MELVILLE, N.Y. - Vaccines have drawn an intense spotlight in recent years, and a study published last month raised a new question: Do Americans overvaccinate?

Scientists writing in the New England Journal of Medicine found that immunity lasts far longer than previously believed, suggesting that fewer booster shots may be warranted in adults. Still other doctors are wondering whether new vaccine approaches would better aid children.

At least one doctor would like to see childhood vaccinations spread out over a longer period of time.

Mark Slifka, an associate scientist with the Vaccine and Gene Therapy Institute in Oregon, wanted to know how long immunity lasted after vaccination or infection. He and his colleagues went into the study with a lot of strong hypotheses and "expected to see long-lived immunity following a viral infection and relatively short-lived immunity after vaccination." Those notions, Slifka and his team said, are the reasoning for booster shots.

To his surprise, the research revealed that the immunity the body marshals after vaccination with tetanus and diphtheria lasted far longer than scientists had once believed. Immunity that arose after certain viral infections, Slifka and collaborators discovered, was essentially maintained for life.

Although it is important for the country to abide by vaccination as a vital public-health tool, Slifka said in the journal, it also is important to understand that boosters are not always necessary.

"We also need to mention that overvaccinating the population poses no health or safety concerns," he said. "It may just be unnecessary under certain circumstances."

Len Horovitz, a pulmonary physician at Lenox Hill Hospital in Manhattan, said that while the concept of overvaccination may sound radical and new, doctors have had the power for years to test a person's immunity after initial vaccination. He says he always tests students who come to see him before starting college. If they need a booster, he gives it.

"It is possible to prevent this phenomenon," Horovitz said, referring to overvaccination, "by testing for antibodies." These immune-system proteins develop in the aftermath of vaccination. Antibodies are stimulated in the presence of a key protein called an antigen, a protein introduced by vaccination or infection.

The body "remembers" antigens through highly specialized, all-knowing constituents of the immune system: B cells, whose role is never to forget. When that memory fails, it can be reactivated with a booster shot.

"To determine whether an MMR booster is needed," Horovitz said of the mumps-measles-rubella shot, "antibody testing for each antigen can be done so that an unnecessary vaccine is not administered."

Robert W. Sears, a vaccine expert, says it is possible to spread out vaccinations for children, which would be less traumatic and provide the same level of vital immunity.