The invention provides an influenza virus that demonstrates enhanced growth in Vero cells. The influenza virus includes PB1, PB2, PA, NP, and NS gene segments encoding proteins having amino acid sequences with selected amino acids. Optionally, at least one of the PB1, PB2, and PA gene segments includes a cytosine to uracil promoter mutation at nucleotide position 4. The invention also provides a pharmaceutical formulation containing the influenza virus, as well as a method of eliciting an immune response in a mammal by administering the influenza virus to the mammal, and a method for generating the influenza virus.