Assessment of Reduced Vaccine Dose on Efficacy of an Inactivated Avian Influenza Vaccine Against an H5N1 High-Pathogenicity Avian Influenza Virus

Abstract

Avian influenza (AI) vaccines are a viable emergency tool for use in a comprehensive strategy for dealing with high-pathogenicity AI in developed countries. However, the available doses of inactivated AI vaccine are limited to national vaccine banks and inventory stocks of some commercial biologics manufacturers. To determine if the available vaccine doses could be stretched by using reduced vaccine dose but maintain adequate efficacy, a study was conducted to determine if 3-wk-old specific pathogen-free White Leghorn chickens vaccinated with full, ½, ¼, and 1/10 doses of an inactivated H5N9 AI vaccine would be protected against a high-dose challenge of H5N1 highly pathogenic AI virus given 4 wk later. At all 4 AI vaccine doses, the AI-vaccinated chickens were protected from disease and death, but all the sham-vaccinated chickens developed clinical signs and died. There were no differences between the full, ½, and ¼dose AI vaccine groups for serological titers at 7 wk of age, or for cloacal and oropharyngeal titers of challenge virus shed at 2 d postchallenge. However, the 1/10 dose group had significantly reduced hemagglutination inhibition titers at 7 wk compared with ¼ dose, and the 1/10 dose group had more chickens shedding challenge virus from the oropharynx than the full dose group. Most importantly, the mean protective dose was 1/50 dose, and using the international regulatory standard of 50 mean protective doses for Newcastle disease vaccine as a guide, the full dose of the H5N9 AI vaccine would be the minimum dose acceptable for use in the field. Use of the full vaccine dose is especially important, because protection in commercial chickens in the field is typically less than seen in experimental studies in specific pathogen-free chickens in the laboratory.