What definition of "vaccine" are you using? From Wikipedia[0]:
Vaccines can be prophylactic (to prevent or ameliorate the effects of a future infection by a natural or "wild" pathogen), or therapeutic (to fight a disease that has already occurred, such as cancer). Some vaccines offer full sterilizing immunity, in which infection is prevented completely.
The key word up there is "ameliorate," which is exactly what the Covid vaccines available in United States promise to do.