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Study: Immune System Has Long Memory

Immunity Conferred From infections, Vaccinations May Last Decades Longer Than Thought
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WebMD Health News
Reviewed by Louise Chang, MD

Nov. 7, 2007 -- After a vaccination or an infection, our immune system remembers to keep protecting us against the offending organism for much longer than scientists have believed, according to a new study.

The duration of immunity, in some cases, is more than 200 years, the researchers say.

"Immunity from either infection or vaccination is much more long-lived than previously believed," says Mark Slifka, PhD, associate scientist at the Vaccine and Gene Therapy Institute at the Oregon Health & Science University in Portland, and the study's lead author. His study is published Nov. 8 in The New England Journal of Medicine.

Vaccine Immunity, Infection Immunity

Slifka and his colleagues analyzed 630 blood samples taken over 25 years from the 45 study participants, all staff at the university's Oregon National Primate Research Center.

Most of the samples taken from the participants, who included men and women, average age 52, were from routine scheduled collections. Only about 8% were obtained after exposure to an animal or other unscheduled events.

Slifka and his team used a mathematical model to estimate how long immunity from both infection and vaccination would last based on measurements taken from the stored blood.

"We measured antibody responses to different vaccine and virus antigens," Slifka says, including measles, mumps, rubella, Epstein-Barr virus, smallpox, chickenpox, tetanus, and diphtheria.

Results: Vaccine Immunity vs. Infection Immunity

Substantial differences in the body's immune memory were found between live infections and vaccines, Slifka tells WebMD. Immunity following live infections was maintained much longer, although there was some variability.

Of the viral infections studied, the shortest maintained immunity was for a natural infection with the chickenpox virus.  "The body remembered that virus for over 50 years," he says. But in the study, the body remembered other viruses for more than 200 years, his team found.

The immunity response from vaccinations against measles, mumps, and rubella was also long lasting, but because only a handful of people in the study were vaccinated against these diseases, the results are less certain.

However,the body wasn't as good at remembering to protect itself against tetanus or diphtheria for which it was vaccinated. For instance, the antibody response to the tetanus vaccine was only a few decades. For diphtheria, it was about twice that length of time.

Second Voice

While the study's methods are praised by another expert, he cautions that the study was also small.

"You can't draw conclusions for the general population from this study," says Samuel Stebbins, MD, MPH, professor of epidemiology at the University of Pittsburgh School of Public Health.

"These are people who have worked in a lab," he says, and that may affect their immune memory.

But Slifka disagrees, noting that only about 8% of the samples were obtained non-routinely, such as after exposure to an animal.

Study Implications

Eventually, the research may trigger a rethinking of the current revaccination schedule, Slifka says.

For instance, the CDC now recommends that adults who got immunized against tetanus as a child get a booster dose every 10 years. In the future, Slifka says, that recommendation may change, saving time and money without affecting protection.

But he isn't suggesting avoiding recommended vaccinations. "People really do need to get their primary series of vaccinations," he says. "But once they receive their primary series, their immunity is going to be maintained for a longer period than we previously thought."

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