A homologous booster shot involves the administration of the same vaccine as previously administered, while a heterologous booster shot involves the administration of a different vaccine.[citation needed]
"Heterologous prime-boost immunization is administration of two different vectors or delivery systems expressing the same or overlapping antigenic inserts."[1]
"An effective vaccine usually requires more than one time immunization in the form of prime-boost. Traditionally the same vaccines are given multiple times as homologous boosts. New findings suggested that prime-boost can be done with different types of vaccines containing the same antigens. In many cases such heterologous prime-boost can be more immunogenic than homologous prime-boost."[2]