidity, affinity maturation, antibodies, antigen-presenting cells, B lymphocytes, carrier protein, CD4+ T-helper cells. In this review, we describe updated information on the various vaccines available over the last two decades, along with recent progress in the ongoing battle developing 63 diverse vaccines against SARS-CoV-2. The inspiration of our effort is to convey the current investigation focus on registered clinical trials (as of January 08, 2021) that satisfy the safety and efficacy criteria of international wide vaccine development.


Introduction
One of the most successful therapeutic strategies to prevent or control various diseases is by "vaccination" protocol. 1,2 Millions of lives have been saved because of vaccinations, which cover a number of diseases including certain types of cancer, HIV and many viral infections. 3 Vaccination was initially accomplished by Edward Jenner, who was the pioneer for smallpox in the late 18 th century. 4 In the 1980s, the development of vaccines to ght against pathogenic microorganisms was tentatively introduced. Vaccines are now employed to improve and increase the protection capability (immunity) of the body to ght severe infection and disease, 5 and moreover, primarily intend to make immunity stronger and reduce resistivity of diseases by reducing the reproduction of target pathogens. The majority of vaccines actively exist in the immune system, since anti-bodies are constantly generated in the body to sustain a healthy immunity system. 6,7 General components of vaccines A vaccine consists of an antigen, stabilizer, adjuvant, antibiotic, preservative, and chemical reagents such as formaldehyde. 8 The advantages and disadvantages of vaccinations 9a are listed in Table 1.
Antigen. The component matching the structural array of disease-oriented organisms, wherein, they are identied by the immune system as 'foreign' and cause an active immune response.
Stabilizer. This module is employed to assist the vaccine by sustaining its efficiency during storage. Instability of the vaccine can lead to reduced antigenicity and decreased infectivity of live attenuated vaccine (LAV). Magnesium chloride (MgCl 2 ) for oral polio vaccine (OPV), magnesium sulfate (MgSO 4 ) in measles vaccines, and lactose and gelatin associated with sorbitol, are current representatives of stabilizing factors.
Adjuvant. They are responsible for enhancing the efficacy of the vaccine by motivating the generation of antibodies.
Chemically, adjuvants are a highly heterogeneous group of compounds (including Al salts).
Antibiotics. Used in lower amounts during the development phase, to circumvent bacterial infection during tissue culture cells where the viruses are grown. MMR (measles, mumps, rubella) and IPV (inactivated polio vaccine) associated vaccines have a minimum amount (<25 mg) of neomycin for each dose.
The MMR vaccine was developed by Maurice Hilleman in 1971. Mumps, like measles infections, are caused by an RNA based virus from the Paramyxoviridae family. Moreover, measles and mumps belong to the genus Rubulavirus, it is a human disease with no animal reservoirs. Generally, the MMR vaccine exhibits side effects of a painful arm from the shot, minor rashes, generally in teen/adult women who have no earlier immunity; then the rubella vaccine component can result in joint and tendon stiffness. This vaccine is also associated with the minor threat of seizures/jerking instigated by fever, but is not connected with any enduring effects. The threat of febrile seizures increases as infants get older, hence this vaccine is recommended at a young age. Some people may experience cheek/neck inammation, impermanent low platelet counts that generally do not require treatment and are also not life threatening. 9b In 1955, Jonas Salk initiated an inactivated polio vaccine (IPV), aer that, Albert Sabin further developed the live, OPV. Even though poliovirus has three serotypes, both vaccines are trivalent and offer good resistivity against poliomyelitis, a limited number of countries have continuously provided IPV. Sabin's OPV vaccine is underused in most countries due to minimising oral management, but it advances immunity in the intestine, which is capable of spreading to others, and is associated with lower cost. 9c Preservatives. They are added in multi-dose vaccines, which can be used to control bacterial and fungal growth, including thiomersal (sodium(2-carboxylatophenyl)sulfanyl-ethyl mercury), formaldehyde, or phenolic derivatives. In fact, formaldehyde is responsible for inactivating the viruses (e.g. IPV) and removing toxicity in bacterial strains, including the toxin employed in diphtheria and tetanus vaccines.

