Final answer:
Replication deficient smallpox vaccines are derived from non-infectious or attenuated strains of vaccinia virus and are used to introduce recombinant antigenic epitopes for protection against smallpox.
Step-by-step explanation:
Replication deficient smallpox vaccines are vaccines that are derived from strains of vaccinia virus that are non-infectious or attenuated. These vaccines are used to introduce recombinant antigenic epitopes to provide immunity against smallpox. Other attenuated vaccine vectors, such as the 17D strain of yellow fever virus, Salmonella typhi murium, Bacillus Calmetta Guerin (BCG), and poliovirus, are also used for delivery of sub-unit vaccines.