Currently, representatives of a wide range of virus families are under intensive development as vaccine vectors for human or veterinary use. Of these, FWPV and CNPV appear to be of great interest as vectors, and some veterinary APV-vectored vaccines are already licensed and in commercial use in North America, South America and Europe ( Table 2 ). The most important characteristics of APVs as vaccine vectors are that unlike most other DNA viruses, APV replicate in the cytoplasm of the infected cell and enzymatic functions used for transcription and replication are provided by the virus itself. This has several consequences regarding the use of these viruses as vaccine vectors. For example, APV promoters must be used for efficient transcription of recombinant genes and as APV transcripts are not spliced, genes cloned into APV vectors cannot contain introns [70, 104] . Other reasons include (1) their ability to accommodate and effectively express large amounts of foreign DNA or multiple genes that encode antigens [47] , (2) their inability to conduct a full replication cycle in non-avian species [105] [106] [107] , (3) antisera against orthopoxviruses do not neutralize APV and thus, prior Table 2 . Notably among them is the Trovac AI H5, a recombinant FWPV that express the H5 antigen of avian influenza virus. This product has had a conditional license for emergency use for chickens in the United States since 1998 and has been widely used in Central America, with over 2 billion doses administered [110] . ALVAC vectored vaccines have recently been registered for veterinary use in the European Union (Proteq-Flu) [111] and the United States (Recombitek). The equine influenza virus vaccine with CNPV vector expresses the hemagglutinin genes of the H3N8 Newmarket and Kentucky strains and contains a polymer adjuvant (Carbopol; Merial Ltd.). With the induction of both cell-mediated and humoral immunity, it is claimed that the vaccine produced sterile immunity 2 weeks after the second of two doses. The new vaccine is also designed to protect horses against the highly virulent N/5/03 American strain of equine influenza virus and to prevent the virus from spreading through the elimination of viral shedding.