Types of vaccines
Various kinds of vaccines are available and those which are administered to infants and adults can be classied (Fig. 1) as follows: Live-attenuated. Inactivated. Toxoid. Conjugate. Subunit. Live attenuated vaccines. These vaccines are adapted from the existing bacteria or virus, they have been weakened, and thus will not result in serious disease in people with strong immunity. LAV vaccines are more similar to the actual infection. A few representative examples are the MMR and varicella (chickenpox) vaccines. Albeit, it is efficient, but not everyone can be administered these vaccines, including children and patients undergoing chemotherapy as their immune system is too weak.
Inactivated vaccines. These vaccines are made from inactivated or dead organisms. The polio vaccine is an example of an inactivated vaccine, which generates an immune response through various routes different to the live attenuated vaccine. It requires a higher dosage to increase and/or sustain immunity.
Toxoid vaccines. This vaccine prevents the disease resulting from the toxins released by a virus into the body. Since the toxic substances administered are weak they are unable to cause the illness, while the immune system which encounters this toxoid vaccine, becomes able to repel the natural toxin. The DTaP vaccine for diphtheria and tetanus toxoids falls into this category.
Conjugate vaccines. They defend the system against different kinds of bacteria which have antigens that are surface coated with polysaccharide units. It is obvious, that this sugar coated unit masks the antigen. These vaccines append (or conjugate) themselves to the polysaccharide units on the antigens. Haemophilus inuenzae type B (Hib) vaccine is an example of the conjugate type.
Subunit vaccines. These vaccines are made up of simple fractions of the virus/bacteria or subunits, rather than the complete germ. Since the subunit vaccines have an essential antigen only and are not the molecular array constituting the germ, they have less side effects. The DTaP vaccine-pertussis (whooping cough) module is a paradigm of subunit vaccines.

Common characteristics of vaccines
Live attenuated vaccines are typically grown in animal cell lines under poor development conditions. The development of an inactivated vaccine involves use of thermal or chemical methods in the beginning, and its mode of action involved in conferring immunity is not fully known. However, the live attenuated or execution of entire organism-supported vaccines have shown a lot of success in the control and inhibition of severe transmittable diseases in human, including animal infectious cattle plague, classic swine fever, equine infectious anaemia, measles, mumps, polio, rubella, smallpox, and so on. In recent times, the use of LAVs, subunit and peptide based vaccines have become possible because of progressive technologies in molecular biology. LAVs are based on the mechanism of action associated with the immune response. Inactivated vaccines, based on antibodies have been mostly used to prevent and manage microbial infectious diseases. LAVs introduce stronger cell immune responses that are decisive to remove several intracellular viral pathogens. However, these pathogens sometimes bypass inactivated vaccines 9d by mutating peripheral antigens. On the other hand, subunit and peptide based vaccines are less efficient in drawing a strong CD8 + immune response.   Protection of inhabitants against disease  Uncertainty about complete protection  Preventing epidemic and pandemic diseases  May have some possible side effects  Prevents diseases spreading to others  Requires NHS/individual outlay  Avoids large cost for the treatment of infected patients  Injections are not pleasant/more immune booster injections are inconvenient Progressive vaccination involves the use of non-viral distributed nucleic acid-supported vaccines, which imitate live microorganism infection-or immunization. This leads to T-helper cellular immune responses. In addition, this vaccine development 9e,f is harmless and consumes less. It does not require extreme infectious organisms, so is safe from infectivity through live transmittable agents and the discharge of harmful pathogenic organisms. These vaccines ll the gap between a virus outbreak and design of a desirable vaccine, wherein they are classied as DNA/RNA based pentose-carbon sugar motif. The remarkable growth of RNA-associated vaccines resulted in the growth of mRNA based vaccines. It is quite signicant to note that mRNA vaccines provide many valuable benets when compared to viral vectored-and DNA vaccines. 10

Proceedings of vaccination against viral diseases
In 1970, to evade the prevalent spread of foot and mouth disease, scientists discovered a vaccine using a single protein from the virus. Despite their achievements in virology, in particular vaccine studies and their development, the lack of understanding of immunological mechanisms of action during induced defensive immunity, has prevented the use of existing vaccines during global pandemic outbreaks of related diseases with similar viral pathogens. The immune system protects against various pathogens, including distinctive units of Thelper cells that are useful to protect against various unusual pathogens. Besides, the follicular T-helper cells (T FH cells) generate interleukins (ILs) and support the partition of B cells (they are lymphocytes, and take part in the humoral immune response) and generation of Memory B cells. Furthermore, Memory T cells can be sub classied as CD4 + /CD8 + T cells (Cluster of Differentiation) and their functionalities are (a) central memory and (b) effectors memory, which provide various responses upon vaccination against different pathogens. 11 Different vaccines attempted for identical pathogens depend on the perceptions of the scientist, 12 many healthcare professionals do not pay sufficient attention to vaccines, which may result in uncertainty in their efficacy, side effects and toxicity. Table 2 lists the various vaccines discovered that are available in the current market for protection against viral infections. They are listed on the basis of various factors, 13,14 including the apparent protection level, plausible mechanism of action, possibility of usage for other diseases. 15,16 Mode of action stimulated by vaccines Normally antibodies prevent/minimize infections from extracellular pathogens: (a) use enzymatic active sites to fuse to toxins to break their diffusion; (b) prophylactic action preventing viral replication; (c) facilitate opsonophagocytosis of extracellular bacteria; (d) inducing the complement cascade.
