Nov 16 (Reuters) - U.S. President Joe Biden"s administration is expected to announce this week the purchase of 10 million courses of Pfizer Inc"s (PFE.N) experimental COVID-19 pill, the Washington Post reported on Tuesday, citing people familiar with the matter. Earlier this month, Pfizer, which also makes one of the mostly widely used COVID-19 vaccines, said the pill cut the chance of hospitalization or death for adults at risk of severe disease by 89% in its clinical trial. Pfizer expects to submit interim trial results for the pill, branded Paxlovid, to the Food and Drug Administration (FDA) before the Nov. 25 U.S. Thanksgiving holiday. The pill will compete with Merck & Co Inc"s (MRK.N) similar oral drug, molnupiravir, which was shown to halve the chances of dying or hospitalization for COVID-19 patients at high risk of serious illness. Merck applied for U.S. emergency use authorization for its drug in October and has already received regulatory clearance in Britain.The price struck between the administration and Pfizer for its drug is less than that for the Merck pill, closer to $500 per treatment, the report said, adding that the final details have not been chalked out. Pfizer did not immediately respond to a Reuters request for comment.