DES MOINES, Iowa – Low-cost vaccines that might help prevent the kind of salmonella outbreak that has led to the recall of more than a half-billion eggs haven’t been given to nearly half the nation’s egg-laying hens.

The vaccines aren’t required in the U.S., although in Great Britain, officials say vaccinations have given them the safest egg supply in Europe. A survey conducted by the European food safety agency in 2009 found about 1 percent of British flocks had salmonella compared with about 60 percent to 70 percent of flocks elsewhere in Europe, said Amanda Cryer, spokeswoman for the British Egg Information Service.

Since Britain’s vaccinations began, the only salmonella outbreaks in eggs have been linked to those imported from elsewhere in the European Union, Cryer said. Overall salmonella cases in the country dropped by half within three years.

There has been no push to require vaccination in the U.S., in part because it would cost farmers and in part because advocates have been more focused on more comprehensive food-safety reforms, those watching the poultry industry said. And the U.S. Food and Drug Administration has not determined how the hens in Iowa became infected.

But Darrell Trampel, a poultry veterinarian at Iowa State University, predicted vaccination will become more common after the recent outbreak.

“I think (vaccination) will move from hit-and-miss to being a standard,” Trampel said.

About 125 million of the 218 million egg-laying hens in the U.S. have been vaccinated, said Gary Baxter, a spokesman for French pharmaceutical company CEVA, which makes some of the vaccines available in the U.S. The salmonella vaccine prevents chickens from becoming infected and then passing the bacteria on to their eggs. It has been available in the U.S. since 1992.

There are two forms. One is a spray that uses a live bacteria, and chickens inhale it. The other contains dead bacteria that are injected.