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* GSK's Shingrix has outperformed Merck's Zostavax in trials
* Consensus forecasts point to $1.05 billion sales in 2021 (Adds sales forecast, background on rival vaccines)
By Ben Hirschler
LONDON, Oct 24 (Reuters) - GlaxoSmithKline has filedits shingles vaccine Shingrix for U.S. regulatory approval, thedrugmaker said on Monday, bringing the potential $1billion-a-year seller a step closer to market.
Shingrix is viewed by analysts as among the Britishcompany's most promising experimental products, since it hasshown greater protection among older recipients than Merck &Co's rival shot Zostavax.
GSK itself highlighted Shingrix as one of its top near-termpipeline hopes last November, during its first research anddevelopment day in more than a decade.
The company, which will see Emma Walmsley take over as chiefexecutive in 2017, is seeking to revitalise a drug portfolio hitby falling sales of best-selling inhaled lung treatment Advair.
In clinical trials, GSK's vaccine remained 90 percenteffective in people over age 70, even four years afterinjections. Zostavax efficacy, by contrast, varies between 18and 70 percent, and it declines noticeably in older people.
People who are 70 years or older are often most at risk fromshingles, a painful, itchy rash that results from thereactivation of latent chickenpox virus.
The impressive trial results should put Shingrix in a strongcompetitive position, analysts believe, even though it requirestwo doses, against just one for Zostavax, and it is also linkedto more injection site reactions.
Analysts, on average, predict that worldwide sales ofShingrix will reach 856 million pounds ($1.05 billion) in 2021,according to consensus forecasts compiled by Thomson Reuters.
Sales of Zostavax, the only shingles vaccine on the marketat present, totalled $749 million in 2015.
GSK's vaccine contains a component from U.S. biotech firmAgenus, which is entitled to royalties on future sales.
GSK said it planned to file Shingrix for European andCanadian approval before the end of this year, with Japanfollowing in 2017.
($1 = 0.8191 pounds) (Editing by Susan Fenton and Jason Neely)
