* Shingrix wins FDA approval for immunising over-50s
* Analysts see eventual annual sales above $1 billion
* "Bubble boy" gene therapy secures UK cost approval (Adds analyst, further details)
LONDON, Oct 23 (Reuters) - GlaxoSmithKline has won U.S. approval for a new and improved shingles vaccine, the second of three key products for which the British drugmaker expects approval this year.
It also secured a recommendation from UK cost authorities for a $700,000 gene therapy for so-called "bubble boy" disease -- a step forward for the field of fixing faulty genes but a tiny sales opportunity. Only three patients have received the gene therapy commercially since its May 2016 approval in Europe.
The green light for GSK's shingles shot Shingrix, which has proved significantly better than Merck & Co's Zostavax in clinical trials, is an important vindication of GSK's product pipeline. The news was announced late on Friday.
GSK's vaccine is considered one of the more pivotal products in its pipeline, with annual sales forecast to reach 1.03 billion pounds ($1.36 billion) by 2023, Thomson Reuters data shows.
"Although largely expected, todays approval is an important one for GSK -- a new product with a clear best-in-class profile," Berenberg analysts said on Monday. "We expect approval in Europe late this year."
The U.S. Food and Drug Administration (FDA) approval comes a month after it cleared GSK's three-in-one inhaler for chronic lung disease.
The third new product GSK expects to launch in the world's biggest drugs market is a dual-drug regimen for HIV, on which the U.S. regulator is scheduled to give a verdict by Dec. 1.
Chief Executive Emma Walmsley, who will present third-quarter financial results on Wednesday, sees the three products as "critical" for GSK's efforts to fill the revenue gap left by falling sales of the ageing lung drug Advair.
The FDA approved Shingrix, which is given in two doses, for adults aged 50 and over. Approval had been expected after an advisory panel to the agency backed the product.
Shingrix is more effective than Zostavax, the only currently available shingles prevention vaccine, which is given as a single dose. GSK said it expects Shingrix to be "available shortly".
The U.S. Centers for Disease Control and Prevention's advisory committee on immunisation practices is expected to vote on a recommendation for the use of Shingrix at its meeting on Wednesday.
The outcome of that meeting may determine just how big the product becomes.
Many analysts expect the committee to recommend routine vaccination of the over-60s, as is currently the case with Zostavax, but an endorsement of use in younger people would represent commercial upside.
Older adults are most at risk of shingles, a painful, often debilitating blistering rash that results from reactivation of the varicella-zoster virus that causes chickenpox and remains latent in those who have had that disease.
Shingrix contains a component from Agenus, which is entitled to royalties on future sales.
Meanwhile, Britain's National Institute for Health and Care Excellence (NICE) said on Monday that it had approved GSK's gene therapy Strimvelis for treating ADA-SCID -- better known as "bubble boy" disease -- despite its steep price tag of 594,000 euros ($698,000).
Infants with the condition need to be kept in isolation to avoid infections and the cost to the state health service can run to millions of pounds, so a one-off genetic cure can prove cost-effective even at a very high price. ($1 = 0.7581 pounds) ($1 = 0.8508 euros) (Additional reporting by Bill Berkrot and Deena Beasley; Editing by David Goodman)