The problems with a high-dose H5 vaccine are twofold:

First, since people have never been exposed to H5 viruses, a "primer" vaccination would have to be given, then a month later another dose would be required. Because of this two-dose regimen, immunity wouldn't take hold for about six weeks, giving the virus a bigger window to infect even immunized individuals, Treanor said.

Second, the higher dose would place a strain on manufacturers' ability to produce enough vaccine, he said.