A complex of a virus-based therapeutic agent with a polymer of formula I: wherein each L 1 and L 2 is independently selected from the group consisting of: O, S, NR x and a bond; wherein R x is independently selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, heteroalkyl, heterocycloalkyl, acyl, aryl or heteroaryl; L 3 is independently selected from the group consisting of alkylene, alkenylene, heteroalkylene, heteroalkenylene, arylene or heteroarylene; or at least one occurrence of L 3 is wherein T 1 is and T 2 is selected from H, alkyl or wherein L T is independently selected from the group consisting of: O, S, NR x and a bond, wherein R x is independently selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, heteroalkyl, heterocycloalkyl, acyl, aryl or heteroaryl, and the remaining L 3 groups are independently selected at each occurrence from the group consisting of alkylene, alkenylene, heteroalkylene, heteroalkenylene, arylene or heteroarylene; L 4 is selected from the group consisting of L5 is independently selected from the group consisting of alkylene, alkenylene, heteroalkylene, heteroalkenylene, arylene or heteroarylene; R 1 and R 2 and R T (if present) are independently selected from an oligopeptide and R y; wherein at least one of R 1 and R 2 and R T (if present) is an oligopeptide; and wherein R y is selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, heteroalkyl, heterocycloalkyl, acyl, aryl or heteroaryl; each R 3 is independently selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, heteroalkyl, heterocycloalkyl, acyl, aryl, heteroaryl and polyalkylene glycols, wherein said polyalkylene glycol is either bound directly to the nitrogen atom to which R 3 is attached or bound to the nitrogen atom to which R 3 is attached via a linker moiety, wherein said linker moiety is an alkylene, cycloalkylene, alkenylene, cycloalkenylene, heteroalkylene, heterocycloalkylene, arylene or heteroarylene group; and n is an integer from 5 to 1,000; or a pharmaceutically acceptable salt thereof. L5 is independently selected from the group consisting of alkylene, alkenylene, heteroalkylene, heteroalkenylene, arylene or heteroarylene; R 1 and R 2 and R T (if present) are independently selected from an oligopeptide and R y; wherein at least one of R 1 and R 2 and R T (if present) is an oligopeptide; and wherein R y is selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, heteroalkyl, heterocycloalkyl, acyl, aryl or heteroaryl; each R 3 is independently selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, heteroalkyl, heterocycloalkyl, acyl, aryl, heteroaryl and polyalkylene glycols, wherein said polyalkylene glycol is either bound directly to the nitrogen atom to which R 3 is attached or bound to the nitrogen atom to which R 3 is attached via a linker moiety, wherein said linker moiety is an alkylene, cycloalkylene, alkenylene, cycloalkenylene, heteroalkylene, heterocycloalkylene, arylene or heteroarylene group; and n is an integer from 5 to 1,000;