Merck & Co Inc. (NYSE: MRK)'s shingles vaccine Zostavax is now approved by the US Food and Drug Administration (FDA) for people aged 50 and older. Zostavax was originally approved on May 2006 to prevent herpes zoster, commonly known as shingles, in individuals 60 years of age and older.

This expanded indication is important for the health of people who are 50 and older because nearly everyone in that age group is at risk for developing shingles, said Jeffrey Silber, M.D., vice president, Merck Research Laboratories.

Shingles is characterized by a rash of blisters, which generally develop in a band on one side of the body and can cause severe pain that may last for weeks, and in some people, for months or years after the episode.

According to the U.S. Centers for Disease Control and Prevention (CDC), in the U.S. around 1 in 3 people will experience shingles in their lifetime and nearly one million cases of shingles occur each year. The incidence and severity of shingles increase with age. Once a person has had chickenpox the varicella-zoster virus (VZV) stays inside the body and can resurface later as shingles.

The FDA's approval was based on a multicenter study conducted in about 22,000 people who were 50-59 years of age. Half received Zostavax and half received a placebo. Study participants were then monitored for at least one year to see if they developed shingles. Compared with placebo, Zostavax reduced the risk of developing shingles by around 70 percent, the U.S. health regulator said.

The likelihood of shingles increases with age. The availability of Zostavax to a younger age group provides an additional opportunity to prevent this often painful and debilitating disease, said Karen Midthun, M.D., director of FDA's Center for Biologics Evaluation and Research.

Shares of Merck ended Thursday's session at $32.72.