CD8 + T cells do not inhibit infection but work to minimize, regulate, and remove intracellular pathogens through: (a) direct destruction of infected cells (discharge of perforina pore forming cytolytic protein present in cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells), granzyme (serine proteases delivered through cytoplasmic granules inside cytotoxic T cells and NK cells); (b) destroy infected cells with antimicrobial cytokine release. CD4 + T cells do not inhibit infection, however they contribute in the minimization, regulation, and renement of extra-and intra-cellular pathogens with their control and cytokinedevelopment capabilities. The main examples of CD4 + T cells are: (a) Follicular T-helper (T) cells yielding predominantly interleukin (IL)-21 and providing assistance to B-cells; (b) T-helper 1 (Th1) effector cells yield interferon (IFN)-g, (TNF)-a/TNF-b, IL-2, and provide a major role in controlling intracellular pathogens e.g., viruses and bacteria such as M. tuberculosis; (c) Th2 effector cells generate IL-4, IL-5, IL-13, and impact extracellular pathogens such as bacteria and helminths; (d) Th9 effector cells generate IL-9 and also defend against extracellular pathogens; (e) Th17 effector cells generate IL-17, IL-22, and IL-26 and participate in mucosal protection (for example against, S. pneumoniae, B. pertussis, M. tuberculosis).

Main effectors of vaccine responses
Vaccines prevent disease by inducing effector modes of action in cells/molecules to reduce the development of pathogens and deactivate their toxic effects. Vaccine-stimulating immune effectors are resourceful antibodies generated by B lymphocytes' ability to interact to a particular toxin/pathogen. A pictographic representation of vaccine immunological function is shown in Fig. 2.
Isotype switching. Control of immunoglobulin (Ig) expression and production from IgM yielding IgG, IgA, or IgE that ensues through B-cell differentiation by DNA recombination.
Marginal zone. The zone between the spleen's red pulp and white pulp is known as marginal zone. Its main function is to catch particulate antigens from the circulation and distribute them to lymphocytes.
Pattern recognition receptors. These germline-encoded receptors sense the existence of infection through the identication of pathogenic microbe molecular arrays, and stimulate innate immune responses.
Regulatory T cells-t. T cells secrete cytokines (IL-10, transforming growth factor [TGF]-b/surface markers) and react to reduce immune system response through different modes of action, this sustains immune homeostasis and tolerance to self-antigens.
Resident memory T cells. Effector memory T cells exist in particular tissues (such as the lungs, gut, and skin) and are an instant and early line of protection against various viral and bacterial pathogens.
Somatic hypermutation. This is a process which intercalates unsystematic mutations in the B-cell receptor region (i.e.,       immunoglobulin) at a particularly high rate throughout the proliferation of B-cells. This mechanistic process occurs as an effect of the cytidine deaminase enzyme and results in antibody diversication.
T lymphocytes. Cells that mature in the thymus, and become stimulated in the spleen or nodes if their T-cell receptors interact with an antigen marked with MHC molecules, and they obtain supplementary co-stimulation indications motivating them to kill the infected cell (mainly CD8 + T cells)/secondary (mainly CD4 + T cells) roles.
