Good news on immunity: vaccines might last longer than thought

Scientists are finding that the human immune system has a much longer memory for pathogens than once thought.

Most recently, researchers have discovered that many people in their 60s and older seem to have at least some residual immunity to the H1N1 swine flu virus, probably from previous exposure to a similar virus more than 50 years ago.

And vaccine experts are finding that immunity persists much longer for various inoculations than once thought.

One report, published in October, found that immunity to whooping cough (pertussis) lasts at least 30 years, on average, among adults, and could extend for up to 70 years after a natural infection. Researchers from several institutions estimated the new timeline after analyzing historic outbreaks of the illness, as well as periods when the disease was dormant in North America - when no cases were reported.

"This is surprising because epidemiologists currently believe the duration of pertussis immunity is somewhere between four and 20 years," said Pejman Rohani, an infectious disease analyst who started the project while at the University of Michigan and is now at the University of Georgia.

The findings are particularly important because whooping cough, once thought to be controlled by widespread childhood vaccination, has been on the rise in the U.S., Great Britain and several other countries since the 1980s.

Another report, done by scientists at the National Institutes of Health and published last year, looked at 246 older adults taking part in an aging study. They found that people vaccinated against the smallpox virus retained immunity for up to 88 years after just one vaccination.

Although naturally occurring smallpox was eradicated in 1977, there is still some concern that the disease might re-emerge as an agent of terrorists, so the discovery that one shot appears to last a lifetime is important to knowing how to protect a population in which routine smallpox vaccination ended more than 30 years ago.

But if the elderly retain immune system memory, another recent study suggests that aging also can make the immune response too active - and that this may be the mechanism that causes the old to die from many viral infections more readily than younger people.

The research, published in November by scientists at the Yale School of Medicine, was based on studies of mice of various ages infected with herpes virus.

They found that the infections produced a rapid increase of an inflammatory agent, called cytokines, which can cause damage to the liver and other systems with an excessive response to infection.

Dr. Daniel Goldstein, an associate professor of internal medicine who led the study, said the findings indicate that aged individuals succumb to viral infection due to exaggerated immune responses rather than to declining immune response, which had been conventional thinking for decades.

He noted that when the team inhibited the inflammatory agent before or after infection, the oldest mice no longer exhibited liver damage and survived the infections.

"Aged mice do have defective immune response, but instead of trying to boost their immune response, we should try to inhibit certain inflammatory pathways," Goldstein said.

He added that the findings could explain why older people are more susceptible to seasonal flu viruses, which, being relatively new each year, could draw a more intense response.

The same reaction may also explain why some younger people exposed to the novel strain of H1N1 have been more likely to become severely ill and die.