Dec 21 (Reuters) - Walmart Inc (WMT.N), Walgreens Boots Alliance (WBA.O) and CVS Health Corp (CVS.N) have limited sales of at-home COVID-19 testing kits as demand surges due to the rapid spread of the Omicron variant of the coronavirus. The variant has become dominant in the United States with lightning speed, dashing hopes for a more normal holiday season, resurrecting restrictions and stretching the country"s testing infrastructure ahead of holiday travel and gatherings. read more Walgreens said in an emailed statement on Tuesday that it would limit both online and in-store purchases of test kits to four per customer. The company also said it was working with suppliers to catch up with demand. Walmart set a limit of eight tests for each online order, but allowed its physical stores to set their own limits depending on inventory levels. "We do have strong inventory levels nationally in store," Walmart said in a statement. "However, inventory is more limited online depending on the zip code." CVS limited sales of test kits to six per person and said the products might be temporarily out of stock on its website as it priorities inventories at its stores. The Biden administration said earlier in the day it would buy 500 million at-home rapid tests that Americans can order online for free starting in January. read more