Poor efficacy of avian influenza virus (AIV) vaccines in chickens has been documented in the field in spite of good results in experimental settings. Although the causes are multifactorial and complex, one contributing factor may be prior infection with immunosuppressive viruses. In an effort to evaluate the role of immunosuppressive agents on AIV pathogenesis and vaccine efficacy, the effect of prior infection with infectious bursal disease virus (IBDV), a ubiquitous immunosuppressive virus of chickens, was evaluated. Specific-pathogen-free white Plymouth Rock chickens were exposed to variant E IBDV at 1 day of age and were subsequently vaccinated with an inactivated H7 AIV vaccine 2 wk later. Vaccinated chickens exposed to IBDV had a geometric mean antibody titer to AIV of 1:1.7 by hemagglutination inhibition assay compared to a geometric mean titer of 1:47.5 from chickens that were vaccinated but not exposed to IBDV. Three weeks postvaccination, the chickens were challenged with one of six different doses of highly pathogenic (HP) AIV homologous to the vaccine. Within challenge virus dose groups, vaccinated chickens exposed to IBDV had similar mortality rates to nonvaccinated chickens that were not exposed to IBDV. In contrast, vaccinated chickens that were not exposed to IBDV were protected from mortality. Exposure to IBDV also decreased the mean death time (2.3–3.7 days depending on dose) compared with vaccinated birds not exposed to IBDV (4–7 days depending on dose). Neither vaccination nor IBDV infection had an effect on mean bird infection dose with HPAIV, but the 50% bird lethal dose was reduced from >10⁶ 50% egg infective dose (EID₅₀) in the vaccinated, IBDV-nonexposed group to 10^3.3 EID₅₀ in the vaccinated group exposed to IBDV. These results are consistent with IBDV exposure contributing to poor vaccine efficacy in the field.