UPDATE 1-Sanofi pandemic flu vaccine rebuffed by EU agency

LONDON, March 19 (Reuters) - An experimental pandemic flu vaccine from Sanofi-Aventis has been rejected by European regulators because of worries about its effectiveness.

The European Medicines Agency said on Thursday its experts were concerned over the ability of Sanofi's H5N1 shot Emerflu to trigger the production of enough antibodies against the flu virus.

Emerflu was expected to be used in adults to protect against pandemic flu, which many experts fear could be triggered by mutations in the H5N1 strain of virus now circulating in birds. It was only intended for use once a pandemic had been declared.

(Reporting by Ben Hirschler; Editing by David Holmes) Keywords: BIRDFLU SANOFI/ ([e-mail]; Tel: [phone]; Reuters Messaging: [e-mail]; blogs.reuters.com/ben-hirschler/) COPYRIGHT Copyright Thomson Reuters 2009. All rights reserved. The copying, republication or redistribution of Reuters News Content, including by framing or similar means, is expressly prohibited without the prior written consent of Thomson Reuters.