T-Independent B-cell responses. This B cell differentiation pathway is mostly triggered by polysaccharides, in the marginal zone and extra-follicular regions of the spleen or nodes. It provides a fast response (days), and generates transient (over months) low affinity antibodies without impacting immune memory.
T-Dependent B-cell responses. This B cell differentiation occurs due to protein antgens which recruit T and B cells into germinal centers of the spleen/nodes. It is slower (weeks), but provides enduring stimulation (years) with high affinity antibodies formation and immune memory.
Toll-like receptors. A cluster of 10 receptors (TLR1 to TLR10) existing at immune cell external regions, which identify pathogens and trigger characteristic immunity.
The main target of immunization through vaccination is to inhibit specic infections and their unavoidable difficulties. The best vaccine is the one which concomitantly accomplishes the following criteria such as: (a) Actively inhibit the infective disease or else minimize the adverse effects of the disease; (b) Offer a strong and continuing defence against a specic disease; (c) Improves immunity through a minimum quantity of administrations; (d) Deliver abundant antigens to afford wide-ranging safety against infection; (e) Never results in side effects, or keeps them to a minimum; (f) Remains stable under storage conditions, preferably mild storage conditions, for its shelf-life; (g) Can be produced on a huge scale; (h) Should be economical and easily available. Herein we consider the COVID-19 pandemic, the main resolution of vaccination against SARS-CoV-2 are: (a) Inhibition of characteristic clinical symptoms so hospitalization is avoided, and reduces severe infectivity; (b) Prevention of disease spreading before the corresponding antibodies are produced (sero conversion) (c) Producing a strong neutralizing immune response able to link with the viral protein spike (S) that must prevent it from attaching to human cells.
From this perspective, the various immunological response effects which neutralize antibodies and CD8 + T cells are most signicant.
The antibodies of anti-SARS-CoV-2 alert the host organism's immune response to the presence of the virus; such antibodies are immunoglobulins, which are appropriately split into IgA, IgM, IgG, and less frequently IgD. Prior serological antibody model responses to viral infections have usually proven the subsequent sequence of these antibodies resulting from virus infection: the antibodies of IgA are primary, which are followed by IgM, IgG-type continues at high levels for a longer time than the preceding ones (IgA and IgM). For certain viruses, sometimes the antigen (the virus itself) co-occurs with antibodies, particularly with the antibodies of primary IgA and IgM. Further, viruses have a "serological window," i.e., a period between the initial arrival of the antigen (in the blood) and the antibody response, thus phase intervals of infection occur. Eventually, IgG-type antibodies are specic to the novel coronavirus (SARS-CoV-2) and can be examined through chemiluminescence immunological routes, which is an automated laboratory process using enzyme-linked immunosorbent assays with higher arrangement. The examination mostly identies the body's immune response to SARS CoV-2 infection.  to the International Committee on Taxonomy of Viruses (ICTV), the coronaviruses (CoVs) are sub-classied as Othocoronavirinae, which in turn consist of four categoriesalpha-CoV, beta-CoV, gamma-CoV, and delta-CoV. The alpha-and beta-CoVs transmit disease in mammals such as bats, pigs, cats and mice. Gamma-and delta-CoVs usually infect birds. In addition, the seven different types of human CoVswhich include HCoV-229E, HCoV-NL63belong to alpha-CoVs. The HCoV-OC43, HCoV-HKU1, SARS-, MERS-CoVs, and the current pandemic SARS-CoV-2 belong to beta-CoVs, and the genus of such beta coronavirus occurrences are zoonotic infections. The December 2019 outbreak coronavirus, which leads to the respiratoryassociated syndrome, originated from Wuhan, China, and is called the novel corona virus disease 2019 or nCOVID-19, and its genome is fully sequenced. 