Based on the results published in the current report and other recent similar studies, H5N1 HA DNA vaccines evaluated in the current study should be included in the design of human studies to understand whether results reported here can be reproduced in humans when they are used as part of DNA prime, either individually or as part of the polyvalent HA DNA formulation. The finding from such studies will be very useful in the identification of simple yet powerful approaches to develop vaccines against major influenza pandemics. preference of Homo sapiens. The less optimal codons in HA genes were changed to the preferred codons in mammalian systems to promote higher expression of the HA proteins, as previously described [11] . These codon optimized HA genes were chemically synthesized by Geneart (Regensburg, Germany) with added restriction enzyme sites of PstI and BamHI for subcloning purpose immediately upstream of the start codon and downstream of the stop codon, respectively.