Immune Responses to Live-Attenuated Prime-Boost Vaccination Are Independent of Injection Site in Calves

Vaccination is highly effective in controlling infectious diseases in calves. A prime-boost injection is often administered intramuscularly in the neck and the hip in calves. However, no guidelines have established whether the same vaccination sites could be used for booster vaccinations. This study investigated the influence of intramuscular administration sites for sequential vaccines on immune responses in calves. All calves received the primary live-attenuated viral vaccine into the left side of the neck. Four weeks later, the booster was administered at the same site as the primary vaccine (local boost group) or in the right hip (distal boost group). The neutralizing antibody titers did not differ significantly between the groups. To assess T-cell responses to viral antigens after booster vaccination, the expression levels of lymphocyte activation markers CD25 and CD69 were measured in peripheral blood mononuclear cells (PBMCs). The site of booster administration had no significant effect on the activation of CD4⁺ and CD8⁺ T cells. Furthermore, vaccine-induced production of interferon-γ, tumor necrosis factor-α, and interleukin-6 in PBMC culture supernatants did not differ significantly between the groups. This study detected no significant differences in vaccine-specific immune responses between the local and distal booster vaccination sites in calves.