Current research has not shown a link between the mRNA COVID-19 vaccines and blood clots. Health leaders around the world continue to encourage everyone to get a COVID-19 vaccine when it is available to them.

In their meeting highlights from May 3-6, 2021, the European Medicines Agency suggested that there “is no safety signal for the mRNA vaccines.” A safety signal is a new or known problem that could have been caused by a medicine. A safety signal means that the problem needs to be studied further. They wrote that “the current evidence does not suggest a causal relation” between the Moderna COVID-19 vaccine and blood clots. They noted that the number of blood clots seen in patients who received the shot are lower than in those who were not vaccinated. 

The U.S. Centers for Disease Control and Prevention also continue to state that blood clots have not been linked to the mRNA vaccines, even after more than 210 million doses have been given.

Finally, a study published in June 2021 from Scotland looked at data from 0.82 million people who received their first shot of the Moderna mRNA vaccine. It found that the shot was not associated with blood clots in the first 28 days after the shot. 

Not all blood clots are bad. When the body is hurt, blood cells and other molecules join together to form a solid clot. The clot helps stop blood loss and prevents germs from entering the body.

When blood clots happen inside of a blood vessel in the brain, heart, lungs, or other body part, they can be dangerous. Health problems like infections, surgery, or inflammation can cause blood clots. They have also been linked to age (over 65 years), lifestyle factors like smoking or staying still for very long periods of time, pregnancy, and some medicines (like birth control pills). Blood clots have also been linked to COVID-19 infection.

Clots that block blood from getting to the brain can lead to a stroke. Clots that block blood flow to the heart can cause a heart attack. Clots in the lungs can cause breathing problems, and clots in the legs can cause swelling and pain. If someone has a blood clot that is blocking a blood vessel, emergency medical care is needed.

To diagnose a blood clot, health care providers look at the patient’s symptoms and use clinical tests. Blood tests may include clotting factors, platelets, fibrinogen, and others. Other tests like computerized tomography scans (CT scans) or ultrasound may also be used.

One way to test for blood clots is a blood test called a D-dimer. D-dimer is a blood protein that gets released when a blood clot begins to dissolve or break down. When the D-dimer level is high, it can be because of a clot, but there are many other possible causes. 

The level can also be high because of the following:

• Pregnancy; 
• Cigarette smoking;
• Infection or sepsis;
• Inflammation;
• Autoimmune problems;
• Advanced age;
• Cancer;
• Other causes.

A health care provider should evaluate your blood tests and symptoms together to decide what kind of treatment may be needed.

Scientists have found a possible link between AstraZeneca and Johnson & Johnson COVID-19 vaccines and blood clots for some people. The risk of blood clots from these vaccines for most people is low. Though rare, most cases of blood clots have been seen in women under 60 years of age within two (AstraZeneca) or three (Johnson & Johnson) weeks after the vaccine. The AstraZeneca and Johnson & Johnson vaccines are not mRNA vaccines.