AstraZeneca Vaccine and Blood Clots: What Is Known So Far In rare cases, an immune reaction has led to antibodies that caused a serious clotting disorder. But public health experts maintain the vaccine’s benefits far outweigh the risks for most people. Credit...Yves Herman/Reuters April 10, 2021, 11:01 a.m. ET The AstraZeneca-Oxford vaccine has been deployed against Covid-19 in at least 115 countries, some of them for several months now. But it wasn’t until a few cases of a rare blood-clotting disorder — some fatal — emerged within the past month or so that many European nations began to rethink its use across all age groups.