GlaxoSmithKline's new shingles vaccine, Shingrix, was approved Friday by the Food and Drug Administration, a potential $1 billion-a-year drug to prevent the shingles virus in adults 50 and older.

"The vaccine has shown over 90 percent efficacy across all age groups," said Thomas Breuer, senior vice president and chief medical officer of GSK vaccines. "Shingrix represents a significant scientific advancement in the field of vaccinology."

The final step will be for the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices to sign off for the use of Shingrix at its meeting next Wednesday. Following that recommendation, Shingrix would be available shortly, the company said.

GSK will go head-to-head with rival Merck, which has the only other marketed shingles vaccine, Zostavax.

The CDC advisers next week may vote that Shingrix should be the preferred vaccine over Merck's shot. The GSK vaccine produces a stronger immune response and lasts longer, with no major safety issues.

Data reported in the New England Journal showed that in adults 70 and older, two doses of the GSK vaccine had 90 percent protection compared with a placebo. The effectiveness went down only slightly, to 88 percent, after four years.

By contrast, Merck's vaccine, according to a Kaiser Permanente study of adults 60 and older, decreased from 68.7 percent effectiveness initially to 4.2 percent in the eighth year after vaccination.

"With Merck's Zostavax, you begin to lose protection even after the first year," said William Schaffner, professor of preventive medicine at Vanderbilt University School of Medicine. "By the time you get out to year seven, many people are down to their pre-immunization baseline again."

"Zostavax was shingles vaccine 1.0. Shingrix is shingles vaccine 2.0. It's better," noted Schaffner, who said he has no financial ties to GSK or involvement in the studies. Shingrix has been tested in more than 38,000 people, GSK said.

Based on the efficacy and safety profile, and incidence of shingles — one in three people develops the virus — life science analysis firm EvaluatePharma predicts Shingrix will have blockbuster sales of $1.126 billion by 2022. Sales of Merck's Zostavax were $685 million last year.

Canada this month became  the first country to approve the vaccine for people 50 and older as a prevention against the painful rash caused by the reactivation of the varicella zoster virus, the same virus that causes chickenpox.

People who live to 85 have a 50-50 chance of developing shingles, which can result in a complication called postherpetic neuralgia, a debilitating nerve pain. The risk of shingles increases with age as the body's immune system declines.

However, there are a couple of issues with the GSK vaccine: It requires two shots, two to six months apart. (The Merck vaccine is one shot.) And it's "more ouchy" at the time of injection, Schaffner said.

Insurance companies usually cover what the CDC immunization committee recommends.

But for people 65 and older on Medicare, the shingles vaccine is covered only under Medicare Part D, the prescription drug benefit. As a result, only 31 percent of people older than 65 have received Merck's vaccine, "which is much less than it ought to be," Schaffner said. "Not every Medicare recipient elects a Part D coverage."

The CDC advisers may also decide whether people who were previously vaccinated with Merck's vaccine can safely receive the Shingrix vaccine later, and when.

GSK presented study results in June that 430 adults who received the Merck vaccine five years earlier were revaccinated with GSK's vaccine. They had no serious side effects, and the immune response was strong, the company said.