WASHINGTON, Dec 19 (Reuters) - White House medical adviser Anthony Fauci on Sunday urged Americans traveling over the upcoming holidays to get booster shots and always wear masks in crowded public spaces to help slow the spread of the Omicron variant of the coronavirus. The easily transmissible variant is "raging through the world," and traveling will increase the risk of infection even among vaccinated people, Fauci said in an interview with NBC's "Meet the Press." "We are going to see breakthrough infection - there is no doubt about that," Fauci, director of the National Institute of Allergy and Infectious Diseases,...