Long-lasting immunity found for some vaccines

BOSTON (Reuters) - The smallpox vaccine protects for a lifetime, and so does actual an infection of measles or mumps, according to the first long-term study of immunity to childhood diseases.

And, surprisingly, while a tetanus shot is only supposed to guard against the disease for about 10 years, a team at Oregon Health & Science University found that half the antibodies against the bacterium were still present in the blood 10 years later.

That may explain why the tetanus rate in Sweden is comparable to that in the United States, even though the vaccine is only boosted after 30 years in Sweden, as opposed to every 10 years in the United States.

Mark Slifka of the Oregon Health & Science University and colleagues used blood samples regularly collected from 45 people over a span of up to 26 years to track the degree to which protection against a host of diseases waned over time.

After the September 11 attacks and the subsequent concerns about bioterrorism, the U.S. Centers for Disease Control and Prevention worried that even people who were vaccinated against smallpox before routine immunization stopped in 1972 might not have protection any more.

But Slifka's team found that it takes 92 years for half the protective antibodies against smallpox to disappear.

Slifka said previous estimates of vaccine effectiveness have usually been based on studying people for just a few years.

"Here we were able to look over the course of a quarter of a century, without speculation, to determine what was going on," Slifka said in a telephone interview.

WANING IMMUNITY

The findings do not always apply. Outbreaks of mumps and whooping cough, for instance, are known to occur among people who were vaccinated years before and whose immunity waned.

And many of the volunteers in the study were naturally infected and cannot credit their immunity to the vaccines. "Because they were older subjects, they contracted natural measles, mumps, rubella and chickenpox infections," Slifka said.

"It is unknown whether vaccine-induced immunity is as long-lived as that induced by natural infection," the researchers wrote in the New England Journal of Medicine.

The antibody half-life was 11 years for tetanus, meaning it took 11 years for the body's immunity as measured by the number of antibodies to fall by 50 percent.

This immunity lasted 19 years for diphtheria, 50 years for chickenpox, and stretched beyond lifetime limits for rubella or German measles, mumps, measles and the Epstein-Barr virus.

"Measles, mumps and rubella were always described as childhood diseases and now we have one of the reasons why. Our immune system can remember them for a lifetime," said Slifka.

"And even for vaccinations that we didn't think could be maintained for very long, like tetanus and diphtheria, we're still finding memory that can last for decades."

Slifka suggested that some people may not need regular booster vaccinations for some diseases.

"We want to emphasize that proper vaccination is vital for protecting people against infectious disease. We also need to mention that over-vaccinating the population poses no health or safety concerns -- it may just be unnecessary under certain circumstances," he said.

(Reporting by Gene Emery; Editing by Maggie Fox)