Of blood clots and COVID-19


By Elizabeth Sitotombe

THERE is an increased risk of developing serious blood clots for three-to-six months after contracting COVID-19, studies have revealed.

Blood clots have been at the centre of discussion during the ongoing COVID-19 pandemic, both in terms of the risks associated with catching the virus and getting vaccinated against it.

Now, what are blood clots? 

A blood clot is a clump of blood that is changed from a liquid to a gel-like or semi-solid state.

Clotting is necessary as it helps prevent blood loss, when you have a cut for instance. 

But when a clot forms in one of your veins, it won’t always dissolve on its own and that can be dangerous and even life   threatening.

Blood clots, including those associated with COVID-19, can also harm the nervous system. 

Blood clots in the arteries leading to the brain can cause a stroke. 

Some previously young and healthy people who have contracted COVID-19 have suffered strokes, possibly due to abnormal blood clotting.

A team from Sweden, the UK and Finland conducted a study that found that people who have had COVID-19 were more likely to develop blood clots; what is more important to note is that even people with mild COVID-19 were also found to be at an increased risk.

Their results, published in the British Medical Journal on April 6 2020, suggested that COVID-19 infection was associated with a 33-fold increase in the risk of pulmonary embolism, a five-fold increase in the risk of deep vein thrombosis (DVT) and an almost two-fold increase in the risk of bleeding.

DVT occurs when a blood clot forms in one or more of the deep veins in the body and typically form on the thigh or lower leg.

Symptoms include leg pain or swelling. 

The pain often starts in the calf and can feel like cramping or soreness.

One might notice red or discoloured skin on the leg or a feeling of warmth on the affected leg. 

It can also occur with no symptoms.

DVT can be very serious because blood clots in your veins can break loose, travel through your bloodstream and get in your lungs, breaking blood flow.

This can lead to pulmonary embolism (PE).

Pulmonary embolism (PE) is a blockage in one of the pulmonary arteries in the lungs. 

It is, in most cases, caused by blood clots that travel to the lungs from deep veins in the legs or rarely from veins in the other parts of the body. 

Warning signs of PE include:

  • Sudden shortness of breath and this symptom typically appears suddenly and always gets worse with exertion;
  • Chest pain or discomfort that worsen when you take a deep breath or when you cough;
  • Feeling lightheaded or dizzy or fainting;
  • Rapid pulse; Rapid breathing and
  • Coughing up blood.

The researchers wanted to find out when that risk returned to normal. They tracked the health of over one million people who tested positive for COVID-19 between February 2020 and May 2021 in Sweden. 

They compared these to four million people of the same age and sex who had not had a positive test.

After a COVID-19 infection, they found an increased risk of blood clots in the leg, or DVT, for up to three months.

COVID-19 patients were at higher risk of blood clots if they had underlying conditions, had a severe case of the virus or if they were infected during the first wave of the pandemic in early 2020.

During the course of the study, the team saw 401 cases of DVT among the COVID-19 patients, compared to 267 cases among the negative patients.

Meanwhile, there were 1 761 cases of PE among virus-infected patients in comparison to 171 cases among the control patients. 

When the researchers compared the risks of blood clots after COVID-19 to the normal level of risk, they found that:

λFour in every 10 000 COVID-19 patients developed DVT compared to one in every 10 000 people who did not have COVID-19.

λAbout 17 in every 10 000 COVID-19 patients had a blood clot in the lung (PE) compared to less than one in every 10 000 who did not have COVID-19.

λBleeding events occurred in 10 of every 10 000 COVID-19 patients compared to four in every 10 000 who did not have COVID-19.

Why does COVID-19 cause blood clotting?

One theory from the Heart Research Institute is that the virus is directly impacting on the cells lining our blood vessels. When the body fights an infection, the immune system becomes activated to try and kill the invader, and research shows an activated immune system can cause blood clots.

In severe COVID-19, the immune system appears to go into overdrive. 

This could lead to the unchecked activation of cells that typically stop blood clotting. 

Another theory is that the increased rate of blood clots in COVID-19 is simply a reflection of being particularly unwell and immobile. 

However, current data suggests the risk of blood clots is higher in patients with COVID-19 than those in hospital and intensive care units (ICUs).

The study did not investigate clots associated with vaccines but, according to the researchers, their results add to a growing body of evidence about the link between COVID-19 and serious blood clots. 

Other studies have also suggested that, while blood clots can occur after vaccination, the risk is significantly lower, again emphasising the importance of getting vaccinated.

According to Dr Frederick Ho, a lecturer in Public Health at the University of Glasgow and one of the people to write an editorial that accompanied the findings in Sweden: “While the risk of thromboembolic events is increased after vaccination, the magnitude of risk remains smaller and persists for a shorter period than that associated with infection.”

Even though many countries are removing pandemic restrictions and shifting their focus to living with the virus, he urged people to continue being vigilant to the complications associated with even a mild COVID-19 infection.


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