Two reports published in a leading medical journal on Friday explain how AstraZeneca’s Covid vaccine can cause severe and sometimes fatal blood clots on rare occasions.
Scientific teams from Germany and Norway found that people who developed the clot after vaccination had produced antibodies that activated their platelets, a blood component that is involved in clotting. The new reports extensively complement what the researchers have already said publicly about the blood disorder.
It is not known why the rare reaction occurred. Younger people seem more susceptible than older ones, but researchers say there are no known pre-existing health conditions that predispose people to the problem. Therefore, it cannot be determined whether a person is at high risk.
Reports of the clot have already led a number of countries to restrict AstraZeneca’s vaccine to the elderly or to discontinue use entirely. The cases have dealt a major blow to global efforts to stop the pandemic as the AstraZeneca shot – easy to store and relatively cheap – has been a mainstay of immunization programs in more than 100 countries.
Regulators in Europe have emphasized that the coagulation disorder is rare and that the benefits of the vaccine far outweigh the risks. However, when a side effect can be devastating or fatal – like the blood clots in the brain associated with this vaccine – some regulators and parts of the public consider the risk unacceptable, even if extremely rare.
As of Sunday, the European regulatory authorities had received reports on 222 cases of the rare blood clotting problem in Great Britain and the European Economic Area with 30 countries (European Union plus Iceland, Norway and Liechtenstein). They said that about 34 million people in those countries had received the AstraZeneca vaccine and that the coagulation problems occurred at a rate of about 1 in 100,000 recipients.
European regulators said they had carried out detailed reviews of 86 cases by March 22, of which 18 were fatal.
The safety bar for vaccines is set high because they are given to healthy people. The seemingly greater susceptibility of younger people to the coagulation disorder is particularly worrying, as the risk of serious illness from Covid itself is lower than that of older people. These differences suggest that younger people, compared to older people, may have less to gain and more to lose than the AstraZeneca vaccine.
Germany, the Netherlands, the Philippines, Portugal and Spain have recommended that the AstraZeneca vaccine only be given to people over 60. Canada and France have restricted it to those over 55. Australia, over 50; Belgium, over 56. Britain, where the vaccine was developed, was its strongest defender but announced on Wednesday that it will start offering alternative shots to those under 30.
Cameroon, the Democratic Republic of the Congo, Denmark and Norway have stopped using the vaccine.
Two doses are required for full vaccination with the AstraZeneca vaccine. However, regulators in France have recommended that people under the age of 55 who have received one dose should be given a different vaccine for their second shot.
The AstraZeneca vaccine is not available in the US, despite the fact that the company has announced that it will file for approval with the Food and Drug Administration.
On Wednesday, the European Medicines Agency announced that the vaccine labeling should be revised to include the coagulation disorder as a “very rare” side effect of the vaccine.
In a statement on its website, AstraZeneca said it “is actively working with regulators to implement these changes to the product information and is already working to understand the individual cases, epidemiology and possible mechanisms that could explain these extremely rare events. “
The two new reports were published by the New England Journal of Medicine. One from Germany describes 11 patients, including nine women between the ages of 22 and 49 years. Five to 16 days after the vaccination they were found to have one or more blood clots. Nine had cerebral vein thrombosis, a clot that blocks a vein that drains blood from the brain. Some had blood clots in their lungs, abdomen, or other areas. Six of the eleven died, one of a brain haemorrhage.
One patient had pre-existing conditions that affected clotting, but during a news conference on Friday, Dr. Andreas Greinacher, an author of the report, stated that these conditions most likely played a minor role in the disorder that occurred after vaccination.
All of the patients, as well as 17 other post-vaccine blood clots whose blood was tested, had antibodies known to activate platelets.
The antibodies resulted in a condition called thrombotic thrombocytopenia, which causes both clotting and abnormal bleeding. The researchers suggested calling the newly identified version “vaccine-induced immunothrombotic thrombocytopenia” in these patients.
The article described special blood tests that can be used to diagnose the disorder and suggested treatment with a blood product called intravenous immunoglobulin, which is used to treat various immune disorders. In some cases, medicines called anticoagulants or blood thinners may also be given, but not commonly used heparin – since the vaccine-related condition is very rarely very similar to that in people given heparin.
The second report from Norway described five patients, one male and four female health workers aged 32 to 54, who had blood clots and bleeding seven to ten days after receiving the AstraZeneca vaccine. Four had severe blood clots in the brain and three died. Severe headache was one of her early symptoms. Like the German patients, they all had high levels of antibodies that could activate blood platelets.
The team from Norway also recommended intravenous immunoglobulin treatment. The researchers said the disorder is rare but “a new phenomenon with devastating effects on otherwise healthy young adults,” and they suggested it might be more common than previous studies with the AstraZeneca vaccine had shown.
On Friday, European regulators also said they were reviewing reports of some blood clot cases that have occurred in people who had received the Johnson and Johnson vaccine. In the United States, federal agencies are investigating reports of another type of unusual blood disorder that resulted in a steep drop in blood platelets that occurred in a few people who received either the Pfizer-BioNTech or Moderna vaccines.
Benjamin Mueller and Melissa Eddy contributed to this.