PROS1 and COVID-19: More than 89% of these vaccines contained recombinant antigens.2 Full-length S protein and RBD are the most commonly used amino acid sequences for recombinant COVID-19 vaccine candidates (see text footnote 2) (Corbett et al., 2020; Folegatti et al., 2020; Keech et al., 2020; Kuo et al., 2020; Logunov et al., 2020; Yang et al., 2020; Zhu et al., 2020; Chen et al., 2021; Lamb, 2021; Lee et al., 2021; Liang et al., 2021; Sadoff et al., 2021).