Jan 7 (Reuters) - The U.S. government has signed the first contract involving 500 million rapid COVID-19 tests that President Joe Biden promised in December would be free to Americans who request them, an official said on Friday. The following describes how the distribution of tests will work in the fight against the rapid spread of the Omicron coronavirus variant: TEST IN HOMES IN "WEEKS" In a briefing on Wednesday, White House COVID-19 coordinator Jeff Zients said the contracting process for the 500 million tests, being handled by the U.S. Army, had closed on Tuesday. The 500 million tests won"t "cannibalize" access to other tests on pharmacy shelves, Zients said. The deliveries of tests from manufacturers to the U.S. government will begin over the next week or so, Zients said, with Americans starting to receive free tests in the coming weeks. WEBSITE TO REQUEST TESTS Biden spokesperson Jen Psaki said in a Dec. 23 briefing that the White House was finalizing designs for a website, which would be live when the first tests were ready. Zients said this week the system would be "free and easy." The Washington Post reported on Thursday that the White House was finalizing details with the U.S. Postal Service to mail the tests. HOW MANY, HOW SOON? Psaki said the White House expected to get the first set of tests in early January. The government contract asks that manufacturers be able to supply antigen tests to detect Omicron and other variants at the rate of 7.5 million per month between January and March if they are based in the United States and 20 million per month if they are not. WHICH TESTS The tests will come from a list of 13 approved by the U.S. Food and Drug Administration, from 12 different companies including Abbott Diagnostics Scarborough Inc, Quidel Corporation and ACON Laboratories Inc.