Risk of blood clots due to Covid-19 much higher compared to risk associated with vaccine - experts

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  • The risk of developing a rare type of blood clot after getting a Covid vaccine is extremely low, experts from the AHA say
  • Their analysis revealed that chances of developing blood clots due to the vaccine are up to 10 times lower than from Covid-19 infection
  • Those with medical conditions are advised to consult their healthcare adviser before seeking vaccination

Recent events surrounding the rare cases of blood clots among people who received a Covid-19 vaccine have triggered panic, fear and heightened vaccine hesitancy all over the world. But many people are unaware that blood clotting is also a feature of Covid-19 disease.

Now, a Special Report from the American Heart Association and the American Stroke Association Stroke Council Leadership indicates just how rare the vaccine-induced blood clots are – 8 to 10 times higher as a result of a Covid-19 infection compared to the risk associated with a Covid vaccine.

Published in the journal Stroke, lead author of the special report and chair of the department of neurology at The Warren Alpert Medical School at Brown University, Doctor Karen Furie, said in a news release: “The public can be reassured by the CDC’s and FDA’s investigation and these statistics – the likelihood of developing CVST (cerebral venous sinus thrombosis) after a Covid-19 vaccine is extremely low. We urge all adults to receive any of the approved Covid-19 vaccines.”

No major safety concerns

In April this year, the Johnson & Johnson vaccine rollout was halted in South Africa after reports of six cases of blood clots among 6.8 million people who received the single-dose jab in the US, Health24 reported.   

However, the South African Health Products Regulatory Authority (Sahpra) confirmed that no major safety concerns had been identified in the healthcare workers who took part in the Sisonke trial in SA, which led to the programme being resumed.

The US Food & Drug Administration (FDA) also lifted the pause, and the European Union drug regulator, who investigated the cases overseas, said a warning should be added to the jabs indicating a possible link to rare blood clots, but that the product’s benefits far outweigh the risks.


The small number of vaccinated individuals were reported to have a serious condition known as cerebral venous sinus thrombosis (CVST) – which refers to blood clots in the brain’s veins – in combination with thrombocytopenia (low blood platelet count).

According to the report, CVST and thrombocytopenia together are called thrombosis-thrombocytopenia syndrome (TTS). When TTS is linked to receiving a Covid vaccine, it is called vaccine-induced immune thrombotic thrombocytopenia (VITT).

“Covid-19 infection is a significant risk factor for CVST. A preliminary analysis of US data during the Covid-19 pandemic, available online ... found that the risk of CVST due to infection with Covid-19 is 8 to10 times higher than the risk of CVST after receiving a Covid-19 vaccine,” said Furie.

The US Centers for Disease Control and Prevention (CDC) also previously put things into perspective: In the US, up to 600 000 people a year develop blood clots in their lungs or in leg veins or other parts of the body, which means that around 1 000 to 2 000 blood clots occur in that population every day.

The CDC and FDA’s report in April 2021 confirmed that the agencies investigated 15 reported cases of TTS in the US. The 15 cases – among the nearly seven million adults who received the J&J vaccine – were in women ages 18 to 59 years.

Only 20 Covid patients with CVST

The analysis included data from 59 healthcare organisations and 81 million patients, the majority from the US. Among the nearly 514 000 patients in the database who were diagnosed with Covid from January 2020 through March 2021, a total of 20 patients were diagnosed with CVST. 

The news release notes that these data were compared to the incidence of CVST in adults who received either the Pfizer or Moderna jab before March 2021, and that no cases of thrombocytopenia (low platelets) were diagnosed among almost 490 000 vaccinated adults.

“CVST blood clots are very rare adverse events. We recommend immediate screening of all patients who arrive in the ER with a suspected clot – did they receive a Covid-19 vaccine during the recent weeks prior to this event?” advised Furie. 

She added that with the right treatment, most patients can have a full recovery after CVST, TTS or VITT.

Further research needed

There is still much to be investigated, the authors wrote. Some of the questions that remain include whether there are preexisting conditions that may predispose someone to CVST or VITT, and what the true prevalence and risk of low platelet count and CVST are after receiving the vaccine. 

“We are learning the various intricacies of Covid-19 live, in real-time, with the patients we see in our hospitals every day,” said Furie. 

“CVST is extremely rare, however, further research and investigation are necessary as the pandemic continues. We will need data and robust research on the people who did not develop blood clots after the vaccine too so that we can fully understand the molecular and cellular mechanisms underlying CVST related to Covid-19 infection or after vaccination,” she concluded.

CVST symptoms

CVST clot symptoms can include the following:

  • Severe headache
  • Blurry vision, fainting or loss of consciousness
  • Weakness
  • Sensory changes
  • Confusion or trouble speaking
  • Seizures
  • Abdominal pain
  • Leg pain
  • Difficulty breathing or shortness of breath

The most common symptoms identified in the US cases were severe headaches, vomiting, back pain, fatigue, weakness or the inability to move one side of the body (hemiparesis), an inability to speak or understand speech (aphasia), loss of consciousness, and abdominal pain, Furie said.

If it was associated with the J&J vaccine, cases of TTS/VITT occurred several days up to 2.5 weeks after vaccination, the report said.

AHA recommendation

In the Special Report, the authors stated: “We are confident the benefits of vaccination far exceed the very small, rare risks. The risks of vaccination are also far smaller than the risks of Covid-19 and its potentially fatal consequences.” To date, the disease has killed more than three million people worldwide.

For those who have medical conditions, especially those with the propensity for blood clotting, the AHA advises they consult a healthcare professional before seeking vaccination so that they can discuss the very rare potential increased risks.

More importantly, the Association reiterated the importance of handwashing, physical distancing and wearing face masks as vaccinations continue, particularly for people at high risk of Covid infection and severe disease. “These simple precautions remain crucial to protecting people from the virus that causes Covid-19. They also are crucial to getting us all back to normal activities," they said.

*For more Covid-19 research, science and news, click here. You can also sign up for our Daily Dose newsletter here.

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