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AstraZeneca's COVID-19 vaccine is linked to rare blood clots in rare cases

In another sob for AstraZeneca’s COVID-19 vaccine, the data suggest that it is in fact related to blood clots that have formed in the brains of some vaccinated people, the European Medicines Agency announced on 7 April.

Blood clots are incredibly rare, EMA experts say. But because COVID-19 itself is deadly and can put people in the hospital, the benefits of the vaccine still outweigh the risks, they say. “We need to use the vaccines we have to protect ourselves from the devastating effects,” of COVID-19, Emer Cooke, executive director of EMA, told a news conference on April 7th.

The EMA has already concluded that the vaccine, developed by AstraZeneca and Oxford University, was not linked to blood clots in general (SN: 18/03/21). But experts were unsure of 18 cases of blood clots in the sinuses that drain blood from the brain, a rare disease called cerebral venous thrombosis or CVST.

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Sufficient data has now been accumulated to implicate jab involvement in these rare blood clots, meaning that CVST and similar conditions should be listed as a possible rare side effect of vaccination. the European Union. As of March 22, countries had reported 62 cases of CVST in approximately 25 million people who received the AstraZeneca vaccine. There have also been 24 cases of clots in veins that drain blood from the digestive system, called splanchnic vein thrombosis or SVT. Eighteen of the people with CVST or SVT died.

It is still unknown how the vaccine can cause blood clots. One potential explanation is that some people develop an immune response that attacks platelets, causing them to clump together, Sabine Straus, chair of the EMA committee, told the news conference. This would make the condition look like low platelet levels and blood clots caused by an immune response to the anticoagulant drug heparin. A preliminary study of four people who died from blood clots after vaccination had platelet-binding antibodies in their blood, investigators reported on March 29 in Research Square, a prepress server. The study has not yet been reviewed by other scientists.

If that's the way those clots form, there are ways to treat them, said Beverley Hunt, a hematologist at King's College London, in a call with reporters. A therapy called IVIG, which contains portions of antibodies that interact with platelets, administered with non-heparin anticoagulants can help break down clots.

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Why the vaccine can cause clotting is also unclear. It could be related to the technology used by AstraZeneca in the vaccine or something about the coronavirus protein that uses the shot, said Adam Finn, a pediatrician and vaccine expert at the University of Bristol in England in a call with reporters. "At the moment we don't know."

Some people who have received other vaccines against COVID-19 have also developed rare clots; for example, three people in the European Union and the UK developed after receiving the Johnson & Johnson jab. But these shots were not related to clotting problems, Peter Arlett, head of data analysis and methodology at EMA, told the news conference.

It is difficult to determine how often people develop blood clots after the shootings, as the committee relies on vaccinated people to report their symptoms rather than routine follow-up by experts, Straus said. So far, the reported rate differs between EU countries. In Germany, for example, the reported rate is about 1 case of blood clots per 100,000 people while it is about 1 case per 600,000 people in the UK, Straus said. Typically, there are 1 to 2 cases per 100,000 people.

Although the figures are difficult to analyze, the rate of post-vaccinated blood clots is still lower than that of other common drugs that are at risk for the same medical problem. For example, for women who take oral contraceptives, the blood clot rate is 4 cases per 10,000, Arlett said.

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Although most rare blood clots have occurred in women under the age of 60, it is still unclear what factors could put people at risk for clots forming, Straus said. This is partly because some of the records do not provide all the necessary information about people who have developed clots, such as age or gender. More women than men were shot in the European Union and the UK, which could also distort the results.

Even without clear information on risk factors, many countries have established restrictions on the use of the AstraZeneca vaccine. Some countries, including Denmark and Norway, stopped using the shot. Others, such as Canada and Germany, use it only to vaccinate older adults. On April 6, the UK stopped an ongoing trial with the AstraZeneca vaccine in children aged 6 to 17 years. UK officials now recommend that health experts offer other COVID-19 vaccines to people under 30.

In the United States, AstraZeneca plans to apply for emergency use authorization from the U.S. Food and Drug Administration soon.

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