DES MOINES, Iowa — Low-cost vaccines that may have helped prevent the kind of salmonella outbreak that has led to the recall of more than a half-billion eggs haven’t been given to half of 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 that about 1 percent of British flocks had salmonella compared to about 60-70 percent of flocks elsewhere in Europe, said Amanda Cryer, spokeswoman for the British Egg Information Service.

There’s 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.

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.

The 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’s injected. Jewanna Porter, a spokeswoman for the Egg Safety Center, an industry group, said both forms provide good protection. The injected vaccine lasts longer, but veterinarians recommend both be updated.

In most cases, laying hens are vaccinated at between 10 and 16 weeks old, which is before they are put into production.

The U.S. Food and Drug Administration said last month it doesn’t believe mandatory vaccination is necessary, but it supports farmers doing it voluntarily.

Doug Grian-Sherman, senior scientist at the Union of Concerned Scientists, said the vaccine deserves additional study, but it would likely have only have limited effectiveness against a bacteria like salmonella, which has many different strains.

“It’s only going to be a Band-Aid on a much bigger problem,” he said.

It would be more effective to give the FDA additional authority to stop repeat offenders and pull contaminated products off shelves and to move away from big production facilities that ship across the nation and can quickly spread disease, he said.