The present study explored the immunological correlates of protection mediated by a live bivalent vaccine consisting of herpesvirus of turkeys (HVT) and SB-1 against infection with the RB1B strain of Marek's disease virus (MDV). Compared to unvaccinated infected chickens, vaccinated protected birds had lower expression of interleukin (IL)-6, IL-10 and IL-18 genes in spleen. However, there was no difference between these two groups of birds in the expression of interferon (IFN)-gamma, IL-4, IL-12 and inducible nitric oxide synthase (iNOS) genes on day 21 post-infection. Furthermore, protection was associated with lower MDV genome load in spleen but not in feather tips, suggesting that vaccination had little or no effect on curtailing virus transmission. In conclusion, vaccination with a bivalent MD vaccine was associated with distinct cytokine expression patterns in spleen and modulation of cytokine responses by the vaccine may play a role in mediation of protection.