However, as shown in the current study, not all H5 HA vaccines can elicit the same levels of cross protective antibodies, and more significantly, maximum levels of protective antibodies were usually detected against the autologous viral isolates. Given the knowledge that protective anti-flu antibody responses in humans are much There were 3 rabbits/group in Vector, HK, VN, AH, and Ind groups, and 4 rabbits/group in 3-valent groups. Rabbit sera tested by the pseudotyped NAb assays were collected at 2 weeks after the 4 th DNA immunization. '','' denotes below detection level. The arrow denotes neutralization against the autologous H5 pseudotyped virus. The statistical differences between the testing group and the autologous neutralization group or 3-valent group are indicated by ''*'' when the p value was less than 0.05. doi:10.1371/journal.pone.0028757.g007 lower than in experimental animals, cross protection may not be very high in humans with one randomly selected H5 HA vaccine. In the current set of studies, it was encouraging to observe that the polyvalent H5 HA DNA vaccine was able to elicit high level protective antibody responses against multiple key H5N1 viruses. Such a polyvalent flu DNA vaccine can be used for stockpiling against a potential H5N1 pandemic even before information is available on which viral isolates may cause a human outbreak. At the same time, there are alternative approaches including the use of consensus HA antigen designs to achieve a broad coverage of various viral strains (personal communication with David Weiner). It will be interesting to compare the relative efficacy between polyvalent and consensus HA DNA vaccines in their abilities to elicit protective antibody responses. One unique advantage of the polyvalent formulation is its flexibility; one alternative HA antigen can replace or be added to the earlier polyvalent formulation in the event that a new strain of virus becomes a threat while it will be necessary to re-design the whole consensus HA insert in order to allow for broader coverage.