Safety and immunity of Covid‐19 aAPC vaccine  Biological: pathogen‐specific aAPC  China/Covid‐19 infection  100, all, 6 months to 80 years  Treat and Prevent Covid‐19 Infection  Experimental: the subjects will receive three injections of 5×10^6 each Covid‐19/aAPC vaccine via subcutaneous injections  I  Primary (0–28 day): frequency of vaccine events, frequency of serious vaccine events, proportion of subjects with positive T cell response