Strategies and challenges for eliciting immunity against avian influenza virus in birds.

Vaccines and vaccination have emerged during the past two decades as essential tools in avian influenza (AI) control for poultry, because they increase resistance to infection, prevent illness and death, reduce virus replication and shed from respiratory and alimentary tracts, and reduce virus transmission to birds and mammals, including humans. Such protection in birds is primarily mediated by homosubtypic humoral immunity against the hemagglutinin protein, but cell-mediated and innate immunity contribute to protection in some bird species. The immune response to the neuraminidase protein can contribute to protection, but immunity to the viral internal proteins is generally not protective. Although, some preliminary studies with M2e protein in chickens suggest partial protection may be achievable. Historically, the H5 subtype AI vaccines have demonstrated broad homosubtypic protection, primarily against H5 high-pathogenicity (HP) AI viruses isolated in the early stages of outbreaks. However, as H5 viruses have become endemic and outbreaks prolonged, some drift variants with resistance to earlier H5 AI vaccines have emerged in Central America, China, Egypt, and Indonesia. How widespread such drift variants are will remain unknown until more detailed genetic and antigenic analyses are conducted on field isolates. Future vaccines will utilize biotechnology to produce new AI vaccine seed strains using HA genes more closely matching circulating field viruses. In addition, newer technologies for AI vaccines will improve vaccine coverage by using mass application technologies for example by drinking water, by spray, or via injection in ovo or at the hatchery.