88 The genetic sequential arrangement of SARS-CoV-2 has an identical genomic array of SARS-/ MERS-CoV. 89, 90 Initially, the taxonomy of coronaviruses are split in to three groups, on account of genetic and serological interactions, the rst set (Group-1) comprised several viruses including the porcine epidemic diarrhoea virus (PEDV), porcine transmissible gastroenteritis virus (TGEV), canine coronavirus (CCoV), feline infectious peritonitis virus (FIPV), the previously identied coronavirus of HCoV-229E, and HCoV-NL63. Whilst the second combination (Group-2) consists of murine hepatitis virus (MHV), bovine coronavirus (BCoV), human coronavirus OC43 (HCoV-OC43), rat sialodacryoadenitis virus (SDAV), porcine hemagglutinating encephalomyelitis virus (PHEV), canine respiratory coronavirus (CRCoV), and equine coronavirus (ECoV). In a similar pattern, the third group (Group-3) contains the avian infectious bronchitis virus (IBV) and Turkey coronavirus (TCoV). Now, the SARS coronaviruses (SARS-CoV) cannot be associated with any of these representative groups, however they possess some similarities along with the second group coronaviruses. 91a The battle between scientists and viral infections is a perpetual process and thus the identication of specic potential drugs with high efficiency and low toxicity is a continuous aim. Globally, this is the rst time, that scientic researchers, diplomats, politicians, and capitalists have convened to work towards a common objective. The FDA approved drugs chloroquine and hydroxychloroquine to be utilized in critical illness cases, but clinical practices were still becoming overwhelmed by CoV cases. The suggestion to employ extensive use of these antiviral drugs was not sufficient. Certain polymerase nucleoside/nucleotide inhibitors are promising agents. Favipiravir a selective viral RdRp inhibitor, has been tested in clinical trials against COVID-19. Furthermore, the antiviral drugs lopinavir, ritonavir, remdesivir, nelnavir, serine based protease inhibitors of nafamostat, camostat, efficient lipid reducing statin, rosovastatin, TNF alpha inhibitors, interleukin 1 receptor antagonists; Janus associated kinases (JAK) as well as monoclonal antibodies, tocilizumab, baricitinib and ruxolitinib etc., along with their combinations and other antiviral components, are also under investigation in clinical studies to combat COVID-19. In the future, researchers are requested to accumulate their results to provide more knowledge to repurpose signicant drugs appropriately, and provide cheap drugs with the minimum toxicity prole. 91b,c

The manufacture of SARS-CoV-2 vaccines through various approaches
Vaccines have to be approved based on sufficient evidence. Many scientic researchers have reviewed the current literature and worked towards the development of a specic treatment for COVID-19. Current studies have exposed several benecial opportunities, a few of them are more established and are in preclinical trials in addition to some in clinical trials. 92, 93 Vaccines endeavor to represent the antibody to the antigen, and they should help the immune system (innate and adaptive immune responses, Fig. 3).

Virus vaccines
A number of vaccines are being developed for SARS-CoV-2, including inactivated and weakened, replicating and nonreplicating viral vectors, and the tentative use of nucleic acid in the form of DNA and RNA. Now, we can distinguish each vaccine characteristic as follows: Weakened virus. Viruses cause disease by reproducing in a rapid manner. A weakened virus reproduces inside the human or animal host very poorly thereby decreasing its virulence and disease causing ability. One pharmaceutical company in the USA (Codagenix, New York) is collaborating with an Indian research team (Serum Institute at Pune) for the manufacture of SARS-CoV-2 with changed genetic sequencemaking viral proteins with lower potency.
Inactivated virus. Chemicals such as formaldehyde, or thermal heating, are applied to inactivate a virus which is then used as a vaccine.
Viral-vector vaccines. In this technique other virus types are modied to render them safe and then used as a vaccine. They combine the qualities of DNA vaccines with those of live attenuated vaccines. The viral vector vaccine consists of a live attenuated virus which is genetically modied to carry the DNA encoding protein antigens from a different organism. For example, the virus belonging to measles/adenovirus is genetically modied and thus made safe. Currently there are products for veterinary use but not for human use. There are two types of viral vectors, 94 one is still able to replicate inside the cells, while the other is non-replicating, since the key genes have been rendered inoperative.
Replicating viral vector (weakened measles). One of the approved Ebola vaccines is a typical paradigm for a viral-vector vaccine. Even if replication occurs inside cells, this vaccine is safe and will intensify the immune response.
