Scientists have said it is unlikely that the AstraZeneca vaccine causes blood clots after a number of EU countries temporarily stopped using it because of safety concerns.
On March 11, Denmark, Iceland, and Norway all said they were suspending use of the vaccine amid reports a small number of people had developed blood clots after receiving it.
The European Medicines Agency said later that day it was investigating the cases—referred to as thromboembolic events—but added that there was no indication the vaccination was the cause and that countries could continue to use the jab while the investigation was ongoing.
The EMA said as of March 10 there had been around 30 cases of thromboembolic events reported out of 5 million people who had received the AstraZeneca vaccine in the European Economic Area.
Professor Jon Gibbins, director of the Institute for Cardiovascular and Metabolic Research at the University of Reading, said in a statement that venous thrombosis is "relatively common affecting 1 to 2 in 1,000" and that clotting can occur for a variety of reasons."
He said: "I think that it is completely understandable that people may be worried about this but there are several factors that mean we need to be cautious about the link between the vaccine and clotting.
"Importantly, there is good emerging evidence that severe COVID-19 infection itself can cause harmful blood clots through the body's own immune system, and so it may be the case that people are more at risk of clotting from not being immunized.
"We will need to see the detailed data in order to assess this properly, but at this point we also need to be careful not to cause unwarranted panic or resistance to vaccination."
Some vaccines are more subject to skepticism than others, as the below graph from Statista shows.
Professor Anthony Harnden, deputy chair of the Joint Committee on Vaccination and Immunisation in the U.K., said: "Vaccine safety is critically important. Our U.K. regulator, [the Medicines and Healthcare products Regulatory Agency], reviews all reports of adverse events for both vaccines as they are reported.
"There have been more than 11 million doses of the Oxford-AZ vaccine administered in the U.K. and no excess reports of blood clots amongst those receiving the vaccine, compared with the expected rate in the population."
Health organizations keep track of vaccine safety using reporting systems that allow people to declare any side effects they or their patients have experienced. Examples include the Vaccine Adverse Event Reporting System in the U.S. and the Yellow Card scheme in the U.K.
Both systems are designed to identify trends in side-effects that might signal an issue with a vaccine or a particular batch, but they do not necessarily identify whether a side-effect is directly linked to a particular vaccine.
This is because the nature of the reporting means some events may have happened anyway, regardless of vaccination. The U.K. Medicines and Healthcare products Regulatory Agency said this was particularly the case when most vaccines have been given to those who are elderly or who have underlying conditions.
Peter English, a retired consultant in communicable disease control and former editor of Vaccines in Practice magazine, said in a statement on the AstraZeneca suspension: "It is by no means unusual for the introduction of a new vaccine to be interrupted by reports of adverse events such as this. This is a sign that adverse reaction monitoring systems are working—but not usually a sign that the adverse reactions are caused by vaccination.
"When vaccines are given to millions of people, it is only to be accepted that bad things will happen to some of them after they are vaccinated. What is important is to be able to assess whether such 'adverse events' are causally associated with the vaccine.
"The jury is still out on whether the events reported in Denmark—and, indeed, the death reported in Vienna—are causally linked to the vaccine. My guess would be that they are more likely to be chance associations, rather than causally linked."
Austria also suspended the use of a particular AstraZeneca batch after a person was diagnosed with multiple thrombosis and died 10 days after vaccination. The EMA said on March 10 there was no indication vaccination had caused the condition and said batch quality was being investigated.
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Update on March 12: This article was updated to include a graph from Statista.
