Blood clot symptoms: signs of clots, does Covid increase risk, what causes them and when to get checked

The study showed that people with underlying health conditions are at most risk

People who have been infected with Covid are at an increased risk of serious blood clots for up to six months, a new study suggests.

Findings showed that people with underlying health conditions are at most risk, along with those who suffered with more severe coronavirus symptoms.

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People infected with Covid are at an increased risk of serious blood clots for up to six months, research suggests (Composite: Kim Mogg / JPIMedia)

What did researchers find?

The study, published in the British Medical Journal, found the risk of deep vein thrombosis (DVT) was “significantly increased” for three months after Covid infection, six months for pulmonary embolism (blocked blood vessels in the lungs), and two months for bleeding.

Experts from Umea University in Sweden assessed data for more than one million people in the country who tested positive for coronavirus between February 2020 and May 2021. They were then compared to more than four million people without the virus.

The team looked at the risk of blood clots in the period after Covid symptoms began, compared to long before people tested positive, and long after their symptoms disappeared.

Results revealed a five-fold increase in the risk of DVT, a 33-fold increase of pulmonary embolism, and an almost two-fold increase in the risk of bleeding in the 30 days after infection.

In absolute terms, this means a first DVT occurred in 401 patients with Covid (absolute risk 0.04%) and 267 patients without (absolute risk 0.01%) over this time period.

Meanwhile, a first pulmonary embolism occurred in 1,761 patients with Covid (absolute risk 0.17%) and 171 without (absolute risk 0.004%), and a first bleed occurred in 1,002 patients with Covid (absolute risk 0.10%) and 1,292 without (absolute risk 0.04%).

Risks continued for most patients for up to a period of six months, and it was found that the risks were higher during the first wave of the pandemic compared to the second and third waves.

Researchers said this could reflect the impact of coronavirus vaccines and treatments, particularly among older people, in reducing their chances of suffering complications.

The team concluded: “Our findings arguably support thromboprophylaxis (preventative treatment) to avoid thrombotic events, especially for high risk patients, and strengthen the importance of vaccination against Covid-19.”

What are the symptoms?

The NHS warns that blood clots can be serious and need to be treated quickly. Common symptoms can include:

  • throbbing or cramping pain, swelling, redness and warmth in a leg or arm
  • sudden breathlessness, sharp chest pain which may be worse when you breathe in
  • a cough or coughing up blood

You are more likely to get blood clots if you:

  • are staying in, or recently left, hospital, particularly if you cannot move around much
  • are overweight
  • smoke
  • are using combined hormonal contraception such as the combined pill, contraceptive patch or vaginal ring
  • have had a blood clot before
  • are pregnant or have just had a baby
  • have an inflammatory condition such as Crohn’s disease or rheumatoid arthritis

When to seek medical advice

If you experience any of the above symptoms and think you may have a blood clot, you should seek medical advice as soon as possible.

Call the NHS on 111, or visit 111.nhs.uk for advice on what to do. Call 999 or go to A&E if:

  • you are struggling to breathe
  • someone has passed out

This could be due to a blood clot in the lungs (pulmonary embolism) which needs to be treated immediately.