Trump had ordered around 170 million doses of Pfizer's vaccine in December with an option to buy another 400 million more. So Biden is executing the pre-purchase order in Trump's deal. Also, Trump ordered from Moderna and others.
First, lets untwist some terminology before straightening out the rest of the facts. A pre-order is
an order you place which will be released and shipped at a later date, but won't be charged until it ships. A pre-purchase is a
pre-order for which you will be charged immediately
upon placing your order, which will ship at a later date. An option to buy is an agreement in which a specific quantity of product is purchased with the
right to order more of that product within a specified period, but
without a requirement or commitment to do so.
An option to buy is not an order, a pre-order, or a pre-purchase order.
Trump ordered 100 million doses (enough to fully vaccinate 50 million people) from Pfizer in July, 2020 and declined multiple options to buy more doses thru November.
In November, BioNTech and Pfizer announced they reached an agreement with the European Commission to supply 200 million doses of the vaccine, with an option to deliver 100 million more doses. Pfizer issued a statement noting that any additional doses beyond the 100 million initially agreed to by the United States are “subject to a separate and mutually acceptable agreement." "Pfizer did offer an additional allotment coming out of that [Michigan] plant, basically the second quarter allotment, to the United States government multiple times, and as recently as after the interim data came out and we knew this vaccine looked to be effective," Gottlieb said on CNBC. [Dr. Scott Gottlieb, former FDA Commissioner and currently Pfizer board member]
In early December, shortly before the Pfizer vaccine became the first to receive FDA emergency use authorization, several news outlets began reporting that the Trump administration had turned down the option to buy more vaccine beyond the initial order. A few days later, the Trump administration announced they had ordered an additional 100 million dose from Moderna. They then scrambled to reach a new agreement with Pfizer for an additional 100 million doses and finally secured that order at the end of December. This brought the total Pfizer order to 200 million doses, enough to fully vaccinate 100 million people.
https://www.cnbc.com/2020/12/14/covid-vaccine-us-passed-on-more-pfizer-doses-in-november-says-board-member-gottliebhttps://www.biospace.com/12-08-20/u-s-government-passes-on-opportunity-to-buy-more-pfizer-vaccine-dosesThere was a delay of several months between the initial orders and additional orders at the end of the year. The additional orders, placed from the middle to end of December, brought the total amount of vaccine ordered from both companies to 400 million doses, enough to vaccinate 200 million people, but still not enough to cover fully vaccinating the U.S. adult population.
Biden quickly ordered an additional 200 million doses, securing enough vaccine for all adults in the U.S., split between Pfizer and Moderna. The order contracts initiated by Biden were negotiated, with both companies, to include expediting the delivery by a month. Of course getting more vaccine, and getting it faster, doesn't help if they're not used expeditiously. So, there has been a major effort to provide federal resources and personnel to states to continually increase vaccination rates.
https://www.npr.org/2021/02/11/967194072/biden-announces-deal-for-200-million-more-covid-19-vaccines