6.2 Subunit vaccines
Subunit vaccines, safer and easier to produce, present one or few antigens to host with strong immunogenicity, but they require adjuvants to elicit a strong protective immune response (Zhang et al., 2020). Subunit vaccines against SARS elicit immune response against the S protein which prevents binding of virus with ACE2 receptor in host lungs (Jiang et al., 2012) and currently, almost all RBD subunit vaccines in process of development against SARS-CoV-2 are based on S protein or RBD of S1 protein as antigens (WHO, 2020; Zhang et al., 2020). The SARS-CoV and SARS-CoV-2 RBDs present the similarity of over 80% in their amino acid sequence which offers an opportunity to develop either of the proteins as subunit vaccine for both and it also offer an advantage of minimum immunopotentiation in the host (Chen et al., 2020; Jiang et al., 2012). After emergence of SARS in 2002, a subunit vaccine consisting of a soluble baculovirus-expressed NTD of S1 protein was developed and immunization in mice elicited high VN antibody titers which conferred highly effective protection against subsequent intranasal challenge with SARS-CoV (Bisht et al., 2004). At Baylor College of Medicine, Texas Children’s Hospital Centre for Vaccine Development developed and tested a subunit vaccine based on the RBD of S1 protein of SARS-CoV which elicited a strong immunity on homologous virus challenge when formulated in alum (Jiang et al., 2012; Chen et al., 2017; Chen et al., 2014). Although some full-length S-proteins of SARS have been observed to elicit increased infectivity and eosinophilic infiltration, Clover Biopharmaceuticals Inc. developed a trimerized S protein based subunit vaccine against SARS-CoV-2 using its Trimer-Tag©technology (Chen et al., 2020; Clover biopharmaceuticals, 2020). Also, Novavax Inc. is assessing the efficacy of its newly developed nanoparticle vaccine candidates based on S protein to arrive at a suitable vaccine candidate for future human trials expected in mid-May (Novavax, 2020). Similarly, the other units which are the process of subunit vaccines development against SARS-CoV-2 are Johnson & Johnson, Pasteur Institute, and Chongquing Zhifei Biological Products Co. Ltd. (Zhang et al., 2020).