A Physician’s Primer on Covid-19, Clotting, and Causality
How to make sense of recent concerns about the AstraZeneca vaccine
Last week, several European countries paused their use of the AstraZeneca vaccine due to concerns about clotting and bleeding risks. Though the World Health Organization (WHO) and European Medicines Agency (EMA) have both said that it is safe to use, most countries have resumed using the vaccine, and the company released data on Monday showing it is 79% effective in preventing symptomatic disease in the United States, many people may still be wondering about the risks. There are five major things to clear up when understanding the concerns about blood clots.
1. What are clots?
When most people think of blood clots, they think of a scab on the skin or clots in menstruation: congealed, thickened blood. In medicine, we’re talking about something more serious, involving the blood that circulates in our veins and travels from the tissues to the lungs to get reoxygenated. Blood clots are a general term for what’s known as deep vein thrombosis (DVT) and pulmonary embolism (PE).
Think of DVTs as blood clots that are often found in the calves or in the arms. Sometimes they resolve on their own, but they become dangerous when they break off and travel through the circulation and into the lungs, causing a PE, which in turn causes chest pain, decreases oxygen, and can lead to death. Sometimes DVTs can break off and travel backward to the heart and through the body again, making their way into the brain and causing a stroke. This is called a paradoxical embolism. A more rare clot in the brain is called a cerebral venous thrombosis (CVST). CVSTs may be the main clot of concern associated with the AstraZeneca vaccine. DVTs, PEs, and CVSTs are medical emergencies.
2. How do clots form?
Most of the time, blood clots form in order to help us heal from wounds — injured tissue, internally or externally. Their formation involves the “coagulation (fancy word for clotting) cascade,” which comprises the extrinsic pathway, intrinsic pathway, and common pathway. The extrinsic pathway refers to factors in the coagulation cascade that are external or extrinsic from blood when studied in a test…