Though the European Medicine Agency continues to maintain that the benefit of AstraZeneca’s Covid-19 vaccine in preventing infection outweigh potential risks, the agency on Wednesday said its review concluded that blood clots should be listed as a “very rare” side effect and AstraZeneca must conduct more studies to better understand the cases reported so far.
Following its own review of the cases, the regulator said most of the reports of blood clots were in women and most were in those younger than 60. The agency, however, stopped short of recommending that certain vaccines be offered to specific demographics as alternatives to the AstraZeneca vaccine.
“It’s very important we use the vaccines available to us,” EMA Executive Director Emer Cooke said during a digital news conference. “We need to beat this pandemic. We need to make sure that people have confidence in these vaccines. These are very rare side effects. The risk of mortality from Covid is much greater than the risk of mortality from these side effects.”
The EMA’s findings about the AstraZeneca vaccine followed a review of the reported cases of blood clots, along with input from a group of outside experts convened by the regulator. The EMA’s safety committee noted the cases involved blood clots in the brain, abdomen, and arteries along with low levels of blood platelets and in some cases, bleeding. The committee reviewed 62 cases of blood clots in the brain and 24 cases of clots in the abdomen, of which 18 were fatal. As of the March 22 cutoff date for the analysis, about 25 million people in the EU had received the AstraZeneca vaccine.
The agency said that most of the cases occurred within two weeks of receiving the first shot of the two-dose vaccine. As for why most of the reported cases were in women, Sabine Straus, chair of the EMA’s safety committee, said that it might be partly due to the fact that more women have been vaccinated. In EU member states, 60% of those vaccinated so far are women. But she added that there may be other factors that the EMA has not yet identified.
The EMA said that one possible cause is an immune response similar to a rare side effect that sometimes occurs in patients treated with heparin, a blood thinner. In these cases, the treatment causes thrombocytopenia, a low blood platelet count. The EMA has asked AstraZeneca to conduct new studies, and to amend ongoing ones, in order to learn more about the potential for the vaccine to cause this side effect.
Peter Arlett, head of pharmacovigilance at EMA, said the agency has directed the company to conduct more laboratory testing to better understand what impact the vaccine has on blood clotting. Next, the company must analyze data from clinical trials to understand the mechanism and risk factors for blood clotting. Ongoing clinical trials should collect data about the side effect. The regulator has also directed the company to conduct epidemiological studies.
In addition to AstraZeneca’s research, the EMA is supporting academic research at Erasmus University Rotterdam and Utrecht University, which will analyze healthcare data to assess the risk factors for blood clots with Covid disease, comparing vaccinated people to those who have not been vaccinated. Arlett said results are expected within the next two months.
In a prepared statement, AstraZeneca said it has been collaborating with regulators to update the vaccine’s product information and it is already working to understand the possible causes of these rare events.
As of April 4, the EMA said that a total of 168 cases of blood clots in the brain and 53 cases of blood clots in the abdomen were reported out of about 34 million vaccinated people in the EU. The agency said that the updated data do not change the safety committee’s recommendations.
The reports of blood clots in the brain are not unique to AstraZeneca’s vaccine. Johnson & Johnson’s vaccine also uses an adenovirus vector. Since that vaccine was authorized, three EU cases have been reported out of 4.5 million vaccinated worldwide, Straus said. For the messenger RNA Covid vaccine from Pfizer and BioNTech, there have been 35 EU cases reported out of 54 million vaccinated worldwide. The Moderna mRNA vaccine has led to reports of five cases out of 4 million vaccinations.
The World Health Organization issued a statement saying that a relationship between the vaccine and blood clots is plausible but not confirmed. It added that the number of reported incidents is low compared to the nearly 200 million people who have been vaccinated worldwide with AstraZeneca’s shot.
“Rare adverse events following immunizations should be assessed against the risk of deaths from Covid-19 disease and the potential of the vaccines to prevent infections and reduce deaths due to diseases,” the WHO said. “In this context, it should be noted that as of today, at least 2.6 million people have died of Covid-19 disease worldwide.”
Based on the EMA’s conclusion that the rare incidence of blood clots is “strongly associated” with the AstraZeneca vaccines, Straus said it’s up to vaccination authorities in EU member states decide whether certain groups of people should be vaccinated with a particular vaccine in lieu of the AstraZeneca shot. Some health authorities are already taking those steps. Last month, Denmark, Norway, and Iceland suspended dosing of the AstraZeneca shot. In the U.K., those under the age of 30 will be offered an alternative vaccine, the BBC reported.
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