WASHINGTON, Dec 18 (Reuters) - U.S. President Donald Trump in a tweet on Friday said Moderna Inc’s vaccine had been approved and would ship immediately, although the U.S. Food and Drug Administration has made no public announcement yet regarding its decision. A panel of outside FDA advisers met to discuss Moderna’s vaccine on Thursday and an agency decision was expected as soon as Friday. “Moderna vaccine overwhelmingly approved. Distribution to start immediately,” Trump said in a post on Twitter. Representatives for the FDA could not be immediately reached for comment. (Reporting by Susan Heavey; Editing by Alex Richardson)