Avian influenza vaccination in North America: strategies and difficulties.

Vaccination with high quality efficacious vaccines that are properly delivered can contribute to the control of avian influenza (AI) outbreaks when used as part of a comprehensive control programme that includes quarantines, animal movement controls, increased biosecurity, enhanced surveillance, and education. In North America both whole virus killed adjuvanted vaccines and fowlpox recombinant vaccines have been used to aid in the control of AI. The fowlpox recombinant vaccine is licensed in several countries including the United States (U.S.), but it has only been used in the field in Mexico and some Central American countries. The U.S., however, has considerable experience with the use of killed vaccines, primarily in turkeys. In the state of Minnesota in the 1980s and early 1990s, outbreaks of AI in range-reared turkeys were common, and vaccines were used successfully as part of a controlled marketing programme. More recently, several large layer flocks in Connecticut were vaccinated as an alternative to immediate depopulation after an H7N2 low pathogenic AI outbreak. The vaccinated flocks were intensively monitored for virus shed using sentinel birds, dead bird testing, and eventually some serological surveillance using a neuraminidase DIVA (differentiation of infected from vaccinated animal) approach. With these successes, vaccination is being considered as a valuable tool in comprehensive AI control strategies. Consideration for matching the vaccine to the field strain should also be considered to provide optimal protection including reduced shedding of virus. Antigenic drift of AI viruses after extended vaccination programmes has been observed in chickens, similar to what has been observed with human influenza viruses. Therefore, periodical evaluation of the vaccine to the field strain is necessary to maintain good protection from clinical disease and virus shedding.