The native bacterial microflora inhabits the gastrointestinal tract (GIT) from birth and persists there throughout life. Roughly more than thousands of microbes are existing in the small and large intestine and they are obviously immunogenic [1, 2]. The indication is strong that the massive majority of immunoglobulin (Ig) A plasma cells in normal human GIT are replying to the antigens of the flora, and while the flora is also accountable for fabricating the maximum amount of T cells which are existing in the GIT of in fine fettle people, the types of T cell reaction which the flora provokes are less well agreed [3]. General mechanism of immunity busting via, an antigen prompted stimulation of antigen-presenting cells (APCs); a link of innate and adaptive immune artilleries triggering T cell and B cell immunity by fundamental immunological pathways are characterized to show how vaccine prompted immune responses lead to the eradication of viral infection [4] (Figure 1). Whichever vaccines are presently approved for human use can be classified into two types virus oriented or protein-oriented vaccines. The virus-based vaccines can contain a live-attenuated virus or deactivated virus which is no longer infective [5]. Live-attenuated virus vaccines are characteristically produced by passaging in cell culture awaiting it loses its pathogenic characteristic and may produce merely a mild infection on injection. Protein-oriented vaccines can contain a protein purified from the virus or recombinant protein, or virus-infected cells, or virus-like particles (VLPs). VLPs comprise the mechanical viral proteins essential to form a virus particle, but lack the viral genome and non-structural proteins. Protein-oriented vaccines need the addition of an adjuvant to prompt a strong immune response [6]. Nowadays, the coronavirus disease-19 (COVID-19) vaccines oriented on these classical platforms are in clinical trials as well as available in the market, some based on the whole-inactivated virus and some are based on the recombinant protein [1]. These classical vaccine platforms have subsidized to most important public health breakthroughs [7, 8].

Though, certain restrictions are accompanying numerous of these platforms that make them less responsive to speed up vaccine development in a pandemic. In the case of VLPs, it is only one antigenic protein, conformationally arranged in the shape of a particle to offer important epitopes for the immune system/cells. Large amounts of the virus would require to be grown under biosafety circumstances for a whole-inactivated vaccine; extensive safety study is mandatory to safeguard live-attenuated viruses are safe and do not straightforwardly return to the thoughtful influence of the usual microbial flora of the gut on the mucosal immune system [9–11].

It barely needs highlighting that the colonization of the gut by microbes after birth is probably the largest antigenic challenge an animal will experience. It is also important to emphasize that the normal flora in humans probably does not exist as a single entity. With over four hundred species of microbes in the human ileum and colon, there is significant scope for quantitative and qualitative heterogeneity among dissimilar persons [12]. Laboratory mice often have a limited, specific-pathogen-free flora, are often fed sterile food, and are usually used at a young age. Early in life, most people, experience gut microorganisms and pathogens modifications this response to a type I helper T (Th1) lymphocytes, possibly through the introduction of interleukin (IL)-12 [13, 14]. Among them, vaccination platforms are widely used and they can be developed by using different vaccination platform development techniques (Figure 1). In this review study, we have focus on the various aspects of COVID-19 vaccine development platforms and future COVID-19 vaccine development scope as well [15].

An element that stimulates and triggers our immune system in contradiction of a specific disease and eradicates the pathogen, and protects us is called a vaccine. Numerous different kinds of vaccine development platforms exist, but the important functioning principle in all leftovers is the same. Vaccination permits the immune system to differentiate a pathogen and then initiate an immune response in contradiction of it [17]. At a later stage, when the body is exposed to a similar pathogen, immune machinery works to eradicate it from the body [1, 2]. Nowadays we have so many platforms that are available to develop the vaccine like viral vector base immunization, nucleic acid-base [deoxyribonucleic acid (DNA) acid and Ribonucleic acid (RNA)], whole pathogen base, live attenuated, base immunization, and many more (Figure 1). All types of platforms are useful for the development of any type of vaccine.