Non-replicating viral vector (adenovirus variety). In general, viral vectors are genetically transformed to generate defective replications which are termed non-replicating vectors. Numerous viruses like adenovirus, adeno-linked, measles, and human parainuenza viruses has been extensively utilized as viral vectors. Ultimately, the virus attains an attenuated state in which they can activate the anticipated human immune responses, however, they are unable to reproduce in human cells.
Nucleic-acid vaccines. A number of groups are aspiring to utilize gene sequences (based on DNA or RNA) for a coronavirus protein to instantaneously provide an immune response. The majority of these vaccines focus on the spike protein present in the virus. 95 Protein-based vaccines. Several researchers have focused on injecting proteins present in coronavirus into the body. Also, protein fragments or shells of protein as copies of the coronavirus' surface could also be utilized.
Protein subunits. Nearly 28t groups are working on vaccines using subunits of viral proteins, predominantly on the spike protein at the receptor binding arena. Vaccines for protecting monkeys against SARS virus infection are available which have not been examined for humans. In fact, to employ such vaccines, adjuvant-immune-stimulation drug molecules should be administered together with the vaccine. 96 Virus-like particles. Shells (outer regions) of an empty virus resemble the CoV structure, however, it is non-transmittable due to the absence of the genetic factors. Several research groups are engaged in vaccines based on virus-like particles that can activate a good immune response, however it is quite complex to synthesize such particles, and 70% of the teams directing their research in this way are from industry or private companies rather than academic laboratories. 97,98 Table 3 and 4 provide a comprehensive overview of the current development of vaccines for SARS-CoV-2, their mode of action, their utility and the name of the manufacturer. 99 More than 10 vaccines have almost reached the approval stage, while many are in the preclinical stage. Fig. 4 portrays the number of vaccine aspirants which are under clinical ($63 drug aspirants) and preclinical ($172 drug aspirants) stages against nCOVID-19. 99a Protein-based techniques seem to be the most popular among the various mechanisms (see https://www.who.int/publications/ m/item/dra-landscape-of-covid-19-candidate-vaccines).

Comparison between previous vaccines and COVID-19 vaccines
Comparing COVID-19 vaccine novelty may seem challenging, but it is normal for queries to arise on a vaccine's overall potential, safety, toxicity and its side effects etc., 100a and some of the efficacy, efficiency and safety of the COVID-19 frontrunner vaccines can be determined by comparison with other vaccines, like u vaccines. For example, Pzer/BioNTech released their COVID-19 vaccine initially in the UK and USA, and the rest of the world could see their effectiveness. However, prior to this demonstration the three prominent COVID-19 vaccines -Pzer/ BioNTech claims an efficacy of 95%, the Oxford/AstraZeneca provides 70%, and the Moderna is stated as 94.1% efficacywere compared with other available vaccines such as the u, polio, and measles, which helped provide their effectiveness and efficiency predictions.
It is important to discuss here that "effectiveness" and "efficacy" are different. 100b Efficacy states how a vaccine performs in ideal lab circumstances, like those present in clinical trials. Whereas, effectiveness means how the vaccine works in normal, non-controlled conditions. For example, in a clinical trial, 90% efficacy refers to 90% lower disease rates in the group getting the vaccine compared with the sample group. But, the members in a group selected for a clinical trial have to be in good health and young and they usually have no underlying health conditions. Besides, medical researchers will not generally consider some demographics in these clinical research studies like children/pregnant women. Thus, once a vaccine is able to prevent disease in clinical trials, we may observe the effectiveness drop when directed to different demographics.
Vaccines do not necessarily need high effectiveness to protect several thousands of lives from disease. For instance, the vaccine for u 100c has 40-60% effectiveness according to CDC data. During 2018-19, this vaccine prevented millions of inuenza cases and its associated illness, but determining the exact effectiveness rate is challenging. Dosages can also increase effectiveness for some vaccines. The two doses of a vaccine can give a protection boost, nevertheless this advantage is sometimes limited to only certain groups like children/ organ transplanted people. The booster dosage may not provide an advantage in people aged 65 years.
Through comparing vaccines, like the ones for polio and measles, we see heavy dosages are needed to realize effectiveness.
