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Experts rarely associate blood clot events with COVID-19 vaccines. Photo editing by Stephen Kelly; Cultura RM Exclusive/Rolf Ritter/Getty Images
  • An analysis of postvaccination events in Scotland determined that the Oxford-AstraZeneca COVID-19 vaccine is related to a slight increase in the risk of blood clotting and low platelets.
  • The Pfizer-BioNTech COVID-19 vaccine showed no evidence of increased risk.
  • Experts emphasize that the risk value is lower for the COVID-19 vaccine than for SARS-CoV-2 infection and that the benefits of the vaccine outweigh these extremely rare risks.

A recent study analyzed Scotland’s national cohort of 2.53 million adults who received the first dose of a COVID-19 vaccine, to determine the association of blood clotting and low blood platelet counts with the vaccines.

The study was led by Dr. Aziz Sheikh and his team at the University of Edinburgh in the United Kingdom. The results appear in Nature Medicine.

Out of the cohort, 1.71 million people received the Oxford-AstraZeneca vaccine, and 0.82 million received the Pfizer-BioNTech vaccine between December 2020 and April 2021. The study did not include the Moderna COVID-19 vaccine, mRNA-1273, as its first dose in Scotland was given on April 7, and it thus had a limited data set.

The Pfizer-BioNTech vaccine, BNT162b2, was given emergency use authorization by the Food and Drug Administration (FDA) in December 2020. The Oxford-AstraZeneca vaccine, ChAdOx1, received conditional marketing authorization from the European Medicines Agency (EMA) in January 2021.

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However, the Danish Health Authority began to investigate blood clotting incidents and one case of death that occurred in people who had received the Oxford-AstraZeneca vaccine. Additionally, the U.K.’s Medicine and Healthcare products and Regulatory Agency has so far reported 379 cases of low platelet and blood clotting events.

In April 2021, the EMA’s safety assessment committee concluded that the blood clotting incidents may be linked to the Oxford-AstraZeneca vaccine, and had blood clotting listed on the label as a very rare potential side effect.

Idiopathic thrombocytopenic purpura, recently better known as immune thrombocytopenia (ITP), is a bleeding disorder in which antibodies in the immune system attack blood platelets in the body. This results in lower blood platelet counts, making it difficult to form blood clots and stop bleeding.

ITP often occurs due to a viral infection, and its symptoms include easy bruising, bleeding, and bruise-like spots.

Some of the individuals with ITP simultaneously showed an increased risk of thrombosis, or blood clotting. While this may seem counterintuitive and while the reasons behind this are unclear, these cases have been compared with heparin-induced thrombocytopenia (HIT).

HIT occurs with the use of the blood thinner heparin, which causes the body to use up platelets while triggering the formation of blood clots. More research is needed to understand the mechanisms behind these associated complications.

The current analysis showed an association between the Oxford-AstraZeneca vaccine and a slight increase in the risk of ITP within 28 days of vaccination, with an incidence rate of 1.13 cases per 100,000 doses.

The study states that this rare risk is similar to that of other vaccines, such as those for measles, hepatitis B, and influenza. It also notes that diagnosing for ITP is uncertain and that accuracy may depend on the doctor.

There was no evidence of increased ITP risk for the Pfizer-BioNTech vaccine. A possible explanation for the reports of ITP after receiving this vaccine is that the individuals had mild underlying ITP and that the inflammatory response that occurs after vaccination increased the severity of the condition.

Venous thromboembolism refers to blood clotting in the veins. One specific type, cerebral venous sinus thrombosis (CVST), forms blood clots in the brain and can cause blood leakage into brain tissue. CVST may result in severe complications, including death.

The study did not find evidence of increased risk of venous thromboembolism for either of the vaccines.

Despite the potential risks of receiving the vaccine, the study notes it is important to acknowledge its benefits.

Dr. Doug Brown, CEO at the British Society for Immunology, said that “the occurrence of ITP after the first dose of the [Oxford-AstraZeneca] vaccine appears to be extremely rare — around 1 in 100,000 — and, as with other potential side effects of the COVID-19 vaccines, this risk remains far lower than the risks of serious health effects associated with COVID-19.”

For instance, while neither vaccine was linked to increased venous thromboembolism risk, COVID-19 is associated with a high risk of the condition, which is potentially fatal. Further, the American Heart Association (AHA) states that a person with a SARS-CoV-2 infection is 8-10 times more likely to develop CVST than a person who has received a COVID-19 vaccine.

This study cohort consisted mostly of older adults and people with existing conditions, so further research is necessary to understand the risks for a younger population.

Research is also required for second dose postvaccination adverse events, as many people had not yet received their second dose at the time of the study.

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