Immunity to infectious bronchitis virus from spray vaccination with derivatives of a Holland strain.

Study was made of the protection of chickens from spray vaccination with different modifications of a Holland (H) strain of infectious bronchitis virus (IBV). Noted in each trial were the virus-serum-neutralizing antibody titers induced and the virulence of the vaccine virus at different passage levels. Protection against respiratory signs was good from challenge with a homologous virus serotype as well as with 5 heterologous IBV isolates. This occurred from vaccinating day-old chicks possessing homologous IBV antibodies as well as vaccinating susceptible 4-week-old chickens. The tracheal virus shed-rate following challenge was considered low (20% or less) with 4 of the challenge viruses. The challenge virus shed-rate was somewhat higher from vaccination at one day old than from vaccination at 4 weeks; the virus-serum-neutralization titers were lower, or even insignificant, with early-age vaccination. The immunogenicity and virulence of the vaccine virus was apparently reduced in the higher embryo passages employed. None of the vaccine derivatives was considered virulent under these conditions.