Washington ? Federal health advisers endorsed a proposed vaccine on Thursday to help prevent shingles, an often-excruciating disease that afflicts as many as 1 million adults every year.

The Food and Drug Administration’s advisory panel on vaccines said the vaccine for shingles appeared to be safe and effective in people aged 60 and older. However, panelists worried its effectiveness may not last, and said it appeared to be less effective in people older than 80.

The vaccine, Zostavax, was developed by Merck & Co., which sought to have it approved for use in people aged 50 and older.

But Merck conducted trials mostly on people 60 and older, leaving the members of the FDA panel on vaccines uncertain whether it was safe or effective for those in their 50s.

Whether to approve the vaccine for sale is now in the hands of the FDA, which often follows the advice of its advisory panels. Even if the FDA approves it for use only in those 60 and older, doctors could still prescribe it for younger people.