SUMMARY In recent years, Arkansas Delmarva Poultry Industry (ArkDPI)-derived infectious bronchitis virus (IBV) vaccines have been used to characterize immune responses of chickens subsequent to vaccination on day of hatch or beyond. Probably because ArkDPI vaccines display increased heterogeneity, the results on cell immune responses have shown ambiguity. In the current study, we investigated the effects of vaccination with a highly stable and homogeneous Massachusetts (Mass)-type vaccine on days 1 or 7 of age on Harderian gland responses. Confirming previous studies, both IBV serum antibodies and lachrymal IgA levels were greater upon vaccination on day 7 compared to vaccination on day 1 of age. Unlike results using ArkDPI viruses, a clear trend was detected for both B and T cells in the Harderian gland after Mass-type vaccination. Consistent with antibody responses, B and T-helper (CD3 + CD4 + ) cell frequencies were higher in birds vaccinated on day 7 of age. Cytotoxic T cells (CD3 + CD8 + ) were also increased compared to chickens vaccinated on day 1 of age. We concluded that depending on the age of IB outbreaks commonly occurring in chicken flocks in a particular region, postponing the first IBV vaccination may optimize immune responses.

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