WASHINGTON - The Trump administration opted last summer not to lock in a chance to buy millions of additional doses of one of the leading coronavirus vaccine contenders, a decision that could delay the delivery of a second batch of doses until manufacturer Pfizer fulfills other international contracts.The revelation, confirmed Monday by people familiar with the matter, came a day before President Donald Trump aimed to take credit for the speedy development of forthcoming coronavirus vaccines at a White House summit Tuesday.Pfizer's vaccine is expected to be approved by a panel of Food and Drug Administration scientists as soon as this week, with delivery of 100 million doses — enough for 50 million Americans — expected in coming months.FILE.