Recombinant viruses obtained from co-infection in vitro with a live vaccinia-vectored influenza vaccine and a naturally occurring cowpox virus display different plaque phenotypes and loss of the transgene.

Some poxviruses are very attractive as transgenic vaccine vectors for humans, domestic animals and wildlife. Poxviridae family members circulate in different ecosystems and parts of the world, providing a pool of possible recombination partners for released or escaped genetically modified poxviruses. We performed in vitro double infections with a vaccinia virus strain Ankara (MVA) vectored influenza vaccine and a cowpox virus isolate from Norway, isolated hybrids, and further analyzed three hybrid viruses with different plaque phenotypes. One of the hybrids was genetically unstable, and during adaptation to new host cells its MVA derived influenza gene was deleted at a high frequency. This is significant in a risk assessment context, since the transgene would be the only logical tag for monitoring unwanted spread and non-target effects of a vaccine virus. Putative recombination events involving genetically modified and naturally occurring viruses should be included in health and environmental risk assessments.