The Food and Drug Administration announced Friday that it had licensed the vaccine, Zostavax, to reduce the risk of shingles in adults 60 and older. The Merck vaccine is only for adults who previously have had chickenpox.

There are roughly 1 million new US cases of shingles each year. It typically affects the elderly, though anyone with a weakened immune system is vulnerable. The vaccine is not meant for anyone with HIV or who is on immune-suppressing drugs, including transplant patients, said Norman Baylor, director of the FDA's Office of Vaccines Research and Review.

Long Dormancy in Nerve Cells

Shingles can cause itching, burning and tingling, as well as a distinctive red rash that develops into pus-filled blisters that later break open and form scabs. Pain can persist in some patients for months or years.

"This vaccine gives health care providers an important tool that can help prevent an illness that affects many older Americans and often results in significant chronic pain," said Dr. Jesse Goodman, director of the FDA's Center for Biologics Evaluation and Research.

Shingles, also called herpes zoster, occurs when the chickenpox virus reactivates and resumes reproducing after a long dormancy in nerve cells in the body.

Zostavax does not prevent the initial infection by the virus, nor does it treat active cases of shingles. Instead, the vaccine helps prevent the re-emergence or resurgence of the virus by boosting a patient's immunity, Goodman said.

First Product That Can Prevent Shingles

Merck research showed the vaccine reduced the incidence of shingles by half and pain and discomfort by nearly two-thirds.

"This is going to be the first product of any kind, vaccine or anything else, that can prevent shingles," said Dr. Jeffrey Silber, senior director of clinical research on biologics and vaccines for Merck Research Laboratories.

Zostavax is based on Merck's chickenpox vaccine, Varivax, but is 14 times more potent. The FDA approved Varivax in 1995 and a majority of American children now receive it.

Anyone who has had chickenpox can develop shingles. Merck estimates that half of all people who reach age 85 will develop shingles during their lifetime.

The vaccine's long-term effectiveness remains unclear.