WASHINGTON, Dec 19 (Reuters) - U.S. health officials urged Americans on Sunday to get booster shots, wear masks and be careful if they travel over the winter holidays, with the Omicron variant raging across the world and likely to take over as the dominant strain in the United States. The government is gearing up for the next phase of battle in a two-year fight against a virus that has killed 800,000 and disrupted every aspect of daily life. Dr. Francis Collins, director of the National Institutes of Health, told CBS' "Face the Nation" that the number of Omicron cases will...