Vaccination of zoo birds against H5N1

Since 2001, highly pathogenic avian influenza viruses (HPAIV) of subtype H5Nx (clade have spread globally causing enormous economical losses in poultry farming. Remarkably, the viruses of this clade also affected several wild bird species, posing a considerable threat to small populations of endangered species in some regions. Moreover, H5N1 was also transmitted to captive birds that were kept free-ranging at water reservoirs in several European zoos. To avoid any contact to potentially infected wild birds visiting these water reservoirs, zoo birds (flamingos, pelicans, pinguins, ducks geese, etc.) were kept for several months under close quarantine conditions which resulted in severe health problems in some of these species. The goal of this project is therefore to vaccinate zoo birds against H5Nx viruses, so that are protected from infection by H5Nx and thus can be kept in a species-appropriate manner.

Our group has developed a vector vaccine which is based on a non-replicative vesicular stomatitis virus (VSV) in which the essential VSV G gene has been deleted and replaced with the HA gene of H5N1 (Halbherr et al., 2013). This vaccine resulted in complete protection against H5N1 in vaccinated chickens. The vaccine also allows easy serological differentiation of vaccinated from H5N1-infected animals (DIVA principle). The vector vaccine has now been adapted to express the HA gene of a clade H5Nx virus.

Following approval of the release of this recombinant vector vaccine, immunization of zoo birds has been started in August 2023. Meanwhile, 317 animals representing 27 bird species have been vaccinated twice at the zoos of Bern and Basel Zoo. The vaccine was well tolerated by the animals, and no side effects were observed. Preliminary serological investigations revealed that the animals formed H5N1-neutralizing antibodies already after the first immunization. The ongoing serological investigations will show whether the second immunization will boost antibody titers and how long antibody titers will last.


  • Foundation Animal Hospital Basel


  • Dr. Stefan Hoby, Zoo Bern
  • Dr. Christian Wenker, Zoo Basel