Vaccine control of avian influenza H5N1 in poultry: Need for a positive marker

Highly pathogenic avian influenza (HPAI) H5N1 virus strains have emerged as zoonotic viral pathogens over the last decade and have eluded our serious attempts of control in domestic poultry by vaccination, with numerous countries continuing to have epidemic waves. Although the biology and genomics of H5N1 influenza viruses are well characterised, viral outbreaks still occur in domestic poultry, posing a dangerous threat of human transmission. There are two main types of current vaccines, inactivated whole virus vaccines and virus vaccines engineered by reverse genetics, both of which are administered with adjuvant to hatchlings and optimally require a booster. However, immunological determinants of vaccine efficacy need to be considered distinctly in chickens and ducks. Our futile attempts to control H5N1 viral outbreaks in domestic poultry flocks question our capacity for future achievements of eradicating circulating H5N1 influenza viruses. There is a need for detection of infection and vaccination of domestic poultry to control potentially deadly but silent infection in vaccinated flocks. A positive marker strategy using tetanus toxoid has several advantages over other negative markers to enable differentiation of infected from vaccinated animals (DIVA) for more effective control programs of HPAI in order to minimise the likelihood of an avian H5N1 influenza pandemic