The F.D.A. estimates that each year, 142,000 illnesses in the United States are caused by consuming eggs contaminated with the most common type of salmonella. It has said the new rules would cut that by more than half. People who eat bad eggs that have not been cooked thoroughly to kill the bacteria can get diarrhea and cramps. Rare cases can be fatal.

There are no laws mandating vaccination in Britain. But it is required, along with other safety measures, if farmers want to place an industry-sponsored red lion stamp on their eggs, which shows they have met basic standards. The country’s major supermarkets buy only eggs with the lion seal, so vaccination is practiced by 90 percent of egg producers, according to Ms. Cryer.

Thomas Humphrey, a food safety professor at the University of Liverpool, said that producers in the United Kingdom turned to vaccination after other measures, similar to those now required by the F.D.A., failed to show significant results.

One-half to two-thirds of American farmers already inoculate their flocks, according to industry estimates, and that number is likely to increase. While the new federal rules do not require vaccination, they do require testing for salmonella. If henhouses are found to be contaminated, then eggs must be tested. If eggs are tainted, then they would have to be broken and pasteurized, which would mean producers would get much less money for their eggs.

The F.D.A. has estimated that it would cost farmers about 14 cents a bird to vaccinate, or about $31 million to cover hens at all the large farms in the country. But vaccine company executives said the cost can be just a few cents a bird, depending on the type of vaccine and how many doses are given. A single bird can lay about 270 eggs in its lifetime.

After the current outbreak, “many producers that were trying to decide whether or not to vaccinate will now vaccinate,” said David Zacek, the chief executive of Lohmann Animal Health International, one of three companies that produce the vaccines in the United States.