The bird flu vaccine is made with eggs. That has scientists worried.

"...when the currently stockpiled formulation against the H5N1 bird flu virus requires two shots and a whopping 90 micrograms of antigen, yet provides just middling immunity. "For the U.S. alone, it would take hens laying 900,000 eggs every single day for nine months," Bright said. And that's only if the chickens don't get infected. To make raw material for an influenza vaccine, virus is grown in millions of fertilized eggs. Sometimes it doesn't grow well, or it mutates to a degree that the vaccine product stimulates antibodies that don't neutralize the virus — or the wild virus mutates to an extent that the vaccine doesn't work against it. And there's always the frightening prospect that wild birds could carry the virus into the henhouses needed in vaccine production. "Once those roosters and hens go down, you have no vaccine," Bright said."