440-444Testing of new vaccines in animal models has certain advantages and disadvantages. As we better understand the complexity of the immune response to vaccines, new information may be complicating the assessment of the efficacy of new candidate vaccines. Four possible complications are discussed here, (i) induction of Foxp3+ T cells; (ii) induction of memory T cell subsets; (iii) location of extracellular organisms in lung necrosis; and (iv) protection against isolates of high/extreme immunopathology