The live infectious bronchitis (IB) vaccine, H120, protected chickens against intranasal challenge with a mixture of strains ( Pool) and IB virus (IBV) strains of the same (Massachusetts) serotype as H120; it usually also protected against challenge with the Pool and IBV strains of other serological types. When these challenge strains were themselves used as vaccines they usually protected against challenge with a mixture of the Pool and an IBV strainof the Massachusetts serotype (VF69-149) or an IBV strain not of the Massachusetts serotype (HVI-116). Poor protection, when observed, was most common inthose experiments involving a minority of the IBV strains that had been incriminated in recent outbreaks of disease in vaccinated flocks of chickens. Much lower concentrations of IBV strain VF69-149 and O18 were found in the nose, trachea and spleen of H120-vaccinated chickens killed at different times after they were given a mixture of these organisms than were found in these sites in similarly challenged unvaccinated chickens. Some protection against challenge with IBV and the Pool was also observedin chickens vaccinated with an inactivated IBV strain; it was much less effective than that obtained following vaccination with the corresponding live IBV strain.


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