In recent days there has been great concern around blood clots, especially after hearing an alert from Germany about people who have suffered a cerebral vein thrombosis – a blood clot in the brain – after receiving the AstraZeneca vaccine. Other European countries have expressed concern about the possible increased risk of pulmonary embolism and deep vein thrombosis after vaccination.
However, AstraZeneca states that there is currently no evidence that recorded episodes are associated with the vaccine, and that the number of blood clots reported after receiving the vaccine is not greater than would normally occur.
Coagulation, an extremely important function in the human body, helps prevent excessive blood loss both inside and outside the body when our skin, internal organs, or blood vessels are damaged.
But although it can be a normal and essential process, if a clot breaks away from where it formed, it can be dangerous, especially if it lodges in an organ or an artery that is too narrow. This is what is known as an embolus and it can block arterial blood flow, which is essential for vital organs and can lead to a pulmonary embolism (blood clot in the lungs), an ischemic stroke (clot in the brain) or a heart attack. It can also block veins, such as a deep vein thrombosis in the legs, known as a thrombus.
There are many factors that can increase the likelihood that a person will develop a blood clot. We will highlight five:
1. Obesity
Obese people are more than twice as likely to develop a thrombus (blood clot in the leg) compared to people of normal weight. This is because obesity causes chronic inflammation and a reduction in fibrinolysis (the ability to break down clots).
Chronic inflammation also occurs as a result of having less nitric oxide in the body. Nitric oxide is a molecule that protects the specialized endothelium (the lining of the blood vessels) and prevents cells from attaching to the endothelial surface. Even at a young age, obese people have significantly lower levels of nitric oxide.
It is this reduced amount of nitric oxide in obese people that increases damage to the lining of blood vessels, in turn increasing the risk of blood clots.
2. Tobacco
Smoking increases the risk of blood clots by up to three times.
As in the case of obesity, smoking reduces the amount of nitric oxide in the body and encourages the blood to stick together to form clots. This process is driven in part by significantly increased levels of fibrinogen, an important component in clotting, present in the blood of smokers.
Chemicals in cigarettes also cause blood platelets to stick together. Together, these factors make the blood thicker, making it difficult for the heart to pump through the body, in turn damaging the inner lining of the blood vessels.
3. Flying and inactivity
Traveling long distances by plane or being immobilized for a long period after major surgery can increase the risk of blood clots in the form of deep vein thrombosis (DVT), that is, blood clots in the legs. The typical incidence of this type of thrombosis is one in every 1,000, but it is multiplied by three on flights of more than three hours.
Because the blood does not flow as much, the cells and proteins in the blood settle and form clumps. When the person moves again, these clots can travel through the body and block a blood vessel if they are not undone. Higher body mass index, age and smoking increase the risk of developing a deep vein thrombosis from inactivity or on flights.
4. Trauma and cancer
Up to one in four people who have suffered a major trauma, which causes damage to blood vessels – for example, if large bones have been broken – develop clots. In these cases, clot formation is related both to injury to the blood vessels themselves and to bed rest, often prolonged, associated with treatment and recovery.
Also, people with cancer are five to seven times more likely to develop clots. This is because some cancers produce increasing amounts of clotting factors. Cancer also damages healthy tissues, causing them to swell and clot.
5. Birth control pill
Women who take the combined oral contraceptive pill, which contains estrogen and progesterone, have been shown to have a slightly increased risk of blood clots. Other oral contraceptives show similar levels of increase, with approximately 6-17 additional events per 10,000 women treated, depending on the drug used, compared to women not taking the oral contraceptive.
The ingredients in contraceptives increase the levels of various clotting factors that circulate in the blood, increasing the chances of blood clots forming in the veins.
Covid-19
Research also shows that Covid-19 patients have significantly elevated levels of a molecule that forms when clots are present.
This is because COVID-19 attacks the endothelial cells that line the blood vessels, causing an increase in blood clots throughout the body and presenting as a vascular disease.
A study also found that between 2% and 9% of COVID-19 patients develop pulmonary emboli. And they are three to six times more likely to develop blood clots compared to the rest of the population. Other factors – such as bed rest and age – can increase the risk of blood clots in these patients.
But while many factors can increase a person’s risk of blood clots, there is no evidence yet to suggest a direct link between these and the AstraZeneca vaccine.
Adam Taylor, Professor and Director of the Clinical Anatomy Learning Center, Lancaster University
This article was originally published on The Conversation. Read the original.
