Experts say low-cost vaccines that may have helped prevent the kind of salmonella outbreak that led to the recall of more than a half-billion eggs haven't been given to half the nation's egg-laying hens.

The vaccines aren't required in the U.S, but officials in Great Britain say vaccinations have given that country the safety egg supply in Europe.

The vaccines prevents chickens from becoming infected with salmonella and passing the bacteria on to their eggs.

Iowa State University poultry veterinarian Darrell Trampel predicts more producers will begin vaccinating their flocks because of the recent outbreak.

But a veterinarian with one of the vaccine makers warns the vaccines won't always work if conditions at egg farms are very unclean.