Brady Collins
Since the rollout of COVID-19 vaccines, there has been concern about the adverse effects of the injection, specifically blood clots. While blood clots are a rare occurrence, they have been observed in patients who have received the Oxford-AstraZeneca, Johnson & Johnson, and Janssen vaccines. A study of patients in the United States suggests that 39 people per million who are infected with coronavirus go on to have a CVST within two weeks of a Covid diagnosis, indicating that the infection puts people at a higher risk of developing blood clots than those who have been vaccinated.
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What You'll Learn
Risk of blood clots after COVID-19 vaccination
There is an increased risk of blood clots after the first and second doses of COVID-19 vaccines. However, the risk is deemed to be very rare. The IRR (Incidence Rate Ratio) for blood clots after the first dose is 1.13, and after the second dose, it is 1.23.
Several studies have been conducted to evaluate the risk of blood clots after COVID-19 vaccination. One study found that out of over 18 million people who received the single-dose J&J vaccine, 60 cases of blood clots were reported, with nine resulting in death. This led to the vaccine being pulled from the market. The AstraZeneca vaccine, which has also been linked to blood clots, was withdrawn or restricted in several countries, including Denmark and Norway. Pfizer-BioNTech and Moderna COVID-19 vaccines, which are mRNA vaccines, have not been linked to blood clots.
The risk of blood clots appears to be higher in people after the first dose of the AstraZeneca vaccine. Additionally, young women who use hormonal contraceptives may be more likely to experience adverse reactions to the vaccines. The oestrogen in the contraceptive pill may also cause blood to clot more readily.
The British Heart Foundation (BHF) Data Science Centre in the UK is conducting a rapid nationwide study to understand the vascular complications around blood clots after coronavirus vaccination. The study will access the healthcare records of all people in England to identify those with blood clotting conditions and compare the risk of developing these conditions among vaccinated and unvaccinated individuals.
Anyone experiencing signs of a blood clot or any adverse reaction to a vaccine should seek immediate medical attention.
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Risk of blood clots after COVID-19 infection
COVID-19 appears to increase the risk of blood clots, especially if you are hospitalised or have chronic health issues. Research has shown that the risk of blood clots can remain elevated for nearly a year after infection. The risk is highest within the first week of diagnosis, when the risk of an arterial blood clot is nearly 22 times higher than in someone without COVID-19. This risk drops to less than four times higher in the second week, and between 27 and 49 weeks, there is an approximately 30% increased risk for arterial clots. The risk of venous clots also increases after a COVID-19 diagnosis. In the first week, the risk of such clots was 33 times higher, and this risk remained at about eight times higher in the third and fourth weeks. Between 27 and 49 weeks, the risk was still 1.8 times higher than in someone who had never had COVID-19.
The risk of blood clots after COVID-19 infection has been evaluated using self-controlled case series (SCCS) studies, which reported IRRs between 6.18 and 63.52. However, this method is subject to some risk of bias, which would inflate the estimated relative risk of blood clots after COVID-19 infection.
The increased risk of blood clots after COVID-19 infection has also been studied using a case-control method, which compares the risk of blood clots in infected subjects to control subjects who were hospitalised due to physical injury. This method found that the risk of blood clots was higher in unvaccinated subjects (odds ratio of 2.16) compared to vaccinated subjects (odds ratio of 1.46).
While the risk of blood clots after COVID-19 infection is a concern, it is important to understand the risk and talk to a healthcare provider if you have chronic conditions that may increase the likelihood of clotting. Treatments can be used to minimise the risk.
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Vaccine-induced thrombotic thrombocytopenia (VITT)
VITT is similar to heparin-induced thrombocytopenia (HIT), but it occurs without prior exposure to heparin. Early mechanistic evaluations have identified antibodies directed against the platelet factor 4 (PF4)-heparin complex, which activate platelets, similar to HIT antibodies. These antibodies cause platelets to clump together and form clots, activating other parts of the body's clotting system.
The risk of developing VITT is very low, and it is important to note that the benefits of vaccination against COVID-19 far outweigh any potential risks. Specific risk factors for VITT have yet to be determined due to the extremely low case count. However, symptoms typically appear between 5 and 28 days after vaccination.
To diagnose VITT, it is necessary to verify COVID-19 vaccine details, including which vaccine was administered and when. Imaging and laboratory tests are then used to look for signs of thrombosis and thrombocytopenia. If these tests are normal, VITT is highly unlikely. However, if both imaging and laboratory tests show abnormalities, hospitalisation and further evaluation by a haematologist or thrombosis expert is recommended.
