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  • A new study finds that people who had COVID-19 are at increased risk for blood disorders months after their illness.
  • This may be due to inflammation from the illness.
  • Getting vaccinated can help protect people from developing blood clots or bleeding disorders after COVID-19.

New research published in the British Journal of Medicine (BMJ) finds risk of a potentially severe consequence of COVID-19 can remain elevated for months.

Swedish researchers analyzed national registry data from over one million people to discover that the increased risk of blood clots in the legs and lungs may extend up to six months after experiencing the pandemic virus.

Dr. Richard Becker, professor and director, UC Heart, Lung and Vascular Institute, and UC Division of Cardiovascular Health and Disease at the UC College of Medicine, told Healthline these findings show the importance of data collection.

“They demonstrate the power and potential of national databases to capture the natural history of disease – in this particular case COVID-19,” said Becker.

For this study, researchers identified over one million people with laboratory-confirmed COVID-19 infection from the beginning of the pandemic to mid-2021. They also matched age, sex, and region to over four million people who had not shown positive for the virus.

The data suggest a fivefold increased risk of deep vein thrombosis (blood clot in deep veins, usually in the legs), a doubling of bleeding risk, and a 33-fold increase in the risk of pulmonary embolism (blood clot in the lungs), with the highest risk in patients with pre-existing conditions.

The results also show these risks significantly increased for 70 days after infection for deep vein thrombosis, 110 days for pulmonary embolism, and bleeding risk increased for two months.

The results show “that the risk of deep vein thrombosis and pulmonary embolism persists for at least six months after initial infection,” said Becker.

Clotting risk was greatest during the first pandemic wave compared with the second and third waves and in those who experienced more severe disease.

Researchers say this might be explained by vaccine coverage and improvements in treatment for older patients after the first wave.

Even among non-hospitalized patients with milder symptoms, the researchers found increased risks of deep vein thrombosis and pulmonary embolism. However, no increased risk of bleeding was found in mild cases.

“The severity of infection and risk profile among COVID-19 survivors helped to identify persons most likely to experience venous thromboembolism following hospital discharge,’ said Becker.

He added that the findings might suggest a different risk of blood clots between COVID-19 variants.

Experts pointed out that this was an observational study, so researchers only analyzed the available data but did not establish a cause.

The researchers also addressed several limitations that could have influenced their findings.

These include:

  • Blood clots may have been under-diagnosed in patients with COVID-19
  • During the first wave, testing for the virus was limited
  • Information on vaccination wasn’t available

They noted that older people, who have a higher clotting risk, were initially prioritized for vaccination.

“As an observational study, it must be interpreted with caution due to confounding factors that may not be accounted for,” said Dr. Daniella Kadian-Dodov, a vascular medicine specialist at Mount Sinai Heart in New York. “However it is in line with our understanding of the relationship of COVID-19, thrombosis, and bleeding risk to date.”

The research sheds light on a potentially life-threatening complication for people with COVID-19. One that Kadian-Dodov called “particularly worrisome.”

“Deep vein thrombosis (DVT) most often occurs in the lower extremities and may present with swelling, pain, or discoloration,” said Kadian-Dodov.”

According to Kadian-Dodov, the major risk of DVT is pulmonary embolism (PE), which is when a piece of the DVT breaks off and travels to the lungs.

“This can cause major stress on the heart and be life threatening,” she warned.

Kadian-Dodov explained that treatment with blood-thinning medications can stop the clotting process and allow the body to break down the clot. In some cases, evidence the blood clot doesn’t fully disappear and is visible in imaging studies years after the initial event.

There are multiple reasons why COVID-19 may increase the risk of having a blood clot, especially for people who had to be hospitalized for the disease.

Dr. Nikhil Bhayani, infectious diseases physician advisor at Texas Health Resources, said when there is injury to the inside of the blood vessel lining from COVID-19 infection, it can trigger an inflammatory process leading to blood clot formation.

He said other factors include that some people are immobilized with the disease for an extended time, and some patients have a predisposition to developing blood clots.

“The immobilization of a patient can cause blood flow to become static, potentially leading to clot formation,” said Bhayani.

Kadian-Dodov said it’s established that vaccination against COVID-19 results in milder disease for people who develop the disease.

“As a result, there is a tempered inflammatory response to the illness and patients are protected from the ‘immunothrombosis,’” she said. “Immune system mediated blood clotting due to immense inflammation, described in COVID-19.”

Kadian-Dodov advised that vaccination against the pandemic virus remains the “strongest protection” against COVID-19 and all related consequences of infection.

Researchers looked at data from over one million people to find the risk of dangerous blood clots in our legs and lungs is significantly elevated for up to six months after infection.

Experts say this might be due to the inflammation that COVID-19 infection is known to cause.

They also say that being vaccinated is the best protection we have against developing blood clots caused by COVID-19.