A new vaccine could help millions of Americans over 50 avoid shingles—a painful rash that occurs in one in three adults who have had a childhood case of the chickenpox.

Shingles is caused by reactivation of the virus that causes chickenpox, varicella zoster virus (VZV). After infection, VZV lies dormant, often for years, in sensory nerves. An estimated 99.5% of people over 40 harbor latent VZV. But as the immune system weakens due to age, disease, or medications, the virus can come roaring back to life as shingles. More than half of all adults age 85 and older have experienced shingles at least once.

The new vaccine, Shingrix, is about 97% effective in preventing shingles in adults age 50 to 70 or older. Zostavax, the original shingles vaccine, is just over 50% effective and does not work as well in people aged 70 and older.

The CDC’s immunization advisory committee issued a statement last month recommending Shingrix over Zostavax for adults over 50, including adults previously vaccinated with Zostavax.

The main reason for the new vaccine’s improved efficacy lies in the way it stimulates the immune system, said Anne Gershon, MD, a pediatric infectious disease specialist at Columbia University Medical Center (CUMC) who is an expert on VZV.

Shingrix delivers an antigen—the part of the virus that stimulates an immune response—and an adjuvant that amplifies this response. In contrast, Zostavax uses live, but weakened, virus as its antigen, which multiplies in a vaccinated person to induce an immune response. It does not contain an adjuvant.

“Live, weakened-virus vaccines may be less capable of inducing a strong immune response in people with an immune system that has already been impaired by aging,” Dr. Gershon said. “The adjuvant approach is so much more effective that it could eventually make some live attenuated virus vaccines obsolete.” She also notes that, unlike Zostavax, which contains live virus, Shingrix does not pose a threat to those who are immunocompromised.

The new vaccine is also more effective in reducing the risk of developing one of the most common complications of shingles, postherpetic neuralgia. In most shingles cases, the rash disappears after a few weeks, but about 20% of patients later develop severe nerve pain around the skin where the rash was present. The pain can last for months, or in rare cases, years. Singles can cause other serious complications, such as eye infections, brain involvement, and intestinal ulcers.

“There’s no good treatment for postherpetic neuralgia and other complications, so we rely on vaccines to cut down the risk of having a shingles outbreak in the first place,” Dr. Gershon said.

But efficacy is not the only consideration when deciding whether or not to be vaccinated. Zostavax is given in one dose, although a booster dose may eventually be needed. Shingrix requires two doses, given two to six months apart. Currently, it is unknown whether a booster shot will be needed with Shingrix. “The two-dose schedule could be a deterrent to many older people,” said Evelyn Granieri, MD, a geriatric medicine expert at CUMC.

Cost may be another consideration. The new vaccine is slightly more expensive than the old vaccine. And while Shingrix, like Zostavax, is expected to be covered under Medicare Part D, not everyone with Medicare has a Part D plan. “This could be a deal breaker for seniors who are living on a fixed income and have to prioritize their healthcare expenditures,” said Dr. Granieri.

“But for those who are already paying out of pocket, the improvement in effectiveness may be worth the slight increase in cost,” said Dr. Gershon.