1. Introduction

A vaccine is the best approach for infectious disease prevention [1]. The first effective vaccine developed was against the smallpox virus in the form of live attenuated, and, through worldwide mass vaccinations, smallpox was declared eradicated [2]. Pasteur and his team coined the idea of attenuation and defended its use with pasteurella multocida, responsible for diarrhea in chickens. Later, anthrax vaccine in sheep and rabies vaccine in animals and humans were developed using the attenuation approach [3]. The concept of inactivation of a pathogen without losing immunogenicity was devised at the end of the 19th century. Later, various inactivated (killed, dead) vaccines were formulated against typhoid, cholera and plague diseases. Over time, various novel technology-based vaccines have been developed, such as diphtheria toxoid and recombinant technology-based hepatitis B vaccine [4]. Despite the success, vaccine development is still a challenging task as scale-up productions and funding resources have commonly hindered much of their development. This process is usually challenging, complex and takes 15–25 years for the final approved product, and by then, the patent life would have ended. It often requires expenses of more than $500 million. Pre-COVID-19 era, only 7% of the vaccines developed progressed to human clinical trials from pre-clinical studies [5]. Additionally, of those in clinical trials, only 20% show safety and efficacy [6]. Other considerations required for vaccine development are stability, storage conditions, number of injections, route of immunization, optimal dose, scale-up, manufacturing and distribution of the vaccine globally.

The novel coronavirus strain, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) , emerged from Wuhan, China, in late December 2019 ^[7][8][7,8]. Rapidly, it spread around the globe and was declared a pandemic soon after [9].

In parallel, there was and is a worldwide race for vaccine discovery. A number of different modes of vaccine formulations have been developed at an unprecedented speed ^[4][10][4,10], such as live attenuated, inactivated (dead), subunit, viral vector and genetic (nucleic acid), against the SARS-CoV-2 virus ^[11][12][13][11,12,13]. Currently, seven vaccines using three different approaches have been approved by the WHO for emergency authority use. Generally, the vaccine development process requires 15–25 years for approval. However, COVID-19 vaccines have been approved within 6–9 months in an accelerated development process in response to the pandemic situation [14]. In fact, the speed has been possible due to the method used for vaccine development in the instance of nucleic acid-based vaccines. These vaccine formulations have been around for over 30 years and tested in humans for a number of diseases, such as cancer, HIV and other viruses, but none have been approved for human use. However, the pandemic has enabled their use to be fast tracked, a technology that instigates revolution in the vaccine development process and overcomes the limitations of previously available technologies [15]. It involves the administration of nucleic acid coding for antigens in the body. This nucleic acid coding acts as a set of instructions and guides the host cells to produce antigenic proteins, which, subsequently, stimulate specific immune responses against the gene-delivered antigen. Thus, the nucleic acid vaccine immunizes the host against the specific pathogen [10].

2. Advantages of Nucleic Acid-Based Vaccines

Upon immunization, nucleic acid-based vaccines imitate a viral infection, causing vaccine antigens to be expressed in situ, which tends to result in the initiation of both humoral and cytotoxic T-cell responses ^[16][112]. Nucleic acid-based anti-SARS-CoV-2 vaccines may have advantages over traditional vaccines for the following reasons: (i) The high potency of mRNA vaccines is capable of generating potent antiviral neutralizing antibodies by activating both CD4+ and CD8+ T-cells with only one or two low-dose immunizations ^[17][18][113,114]. (ii) The structural modification of mRNA results in higher immunogenicity by improving its stability and translation efficacy ^[17][113]. (iii) Because of its degradation process in cells, mRNA-based vaccines reduce the risk of infection and insertion-induced mutagenesis ^[19][115]. (iv) These vaccines are easier to design in roughly a day and are also easy to produce on a large scale ^[20][116]. (v) Nucleic acid vaccine candidates generate strong protection via antibody generation and activating the cell-based immune pathways ^[21][117]. (vi) These vaccine candidates, particularly DNA vaccines, have a great amount of shelf-life stability and elicit a potent immune response ^[22][23][102,107]. (vii) Considering the pandemic situation and global vaccine demands to eradicate the same, these vaccines can be scaled up and mass-produced easily as compared to the conventional vaccine formulations ^[23][107]. These vaccine candidates have demonstrated sound immune response with potential efficacy against the novel coronavirus in clinical trials thus far.

3. Challenges for Nucleic Acid Vaccine Development