The European regulator said Tuesday there is "no indication" the drug causes clots, and von der Leyen reiterated her trust in the vaccine on Wednesday.
Ms. von der Leyen didn't rule out using emergency powers in the EU's basic treaties that allow the bloc to seize production in times of severe economic difficulties.
The threat from Brussels comes despite figures from the European Centre for Disease Prevention and Control released on Monday showing the bloc was sitting on over 15 million unsued coronavirus vaccine doses.
The President of the European Commission says it will have to reflect on how to make exports to vaccine-producing countries dependent on their level of openness - saying it's still waiting for doses to come from the United Kingdom, while ten million doses have been exported to the United Kingdom from the EU.
The EU has already set up special oversight of vaccine exports in which manufacturers contracted to supply Europe must declare if they intend to export doses outside the bloc.
She said that 10 million doses have been exported to the United Kingdom in the last six weeks and pointed out that AstraZeneca is manufacturing its vaccine in Britain.
"AstraZeneca has unfortunately under-produced and under-delivered, and this painfully, of course, reduced the speed of the vaccination campaign", von der Leyen told reporters.
With the Pfizer jab being produced in Germany and Belgium, and Moderna being finished off in Spain, Downing Street urged the European Union to "stand by its commitment" not to restrict exports.
The U.S. has said Canada and Mexico are at the top of the list.
The EU had not previously said how many doses of the Pfizer/BioNTech vaccine, which is administered in two shots, it would receive in the April to June period under two confidential supply contracts with the drugmakers. Last week, the United Kingdom summoned a senior EU diplomat in response to accusations by European Council President Charles Michel that Britain had an "outright ban" on exports.
Several countries in Europe have suspended use of the AstraZeneca vaccine in recent days over reports of risky blood clots in some people who received the shot. The country now has four vaccines approved for emergency use, including those made by AstraZeneca and Johnson & Johnson.
Most of the EU's worry is over Britain, home of the AstraZeneca vaccine, where the inoculation campaign has progressed at a much faster pace than in the EU.
Prime Minister Boris Johnson said he is to receive the AstraZeneca jab "very shortly" in an attempt to allay concerns. The regulator is expected to give an update on Thursday.
Foreign secretary Dominic Raab disputed Ms von der Leyen's assertions. "It will certainly be Oxford AstraZeneca that I will be having", the Prime Minister said.
This story has been published from a wire agency feed without modifications to the text.