It is important to note that VITT is distinct from post-vaccine ITP, which is characterised by isolated thrombocytopenia without thrombosis.
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AstraZeneca and Johnson & Johnson vaccines
The AstraZeneca and Johnson & Johnson vaccines are vector vaccines that instruct human cells to make the SARS-CoV-2 spike protein. In April 2021, the Johnson & Johnson vaccine was paused in the US, South Africa, and the European Union after reports of rare blood clotting. Six cases were detected in more than 6.8 million doses of the vaccine, with one fatality. All six cases were in women aged between 18 and 48, and symptoms appeared 6 to 13 days after vaccination.
The AstraZeneca vaccine has also been linked to a number of rare blood clots, with a small number of clot-related deaths leading to its withdrawal or restriction in several countries in 2021. The European Commission withdrew its marketing authorization for the vaccine in March 2024. The complication occurred in about 2-3 people per 100,000 vaccinated with the Astra shot under the age of 60 in Australia.
Scientists are exploring the possible link between the two vaccines and blood clotting issues. One theory is that viral vector vaccines trigger an abnormal immune response, leading to blood clots. Researchers have identified specific antibodies in people who developed clots after receiving either vaccine. These antibodies attach to platelets, the blood cells that form clots.
The clotting disorder is called thrombosis with thrombocytopenia syndrome (TTS), and it is rare. Out of more than 18 million people who received the Johnson & Johnson vaccine, 60 cases of TTS were reported, with nine fatalities. The risk is greatest in women aged 30 to 49, at 1 in 100,000.
While the link between the vaccines and blood clots is not yet fully understood, healthcare professionals are advised to be alert to the signs and symptoms of thromboembolism and thrombocytopenia to ensure prompt treatment.
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Benefits of the vaccine outweigh the risks
There is a growing concern over the rare occurrence of blood clots following COVID-19 vaccination. The Johnson & Johnson, AstraZeneca, and Sputnik V vaccines have been associated with a small number of serious blood clotting cases, with symptoms typically presenting within four days to a few weeks of receiving the vaccine.
However, experts emphasize that the benefits of the vaccine vastly outweigh the risks. Firstly, the occurrence of vaccine-related blood clots is extremely rare. For instance, the Oxford-AstraZeneca vaccine, which has been linked to rare clots, is not yet proven to be the direct cause. Furthermore, the US Food and Drug Administration (FDA) reviewed the Johnson & Johnson vaccine, which had been administered over 6.8 million times, and concluded that it was safe and effective, with a very low overall chance of blood clots occurring.
Additionally, the risk of blood clots associated with COVID-19 infection is significantly higher than the risk associated with vaccination. Studies suggest that contracting COVID-19 increases the likelihood of developing blood clots, and the risk increases during periods of higher infection rates. Thus, the indirect benefit of preventing infection-associated blood clots by getting vaccinated outweighs the direct harm of vaccination.
Moreover, COVID-19 vaccination may offer additional protection against blood clots. As the vaccination rate increases, the overall immunity of the population improves, reducing the spread of the virus and conferring collective protection against infection-related health risks, including blood clots.
While investigations into the link between COVID-19 vaccines and blood clots are ongoing, the available evidence suggests that the benefits of vaccination in preventing severe disease and reducing the spread of the virus outweigh the rare and minimal risk of vaccine-induced blood clots.
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Frequently asked questions
Blood clots have been identified as an extremely rare side effect of certain COVID-19 vaccines, including the AstraZeneca and Johnson & Johnson vaccines. However, the overall risk of developing blood clots from the vaccines is very low, and the benefits of the vaccine vastly outweigh the risks.
In rare cases, the body produces antibodies as a side effect of the vaccine, which leads to uncontrolled activation of platelets, resulting in low platelet counts and blood clots. This rare antibody occurrence is a random event without a known association with a person's medical history or any particular conditions.
Symptoms of vaccine-induced blood clots may include severe headaches that are not relieved by painkillers, worsen when lying down or bending over, and are unusual for the person experiencing them. These headaches may also be accompanied by blurred vision, nausea, drowsiness, and problems speaking. Anyone experiencing these symptoms or other signs of a blood clot after receiving the vaccine should seek immediate medical attention.
The occurrence of vaccine-induced thrombotic thrombocytopenia (VITT) is rare, affecting 2 to 20 people per million. The risk of developing blood clots is higher in individuals infected with COVID-19, with close to 20% of ICU patients developing blood clots, compared to an estimated range of 3 to 10% for patients admitted to the ICU for other reasons.
