Efficacy of four commercial Infectious Coryza vaccines available in Thailand were examined for protection rate against Thai field isolates serovar A, B, and C. Three hundred and thirty six male, layer chickens were divided into 25 groups. Groups 1-18 were vaccinated twice at 9 and 13 weeks old. Groups 18-24 served as positive controls, and group 25 served as a negative control. Then, groups 1-24 were challenged with Avibacterium paragallinarum at 15 weeks old. The result showed that vaccines 1 and 2 provided 100% protection for the birds against serovar A, B, and C Thai field isolates. Vaccine 3 provided 100% protection against serovar B and C, while Vaccine 4 provided 100% protected against only serovar B. No adverse reaction of vaccines was observed in any group. This study revealed that the protection rate of Infectious Coryza vaccines depended on the strains isolated from each country.

Key words: Avibacterium paragallinarum, Cross protection, Infectious Coryza, Vaccine efficacy