The smallpox vaccine was an active infection of cowpox that left you immune to both diseases. Seasonal flu shots use active flu virus, cultured in eggs, then inactivated and concentrated. Mrna vaccines are a set of rna instructions that tell your cells to make a bunch of unique proteins that a virus uses to enter your cells, this triggers your immune system to recognize that protein.

There is no related infection like cowpox and covid mutates too quickly to wait for literally a billion eggs to be laid. Mrna vaccines can be designed in days and manufactured in less than a month . The mortality rate of vaccinated people was a fraction of those who were not vaccinated, the vaccine does not stop you from getting all covid forever, it stops you from becoming severely ill from one particular strain. But every time it mutates a new vaccine has to be developed to match, the same as seasonal flu but 5x faster.