Fort Lauderdale, Fla. ? A new vaccine to prevent painful attacks of shingles in people 60 and older has received federal approval and is expected to be on the market in about a week.

The Food and Drug Administration announced Friday that it had cleared the way for Merck & Co. Inc. to begin shipping the vaccine called Zostavax. A five-year study showed the vaccine can prevent about half the people who get it from developing shingles, a painful rash that can lead to a more devastating painful condition called postherpetic neuralgia.

As many as 1 million Americans develop shingles each year, according to the National Institutes of Health, and about 20 percent of them progress to postherpetic neuralgia, which can cause excruciating pain for months or years.

Shingles can emerge in older people when their immune systems weaken, allowing the virus that causes childhood chickenpox, which has stayed dormant in the body, to reawaken.

Shingles first causes pain, itching or tingling in an area of skin on one side of the body or face. A painful blistering rash then develops and can take up to four weeks to heal.

Anyone who has had chickenpox – most adults in the United States – could develop shingles, but not all will. The two major risk factors are increasing age and declining immunity. About half of those who live to age 85 will get the disease if they had chickenpox as a child.