Regarding one vaccine's early trials...
The Salk vaccine was first tested on "disposable children."
"Salk and team, with the support of the National Foundation for Infantile Paralysis, began its first tests on humans of their killed-virus polio vaccine. The subjects were resident children in institutions for the physically and intellectually disabled. Salk tested vaccine for all three strains of polio, some in combination, and some on their own.
Their findings showed that vaccine recipients produced antibodies to the virus type in the vaccine they were given. Salk’s results provided some evidence on which to base larger trials."