UPDATE 2-Trial suggests Prevnar may also protect ages 18-49

(Adds details on possible expanded demand for vaccine)

Oct 4 (Reuters) - Pfizer Inc said a late-stage trial of its vaccine to protect against pneumococcal bacteria suggested it would also work in adults aged 18-49, thereby possibly expanding its sales.

The vaccine, Prevnar 13, which already has $3.5 billion in annual sales from it use by those over age 50 and under age five, is designed to protect against pneumonia, meningitis and other infections caused by pneumococcal bacteria.

Pfizer said on Thursday that the favorable results from the study will support both its recent European Union application to market the product for that 18-49 age group, as well as applications it plans to make in other countries.

Prevnar 13 - known as Prevenar 13 in Europe - protects against 13 strains of the bacterium, formally known as streptococcus pneumoniae. It generated global, second-quarter sales of $916 million. An older vaccine called Prevnar, which protects against seven strains, had second-quarter sales of $84 million.

A competing vaccine from GlaxoSmithKline , Synflorix, protects against 10 strains of the bacteria.

Pneumonia, often caused by the pneumococcal organism, is one of the biggest causes of death in older people and its incidence begins to increase after age 50.

Pfizer said Prevnar 13 met the main goal of the late-stage study by showing that the immune response to the vaccine in the 18- to 49-year-old age group was not inferior when compared with the response in the 60-to 64-year-old group.

Pfizer's shares marginally higher at $25.37 in midday trade on the New York Stock Exchange.

(Reporting by Ransdell Pierson in New York and Balaji Sridharan in Bangalore; Editing by Roshni Menon and Leslie Gevirtz)

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