As per the available data, numerous approaches are implemented in the development of coronavirus vaccines; the maximum of this targets the surface open spike (S) glycoprotein or S protein as the main inducer of counteracting antibodies. Quite a lot of S protein established approaches have been tried for evolving coronavirus vaccines, for example, the use of full-length S protein and expression in VLP, DNA, or viral vectors and so many [65, 66]. Currently, more than 300 vaccines candidate is under development stage, and from them, 9 are permitted for emergency use in many countries like the USA, UK, India, Australia, China, Russia, and many more COVID affected countries [67, 68]. These approved candidates showing impressive efficiency from 50% to 95%. Recently, several new severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) variants have emerged and are circulating globally, and preliminary findings imply that some of them may escapeimmuneresponses against previous variants and diminish the efficacyofcurrentvaccines. Most of these variants acquired new mutations in their surface protein which is the antigen in most of the approved/under development vaccines. It has been possible, because of tools for fast-tracked testing comprising small and large animal models for vaccine effectiveness analysis, assays for immunogenicity evaluation, critical reagents, international biological standards, these all things are made possible to accelerate the development of vaccines [21]. Following table presenting the compilation on current status of vaccine development throughout the world (Table 2).

Currently, it is noticed that the virus rapidly going through the mutation process. Nowadays, many mutant variants were reported and it’s becoming a new challenge to fight against the COVID-19 pandemic. The mutations take place rapidly in the S and receptor binding motif of the virus, it will affect recombination events and other positive pressures, and that the receptor-binding motif is the most conflicting area of S which may lead to the occurrence of escape mutants. Variants with several S mutations have been noticed in India Brazil, South Africa, Denmark, UK, and the USA. Two Variants B.1.1.7 and B.1.351 which were first started in the UK and South Africa, correspondingly, have produced a great deal of concern regarding their potential impacts on vaccine efficacy [72]. Recent data from in vitro studies establish that vaccine sera from numerous diverse vaccines have meaningfully reduced neutralization activity in contradiction of variant B.1.351 associated with older viral sequesters although the sera display little or no reduced neutralization against variant B.1.1.7. Variant B.1.617 was first detected in India, at the last of October and has meanwhile spread to more than many other countries, including Australia. Phase III trial results from AstraZeneca and Novavax were publicized that the vaccines had high efficiency (85.6% and 74.6%) in contradiction of the B.1.1.7 variant. However, in the future there may be many more mutation variants will arise, and to defeat them we have to develop a vaccine, which eradicates all types of variants in the future [73, 74]. As per current literature, there is a crucial requirement to start the development of efficient forms of prominent vaccines to defend in contradiction of the B.1.351 variant and others, also to improve an agenda for extended genomic investigation and quick analysis of novel variants to produce actionable research. For instant response, the emerging variants, the Coalition for Epidemic Preparedness Innovations (CEPI) set up a combined project named “Agility” in corporation with Public Health England (PHE), National Institute for Biological Standards and Control (NIBSC), and the Global initiative on sharing all influenza data (GISAID), to permit the speedy biological evaluation of developing variants both in vitro and in vivo. The agility project’s purpose is to deliver open-access high-quality reports on the biological suggestions of developing variants and notify the requirement for strain changes or alterations for vaccines to guarantee efficiency is sustained [30, 75–77].

In this review, we have described different vaccine platforms for the development of the vaccine. There are viral and non-viral vector based platforms available for vaccine development. Viral vectors have been explored for their application as vaccine antigen transporters. The use of replicating or non-replicating viruses as a platform to supply vaccine antigen. The main challenge to consider is the likely introduction of vector-specific immunity which consequently impairs the capability of viral vectors to provoke appropriate immune responses in contradiction of the antigen being conceded. Nevertheless, this problem can be prevented by accepting suitable vaccination approaches such as the use of viruses that do not spread in humans and the use of diverse virus serotypes for major and improvement vaccinations. Therefore, many viral-vectored vaccines, non-viral vector vaccines, and immunotherapeutic agents are presently being estimated and used in vaccine developments. Although the additional challenges regarding vaccine development are to be explored in the future.