Described herein is the identification and of a potent West Nile virus (WNV) CD4 positive T cell epitope and its use for increasing the immunogenicity of heterologous flavivirus vaccines, such as dengue virus type 2 (DENV-2) DNA and virus-like particle (VLP) vaccines. Also described are methods for the identification of potent T cell epitopes to enhance immunogenicity of multivalent vaccines.