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Shingles is a painful rash that's triggered by a reactivation of the virus that causes chickenpox. About one-third of Americans develop the disease at some point, according to the U.S. Centers for Disease Control and Prevention (CDC).
There already is a vaccine against shingles, but its effectiveness is limited.
The new study found that the experimental vaccine protected about 90 percent of adults age 70 and up. And the effects were still apparent four years later.
The study results were published in the Sept. 15 issue of the New England Journal of Medicine.
Researchers called the results of the new vaccine trial very encouraging.
"This will hopefully have a high level of efficacy [effectiveness] and a long duration," said Dr. Len Friedland, vice president of scientific affairs at GlaxoSmithKline's Vaccines North America, which is developing the vaccine.
A more effective shingles vaccine would be "welcome," said Dr. Kathleen Neuzil, director of the University of Maryland's Center for Vaccine Development, in Baltimore.
For now, she advises older adults to talk to their doctor about getting the existing shingles vaccine, which the CDC recommends for people age 60 and older.
"It goes to sleep in the nerves," Friedland explained. "There it's kept in check by a good, robust immune system."
But as people age, he said, the immune system tends to weaken -- and that can allow the dormant virus to awaken.
"If you're lucky enough to make it to age 85, you have a one-in-two chance of developing shingles," Friedland said.
Shingles causes a painful rash on one side of the body or face that typically clears up in a few weeks, according to the CDC. But some people develop a complication called post-herpetic neuralgia (PHN), which causes severe pain in areas where the rash appeared.
PHN usually goes away in a few weeks or months, but it can last for years, the CDC says.
And the treatments for the complication "are not very effective," Friedland noted.
"So the best way to manage shingles is through prevention," he said.
Since 2008, the CDC has advised adults age 60 and older to get the existing shingles vaccine, whether they think they ever had chickenpox or not. (Studies show that nearly all Americans age 40 and up have had chickenpox, even if they don't remember it.)
The experimental vaccine uses a weakened live virus to stimulate the immune response to the shingles virus, Neuzil explained. The experimental vaccine -- dubbed HZ/su -- uses just a piece of the surface of the shingles virus, plus an "adjuvant" ingredient that spurs a stronger immune response, she said.
An earlier study had already shown that HZ/su cut the risk of shingles by 97 percent among people age 50 and older, over three years.
The new trial involved nearly 14,000 adults age 70 and older. These participants were randomly assigned to receive two doses of the vaccine or injections of a placebo.
Over the next four years, only 23 vaccine recipients developed shingles, compared to 223 people given placebo shots, the investigators found.
Neuzil called the safety findings "reassuring." There are theoretical concerns about the adjuvant in the vaccine, she said: In people with a certain genetic type, it's possible the ingredient could stimulate the immune system in a "bad way."
"But that's speculative at this point," she said.
Friedland said GlaxoSmithKline expects to apply for U.S. Food and Drug Administration approval of the vaccine by the end of the year.
For now, Neuzil urges older adults to consider the existing vaccine -- which, based on government figures, few Americans have received.
Shingles can cause real suffering, according to Neuzil -- who said she's seen patients who can't sleep, or even tolerate clothing touching their skin.
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"Anyone over 60 should talk to their doctor about getting the vaccine," she said.
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