The F.D.A. said it considered mandatory vaccination very seriously. “We didn’t believe that, based on the data we had, there was sufficient scientific evidence for us to require it,” said Dr. Nega Beru, director of the agency’s Office of Food Safety.
However, Dr. Beru says that the new rules encourage producers to vaccinate if they think it will help fight salmonella.
Another F.D.A. food safety official, Nancy S. Bufano, said that despite the success of vaccination in Britain, the agency thought that because the vaccines used in the two countries were not identical, it made comparisons difficult.
Vaccine company executives, however, said the differences were minor and the drugs used in both countries were equally effective.