Aug 31 (Reuters) - Walmart Inc (WMT.N) on Tuesday became the latest U.S. retailer to say that it is ready to administer millions of COVID-19 booster vaccine doses this fall if U.S. health officials endorse such a shot to improve fading immunity against the coronavirus. Rivals CVS Health Corp (CVS.N) and Kroger Co (KR.N) earlier this month had announced plans to make booster shots available at their stores, pending regulatory guidance. read more U.S. health regulators have authorized a third dose of the vaccines made by Pfizer (PFE.N) and partner BioNTech (22UAy.DE)as well as Moderna (MRNA.O) for those with weak immune systems, and the government has said it plans to make booster doses available to more Americans in September. The need for boosters, however, has not yet been recommended by health regulators. The U.S. Centers for Disease Control and Prevention (CDC) on Monday said it may be difficult to determine at this point whether immunity from prior vaccination is waning over time or if the vaccines are just less able to prevent infection by the highly-transmissible Delta variant. read more Walmart, which runs one of the largest U.S. pharmacy chains, and its Sam"s Club pharmacies are currently administering the third dose to immunocompromised individuals.