Safe and effective vaccines are the best method to defeat worldwide SARS-CoV-2 and its
circulating variants. The SARS-CoV-2 S protein and its subunits are the most attractive
targets for the development of protein-based vaccines. In this study, we evaluated three
lipophilic adjuvants, monophosphoryl lipid A (MPLA), Toll-like receptor (TLR) 1/2 ligand
Pam
3
CSK
4
, and α-galactosylceramide
(
α
-GalCer), in liposomal and nonliposomal vaccines. The
immunological results showed that the MPLA-adjuvanted liposomal vaccine induced the
strongest humoral and cellular immunity. Therefore, we further performed a systematic
comparison of S-trimer, S-ECD, S1, and RBD as antigens in MPLA-adjuvanted liposomes and
found that, although these four vaccines all induced robust specific antibody responses,
only S-trimer, S1, and RBD liposomes, but not S-ECD, elicited potent neutralizing
antibody responses. Moreover, RBD, S-trimer, and S1 liposomes effectively neutralized
variants (B.1.1.7/alpha, B.1.351/beta, P.1/gamma, B.1.617.2/delta, and
B.1.1.529/omicron). These results provide important information for the subunit vaccine
design against SARS-CoV-2 and its variants.