illions of Americans lack immunity to tetanus and diphtheria because they fail to follow federal guidelines that call for regular booster shots against the diseases, a new study finds.

The study, released yesterday by The Annals of Internal Medicine, found that many Americans were immunized against tetanus and diphtheria early in their lives, but became vulnerable to infection by skipping the boosters.

The lead author, Dr. Geraldine M. McQuillan of the Centers for Disease Control and Prevention, and her colleagues said doctors needed to be more aggressive about immunizing.

Although the two diseases are rare in the United States — routine immunizations have been given since the late 1940's — infections do occur, and if the booster problem persists, that could widen the spread, the authors said. Booster shots are supposed to be given every 10 years.

The study was based on analyzing antibodies in blood from more than 18,000 people 6 and older. It found that 60.5 percent of Americans were protected against diphtheria and 72.3 percent against tetanus. But the numbers varied based on factors like income, sex and, above all, age. By 70, 45 percent of men and 21 percent of women had tetanus protection.

Some differences follow broad trends. People with more education were more likely to be protected. But some differences were hard to explain. People who said they had routine access to medical care and regular doctors were less likely to have sufficient immunity.