Medical researchers and government health policymakers, a cautious lot, normally take pains to keep expectations modest when they're discussing some new finding or treatment.
They warn about studies' limitations. They point out what isn't known. They emphasize that correlation doesn't mean causation.
So it's startling to hear prominent experts sound positively excited about a new shingles vaccine that an advisory committee to the Centers for Disease Control and Prevention approved last month.
"This really is a sea change," said Dr. Rafael Harpaz, a veteran shingles researcher at the CDC.
Dr. William Schaffner, preventive disease specialist at the Vanderbilt University School of Medicine, said, "This vaccine has spectacular initial protection rates in every age group. The immune system of a 70- or 80-year-old responds as if the person were only 25 or 30."
"This really looks to be a breakthrough in vaccinating older adults," agreed Dr. Jeffrey Cohen, a physician and researcher at the National Institutes of Health.
What's causing the enthusiasm: Shingrix, which pharmaceutical firm GlaxoSmithKline intends to begin shipping this month. Large international trials have shown that the vaccine prevents more than 90 percent of shingles cases, even at older ages.
The currently available shingles vaccine, called Zostavax, only prevents about half of shingles cases in those older than 60 and has demonstrated far less effectiveness among elderly patients.
Yet those are the people most at risk for this blistering disease, with its often intense pain, its threat to vision and the associated nerve pain that sometimes last months, even years, after the initial rash fades.
Almost all older Americans harbor the varicella zoster virus that causes shingles; they acquired it with childhood chickenpox, whether they knew they had the disease or not.
The virus stays dormant until, for unknown reasons, it erupts decades later. The risk rises sharply after age 50.
Shingles is hardly a minor menace. "A million cases occur in the United States each and every year," Schaffner said. "If you're fortunate enough to reach your 80th birthday, you stand a 1-in-3 to 1-in-2 chance of shingles."
Preventing the great majority of these cases along with the risk of lingering and debilitating nerve pain, called postherpetic neuralgia, would represent a major advance in public health.
So while the old vaccine will remain on the market, the CDC committee voted to make Shingrix the preferred vaccine and recommended it for all adults older than 50, a group younger by a decade than those earlier encouraged to get Zostavax.
The committee also recommended Shingrix for adults who have previously gotten Zostavax, since a smaller study in people older than 65 demonstrated effectiveness and safety in those already vaccinated. The Food and Drug Administration approved Shingrix last month.
Once the CDC's director endorses the committee's recommendations, and the agency publishes them, insurers, including Medicare and Medicaid, will start covering the vaccine.
"By early 2018, it should be broadly available to consumers in the U.S.," said Dr. Thomas Breuer, chief medical officer of GSK Vaccines. (Canada has also approved Shingrix; it awaits approval in Australia, Japan and Europe.)
What makes the new vaccine so promising, especially for older adults?
It provides better protection against shingles from the start. Though Zostavax, introduced in 2006, can reduce shingles cases by about half (and postherpetic neuralgia by two-thirds), that overall rate conceals big differences by age.
That vaccine's effectiveness drops from 64 percent for people in their 60s to 38 percent among those older than 70, and falls still lower for people in their 80s.
But the new vaccine protects nearly as well in older groups as in the middle-age. Shingrix racked up a 97 percent effectiveness rate in adults older than 50 and, in a separate study of people older than 70, prevented 90 percent of shingles in those 70 to well past age 80.
"In groups such as the elderly, who often don't maintain vigorous responses to vaccines, this represents extremely strong disease protection," said Dr. Kathleen Dooling, an epidemiologist at the CDC.
Shingrix's protection appears to last longer. Among seniors, the effectiveness of Zostavax wanes with disappointing speed. "After 11 years, the protection was close to zero," Harpaz said.
Regulators don't yet have 11 years of data on Shingrix, but in some samples, it remained effective for six years or longer, according to GSK. That should greatly reduce the incidence of postherpetic neuralgia, too, assuming the 42 million people in their 50s start getting vaccinated.
The new vaccine may protect people with compromised immune systems.
A substantial number of older Americans have suppressed immunity because they're undergoing chemotherapy or transplants, have HIV or take steroids. For them, the previous vaccine was off-limits because it was made with a weakened live virus.
Yet immune suppression itself leaves the people vulnerable to shingles. Shingrix, a recombinant vaccine made from a glycoprotein and a combination of immunity boosters called adjuvants, doesn't pose the same danger.
The CDC committee held off on recommending Shingrix for the immunocompromised, because GSK is still running trials with these patients. But since the FDA did not declare Shingrix contraindicated for them when approving it, they can get the vaccine once it's available.
Public health advocates do foresee a couple of potential problems.
First, Shingrix requires two doses, administered at least two months apart. Prodding the older population to get a single shot has proved tough: Barely 31 percent of those older than 60 have been vaccinated against shingles. How much harder will it be to persuade people to get two Shingrix injections?
Further, "it tends to be a bit of an ouch-y vaccine," Schaffner cautioned.
In studies, most older recipients said they'd experienced pain, redness or swelling in their upper arms for a day or two after the shot, and 8.5 percent of those older than 70 deemed those symptoms uncomfortable enough to interfere with normal activities.
About half of those older than 70 reported more systemic side effects like fatigue, fever or aching joints, lasting one to two days. Physicians and pharmacists should prepare people for such reactions, Schaffner said.
"If people anticipate it, they'll cope with it better. They'll take a couple of Tylenol" and not worry that something is seriously wrong.
They may feel pocketbook pain, too. Zostavax is the most expensive adult vaccine, and at $140 for each dose (plus the cost of administering the injection), Shingrix will be pricier still.
The 50- to 65-year-old cohort, many of whom have coverage under employee health plans, may not find that much of a barrier. At older ages, cost matters more.
Medicare will cover Shingrix under Part D (like its predecessor), not under Part B like the flu vaccine. That complicates reimbursement for those seeking vaccination in doctors' offices, so Medicare patients will probably find it simpler to head for a pharmacy.
But not all Medicare recipients have Part D, and those that do could face copayments.
Still, it's no contest: The hazards of shingles and its complications dwarf any problems yet reported with Shingrix.
"Compared to shingles, a little arm pain for a day or so is a small price to pay," Schaffner said. "If you know people who've had this illness, you'll be first in line for this vaccine."