Polio vaccines 100d should be up to 100% effective. According to the CDC, "Two inactivated polio vaccine (IPV) dosages have 90% effectiveness; three dosages are 99-100% effective." The IPV vaccine prevents poliomyelitis (poliovirus), which can activate infection in the brain and spinal cord leading to paralysis.
The MMR vaccine 100e defends against measles, mumps, and rubella, which tends to have up to 97% effectiveness at inhibiting measles once directed in two dosages. A single dose is around 93% effective, as reported by the CDC. They suggest to give the initial dose at "12-15 months of age, followed by the second dose at 4-6 years."          In order to determine the similarities and differences between previously available vaccines and COVID-19 vaccines, the Bacillus Calmette-Guérin (BCG) vaccine against tuberculosis is provided as one more example; it confers a wide range of immunity against other infections, and it also may minimize the intensity of COVID-19. Moreover one epidemiological analyses provided universal connections between the vaccinations of BCG and COVID-19 mortality: the suggestion of BCG vaccination results on COVID-19 fatality are dominated by socio-economical and demographical variations between countries. In the wake of reducing the manifold distracting factors, many substantial connections between the BCG vaccination and decreased COVID-19 fatalities were perceived. Obviously this investigation emphasizes the necessity for an intrinsic mechanism of studies supporting BCG vaccination effects on COVID-19, and also for clinical evaluation to control the COVID-19 pandemic. 99b COVID-19 is a serious respiratory related disease, so the scientic and medicinal community are working hard across the globe to develop a vaccine. Presently, around sixty vaccine candidates are on trial in many countries. Now, nearing twenty candidate vaccines are in phase 3 clinical trials. Gratifyingly, seven vaccines have been approved in many countries.

Conclusions
Globally, as of January 2021, there have been around 88 million COVID-19 cases, including nearly 1.9 million fatalities, WHO have registered (see WHO Covid-19 case report). 99d These numbers are expected to increase further, so there is an a Globally, vaccine producers have the ability to achieve rapid development of highly efficient, safeminimum toxicityvaccines. emergency requirement to produce vaccines to protect people. Several candidates vaccines are being developed which are in preclinical and clinical trials. The mechanism of action of these candidates varies signicantly, as better knowledge becomes available 154a about this virus, researchers can adapt their design so if one candidate shows low efficacy, another one may be more active.
Similarly, different demographics may necessitate the need for designing vaccines with different mechanisms of action. The time required for the various stages of clinical trials needs to be shortened (without compromising on the ethics and safety) to achieve the desired goal in a short period of time. 154b Finally, manufacturing such large quantities of the vaccine or vaccines, quickly (without compromising on the quality, purity and efficacy) and distributing them to all parts of the world is another problem which the present planners have not faced before. This requires sufficient manufacturing capacity, availability of raw materials, logistics and several other factors. Also, as all resources are diverted towards SARS-CoV-2, epidemiologists and public health organizations should not lose track of their ght against other viruses. 155 The current assemblage of vaccine developers 156a may reward researchers with increased capability, as numerous basic, transformational and preclinical statistical data have become available during coronavirus exploration. These factors combine together as a substantial promising source for rapid vaccine development. 156b Since, December 2020, some new vaccines have emerged and also have been approved by certain national regulatory authorities for use against COVID-19. Amongst these, as per the universal expectations of the WHO EUL/PQ assessment, the Pzer vaccine and some other candidates have been approved. More studies on vaccine aspirant efficacy and safety results, 157 including on the Moderna and AstraZeneca vaccine, therein have been widely reported, and AstraZeneca have published their results in well reputed journals. Thus we are eagerly expecting more potential COVID-19 vaccine candidates will be offered to governing authorities for approval in the coming years. Gratifyingly, the growth of many efficient COVID-19 vaccine aspirants under clinical trials is fascinating. When the vaccine candidates are proven to be benign and efficient, they need to be acknowledged by the governing authorities, produced to the necessary standard, and distributed. WHO is collaborating all over the world to assist with the roles in this process, which includes facilitating reasonable access to safe and effective COVID-19 vaccines for everyone.

Author contributions
KD and SA equally contributed to the collection of data and developed the entire manuscript. MD corrected the manuscript. SG and ST assisted in draing.

Conflicts of interest
We have no conicts of interest.