Public-health experts cheered last month when a pivotal study showed GlaxoSmithKline's experimental shingles vaccine is much more effective than the established Merck vaccine.

In the long run, the prospect of a better shingles shot could improve vaccination rates among people 60 and older, currently a disappointing 24 percent.

But in the short run, the good news creates a bit of a quandary: Should older adults who want protection wait, probably a few years, for approval of the new vaccine, and hope the herpes zoster virus doesn't rear its excruciating rash in the meantime?

"That's a hard question," said Stephanie Bialek, a herpes virus expert at the U.S. Centers for Disease Control and Prevention. "It's one we expect a lot of people to have.

And one, she added, that cannot yet be answered because the new vaccine is still being studied.

Shingles is related to that childhood bane, chicken pox. After the itchy, unsightly blisters of chicken pox disappear, the varicella-zoster virus goes dormant in nerve cells.

Years or decades later, the virus erupts as shingles in a quarter of infected adults, more than a million a year. People who live to 85 have a 50-50 chance of developing the disease, and the incidence seems to be growing even more than the cohort of seniors.

That's not the worst part. Shingles can recur. The virus can even affect the eye, a sight-threatening emergency called herpes zoster ophthalmicus. And about 10 percent of shingles sufferers are left with a complication called postherpetic neuralgia, a debilitating nerve pain that can persist indefinitely.

Exactly why the virus reactivates is unclear, but age-related declines in immune protection play a role because shingles usually occurs after 50.

The Merck vaccine, Zostavax, was approved in 2006 for people age 50 and over, and is recommended for ages 60 and over. (Most insurers cover it at age 60.)

Zostavax, made with a weakened form of the herpes zoster virus, is less effective in older folks - those more likely to get the disease. It reduces shingles by 70 percent among those in their 50s, by 64 percent among people in their 60s, and by 38 percent among people 70 and older. It also reduces postherpetic neuralgia by about 67 percent in all groups and may lessen the severity of a shingles recurrence.

The Glaxo vaccine, in contrast, is made from a single viral protein combined with a novel immune-stimulating substance known as an adjuvant.

"It's a completely different approach," said Thomas Heineman, head of Glaxo's global vaccine development division, in King of Prussia. "The new adjuvant was developed by [Glaxo] and has been used in trials of vaccines for malaria, hepatitis B, and tuberculosis."

The international study of the shingles immunization, published last month in the New England Journal of Medicine, gave two doses of either the Glaxo vaccine or a placebo to 15,400 people age 50 and over. Only six vaccine recipients in the entire group developed shingles over the three-year follow-up, compared to 210 in the placebo group, making the vaccine about 97 percent effective. Moreover, age did not diminish the potency; among people 70 and older, the vaccine was 98 percent effective.

With any adjuvant, there is a hypothetical risk of overstimulating the immune system, but Heineman said "that is not something we've seen." Though the vaccine group reported more temporary injection-site reactions and body-wide complaints than those who got the placebo, the numbers of serious side effects, autoimmune disease, and deaths were similar in both groups.

Glaxo has not said when it hopes to file for approval, but it is completing studies of the vaccine in people age 70 and older and in immune-compromised patients, for whom the current vaccine is not safe. The U.S. Food and Drug Administration review can take six months or more.

One question Glaxo "has no formal plans at this point" to explore, Heineman said, is whether people who have had Zostavax could also get the Glaxo vaccine.

Shari Rozen, family physician in the Pittsburgh suburbs who recruited some of her patients for Glaxo's international study, is delighted by the results.

But given the knowns - and unknowns - she is not telling patients to delay vaccination.

"At this point, there is only the one vaccine," Rozen said. "I do recommend it."

She added, however, that one of her physician partners has made a personal choice to hold off.

"He is in the [vaccination] age group and he said, 'I'm going to wait,' " Rozen said. "He has made that decision.'"