NEW YORK (NYTIMES) - Millions of people in dozens of countries have received the AstraZeneca Covid-19 vaccine with few reports of ill effects, and its prior testing in tens of thousands of people found it to be safe.

But recently, blood clots and abnormal bleeding in a small number of vaccine recipients in European countries have cast doubt on its safety, although no causative link has been found between the patients' conditions and the vaccine.