Submitted to: American Veterinary Medical Association Abstract
Publication Type: Abstract Only
Publication Acceptance Date: April 12, 2008
Publication Date: July 19, 2008
Citation: Swayne, D.E. 2008. Scientific basis for use of vaccination as a strategy to control avian influenza [abstract]. In: Proceedings of the 145th American Veterinary Medical Association Annual Convention, July 19-22, 2008, New Orleans, Louisiana. 2008 CDROM.
Vaccines have been used to control a variety of piscian, avian, and mammalian diseases. Commercial usage of vaccines against avian influenza (AI) began in 1979, in Minnesota to control H4 and H6 low pathogenicity avian influenza (LPAI) which was causing economically significant disease in turkey breeders and meat turkeys. This was soon followed by AI vaccine use in turkeys of California. Since 1979, AI vaccines have had limited application for AI, mostly as autogenous vaccines against LPAI, but such use has been critical for control and eradication programs. However, in the mid-1990’s the emergence of H9N2 LPAI in developing countries resulted in the first large scale use of AI vaccine. For HPAI, the use of vaccine has been much slower to be accepted and applied; historically, HPAI has been dealt with by stamping-out programs. However, in the mid-1990’s, large scale outbreaks of H5N1 HPAI in Mexico and H7N3 in Pakistan were not controllable with traditional stamping-out strategies. As a result, vaccines were applied in these situations to reduce outbreak virus spread and control the economic impact of the disease. The use of such vaccines has continued today, especially in Central America to control the predecessor H5N2 LPAI virus. However, the greatest quantity of AI vaccine used has followed the emergence of H5N1 HPAI epizootic in Asia, Africa, and Europe. The actual quantity of vaccine doses used is unknown, but from conservative estimates the amount has exceeded 30 billion doses.
Two main aspects are critical for vaccine success in a control program: 1) vaccine efficacy, and 2) vaccination effectiveness. Vaccine efficacy encompasses the safety and purity of the vaccine, having sufficient antigenic content to produce robust protective immune response, and a sufficiently close genetic match to protect from minor genetic drift. Protection is primarily based on an antibody response to the hemagglutinin (HA) protein and is specific to an HA subtype. Vaccination effectiveness involved all aspects of application of vaccines from storage to administration. When properly applied, high quality AI vaccines will protect poultry by increasing resistance to infection, preventing illness and mortality, reduce the number of infected birds, and if infected, greatly reduce the amount and time that virus is shed from respiratory and alimentary systems. This translates into reduction in environmental contamination or viral load by AI virus and thus reduced transmission. However, vaccination alone will not eradicate AI. Vaccination should only be used as one tool in a comprehensive control program that includes enhanced biosecurity, increased surveillance, education of poultry workers, and elimination of infected poultry.
Over the past 40 years, AI vaccines have been primarily based on field outbreak AI strains that were grown in embryonating chicken eggs, chemically inactivated, emulsified in mineral oil adjuvant, and injected into individual birds. Recently, recombinant viral vectored vaccine have been developed and licensed including fowl poxvirus and avian paramyxovirus type 1 (ND) vectored vaccines with AI H5 gene inserts. Advances in biotechnologies may overcome some existing limitations and result in vaccines that can be grown in tissue culture systems for more rapid vaccine production; provide optimized protection as the result of closer genetic match to field viruses through reverse genetics and gene insertions in vector systems; can be mass applied by aerosol, drinking water or in ovo administration; and provide easier strategies for identifying infected birds within vaccinated populations; i.e. Differentiating Infected from Vaccinated Animals (DIVA). However, these new technologies will be licensed only after demonstration of purity, safety, efficacy and potency against AI viruses, and limitation on horizontal transmission in naïve poultry. There potential use in the field will also be determined on the requirement for low cost vaccines to be economically competitive.