Explanation of blood clots and the AstraZeneca Covid vaccine

Millions of people in dozens of countries have received the AstraZeneca Covid 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. The reports have prompted more than a dozen countries to either partly or fully suspend the vaccine’s use while the cases are investigated. Most of the nations said they were doing so as a precaution until leading health agencies could review the cases.

The AstraZeneca vaccine has not been authorized for use yet in the United States, although a review of its U.S. trial is expected soon.

The cascade of decisions to pause the use of AstraZeneca’s vaccine, mainly by European countries, followed reports of four serious cases in Norway, which were described among health workers under age 50 who received the vaccine. Most developed clots or bleeding abnormalities and had low platelet counts, health authorities there said. Two of them have died from brain hemorrhages, and the other two are hospitalized. The death of a 60-year-old woman in Denmark and of a 57-year-old man in Italy also fueled quick decisions, although none of the deaths have been fully investigated to determine whether there is any link to the shots they received.

What is a blood clot and what causes them generally?

A blood clot is a thickened, gelatinous blob of blood that can block circulation. Clots form in response to injuries and can also be caused by many illnesses, including cancer and genetic disorders, certain drugs and prolonged sitting or bed rest. Clots that form in the legs sometimes break off and travel to the lungs or brain, where they can be deadly.

Can the vaccine cause blood clots?

Vaccines have not been shown to cause blood clots, said Daniel Salmon, director of the Institute for Vaccine Safety at Johns Hopkins University.

Blood clots are common in the general population, and health authorities suspect that the cases reported in vaccine recipients are most likely coincidental and not related to the vaccination.

“There are a lot of causes of blood clotting, a lot of predisposing factors, and a lot of people who are at increased risk — and these are often also the people who are being vaccinated right now,” said Mark Slifka, a vaccine researcher at Oregon Health and Science University. ...