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).