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 with about 60 to 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.

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.

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.

The FDA has not yet determined how the hens in Iowa became infected, said Dr. Jeff Farrar, FDA’s associate commissioner for food protection.

Both farms involved in the recall — Hillandale Farms and Wright County Egg — vaccinated some of their chickens.