6. Vaccine strategies against COVID-19
Since, the SARS-CoV-2 reservoir has not been established yet its re-emergence or that of its related virus in future is expected to infect the human populations again. Only the vaccine development targeting this virus will help contain the virus before it cripples the global health and economic system again as observed in the current pandemic. Currently, we do not have any CoV vaccine, approved or licensed, to prevent respiratory infections in humans. Because of increasing COVID-19 cases at an alarming speed, continuous appearance of MERS cases, threat of re-emergence of SARS, and the emergence of new CoV infections in future (Wang et al., 2020), it is important to target the development of a broad spectrum vaccine with adequate safety and efficacy to prevent such infections in humans. Several vaccines have been proposed for COVID-19 which are in various stages of development (WHO, 2020). Different vaccine strategies have been adopted by various institutions and companies based on their previous technological platforms, resource base, and demands imposed by the current pandemic. The approaches and strategies adopted for SARS and MERS vaccines will provide intriguing inputs for the speedy design and development COVID-19 vaccines (Wang et al., 2020). The use of new technologies like recombinant viral vectors and nucleic acid vaccines provide universal vaccine platforms which conveniently develop and present new antigenic targets from emerging viruses (Rauch et al., 2018). Upon immunization they mimic attenuated virus vaccines which infects host cells and induce endogenously produced antigenic proteins, respectively, which elicit antibody as well as T-cell immune responses (Rauch et al., 2018; Roper and Rehm, 2009; Enjuanes et al., 2008; Schindewolf and Menachery 2019). Most of the nucleic acid vaccines based on either DNA or mRNA constructs encode for S or RBD proteins in host cells (Roper and Rehm, 2009; Enjuanes et al., 2008; Schindewolf and Menachery 2019) and the similar proteins are prime targets of subunit vaccines for SARS-CoV and MERS-CoV which can be extended to SARS-CoV-2 too (Jiang et al., 2020). Currently, most of the COVID-19 candidate vaccines under development are S protein or RBD subunit vaccines and vector vaccines which express mainly S protein or the RBD (WHO, 2020). The criteria for any successful vaccine development are safety, efficacy, and duration of immunity. However, in case of pandemics like COVID-19 rapid development of vaccine at a very high production capacity is another important criterion (Rauch et al., 2018). When it comes to traditional inactivated and attenuated vaccines, the rapid production of such vaccines in large quantities under biosafety level-3 conditions is difficult which renders them less suitable as first-generation vaccines under pandemic conditions (Saif, 2020).