Group	1st vaccine (200 μL/mouse)	2nd vaccine (200 μL/mouse)	3rd vaccine (200 μL/mouse)
rRBD protein	20 μg protein in PBS (100 μL) + 100 μL SAS	10 μg protein in PBS (100 μL) + 100 μL SAS	10 μg protein in PBS (100 μL) + 100 μL SAS
PBS control	100 μL PBS + 100 μL SAS	100 μL PBS + 100 μL SAS	100 μL PBS + 100 μL SAS