People who live to age 85 have a 50-50 chance of developing the excruciating rash known as shingles, yet the vaccine that cuts the risk remains unpopular.

In 2015, only about 34 percent of adults 65 and over had ever gotten the shot, the U.S. Centers for Disease Control and Prevention is reporting Wednesday. The rate has been inching up since 2006, when Merck launched the shingles vaccine, called Zostavax. Still, the uptake is far below the pneumonia vaccine (64 percent of seniors) and the annual flu vaccine (69 percent of seniors), the latest CDC data show.

Why hasn't Zostavax caught on? And are the prospects any brighter for GlaxoSmithKline's Shingrix, a better shingles vaccine that is aiming for U.S. Food and Drug Administration approval late this year?

William Schaffner, an infectious-disease specialist at Vanderbilt University Medical Center in Nashville, has studied those questions. As medical director of the National Foundation for Infectious Diseases, an educational organization, he serves as a liaison to the CDC's immunization advisory committee, which makes vaccine recommendations that shape insurance coverage.

After considering many factors, the CDC committee recommended Zostavax for ages 60 and over, although it's approved for people 50 and over.

A big deterrent to Zostavax — one that Shingrix will face ­­— involves the way it's covered by Medicare, the national health insurance for Americans 65 and over, Schaffner explained. While Medicare covers other adult vaccines as Part B medical benefits, the shingles vaccine is covered under the Part D prescription program, which is optional. Some seniors don't buy Part D, and doctors can't easily use it to get reimbursed for their office services. Zostavax's full price is about $200.

"Congress, purely for fiscal reasons, put the shingles vaccine into" the prescription plan, Schaffner said. "That has been far and away the biggest barrier to uptake of the vaccine."

Glaxo has asked the CDC immunization advisory committee to recommend Shingrix starting at age 50. That would enable more people to get the vaccine through private medical insurance before going on Medicare.

A Glaxo spokesman said the price of Shingrix had not yet been determined.

"Shingrix is so much better than Zostavax, Glaxo may indeed receive a recommendation starting at age 50," Schaffner said.

Shingles usually occurs after age 50, and the incidence climbs steadily with age, presumably because immune system protection wanes over time. Shingles is an after-effect of that childhood bane, chickenpox. After the itchy blisters disappear, the chickenpox virus goes dormant in nerve cells. Years later, it gets reactivated as shingles in one out of three older adults — more than a million a year. In addition to causing a searing rash, shingles can affect the eyes and leave sufferers with persistent nerve pain.

Zostavax, made with a weakened form of the herpes zoster virus, cuts the risk of shingles, but the reduction shrinks with age, making the vaccine less effective in those more likely to get the disease. It reduces shingles by 70 percent among people in their 50s, but by just 38 percent among people over 70.

Moreover, the protection fades to almost nothing eight years after vaccination, a Kaiser Permanente study found.

In contrast, Shingrix — made with a single viral protein combined with a novel immune-stimulating substance — has demonstrated both effectiveness and durability. Data reported in the New England Journal of Medicine showed that in adults 70 and up, two doses were more than 90 percent protective compared with a placebo. The efficacy went down only slightly, to 88 percent, four years after people were vaccinated.

"I think providers are going to be more enthusiastic about the Glaxo vaccine than the Merck vaccine," Schaffner said.

Last month, Glaxo addressed the question of whether people who have already been vaccinated can opt for the newer immunization. The company presented data to the CDC that showed Shingrix produced a strong immune response and no serious side effects in adults who had previously received Zostavax.

Ultimately, Schaffner said, boosting shingles vaccination rates will require that staple of public health: raising awareness.

"You have to educate providers and patients," he said.