WASHINGTON, Dec 29 (Reuters) - White House COVID-19 coordinator Jeff Zients said on Wednesday he expects late next week to complete contracts with companies to deliver the 500 million antigen tests promised by U.S. President Joe Biden to help address the surge in coronavirus cases. Speaking on a briefing call with reporters, Dr. Anthony Fauci, the U.S. government"s top infectious disease expert, also said it is conceivable in the future that an additional shot might be needed to bolster immunity against the coronavirus.