Now, consumers have been shaken by one of the largest egg recalls ever, involving nearly 550 million eggs from two Iowa producers, after a nationwide outbreak of thousands of cases of salmonella was traced to eggs contaminated with the bacteria.
The F.D.A. has said that if its egg safety rules had gone into effect earlier, the crisis might have been averted. Those rules include regular testing for contamination, cleanliness standards for henhouses and refrigeration requirements, all of which experts say are necessary.
However, many industry experts say the absence of mandatory vaccination greatly weakens the F.D.A. rules, depriving them of a crucial step that could prevent future outbreaks.
Salmonella bacteria is passed from infected hens to the interior of eggs when they are being formed. The salmonella vaccines work both by reducing the number of hens that get infected and by making it more difficult for salmonella bacteria to pass through to the eggs.
“They are the only thing I’m aware of that really controls the problem from the inside out, at the source,” said Ronald Plylar, the former president of a company that developed an early salmonella vaccine.
Many people in the American egg industry say they believe that the current outbreak and recall will tip the balance and force nearly all producers in the United States to begin vaccinating hens to reassure consumers.
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.
The drop in salmonella infections in Britain was stunning.
In 1997, there were 14,771 reported cases in England and Wales of the most common type of the bacteria, a strain known as Salmonella Enteritidis PT4. Vaccine trials began that year, and the next year, egg producers began vaccinating in large numbers.
The number of human illnesses has dropped almost every year since then. Last year, according to data from the Health Protection Agency of England and Wales, there were just 581 cases, a drop of 96 percent from 1997.
“We have pretty much eliminated salmonella as a human problem in the U.K.,” said Amanda Cryer, director of the British Egg Information Service, an industry group.