chicken eggs

(MedPage Today) -- Two flu vaccines -- one grown in cell culture and one in chicken eggs -- were both highly effective against placebo in a large randomized trial, researchers said.

After a lethal bird flu virus emerged in Asia, U.S. officials launched an intense effort to build new defenses against a pandemic, including replacing an antiquated vaccine system, which depends on millions of chicken eggs.

The U.S. Food and Drug Administration announced that it has approved four vaccines against the 2009 H1N1 influenza virus.

The vaccines will be distributed nationally after the initial lots become available, which is expected within the next four weeks.

People with severe or life-threatening allergies to chicken eggs, or to any other substance in the vaccine, should not be vaccinated.