COVID vaccine and blood clots: connected?

Germany, France, and Spain have just joined a growing number of European nations pausing the roll-out of the Oxford-AstraZeneca vaccine, after reports of blood clots in people who had been vaccinated. 

But according to both scientists and medical regulators, there is no evidence of a link between the two.

What do the reports say and how are countries reacting?

The first reports of blood clotting following vaccination originated in Austria. One woman died 10 days after her shot, having been diagnosed with multiple thrombosis (the formation of blood clots within blood vessels); another person is now recovering from pulmonary embolism (a blockage in the arteries in the lungs).

Further reports – both deaths and hospitalisations – have also come from Denmark, Italy, Norway and Ireland in low numbers. According to a review of 17 million vaccinations across the EU and the UK from AstraZeneca, there have been 15 reports of deep vein thrombosis and 22 reports of pulmonary embolism.

Still, this has prompted Germany, France, Spain, Denmark, Norway, Iceland, Ireland, the Netherlands, Cyprus, Slovenia and Bulgaria to temporarily suspend their AstraZeneca vaccine programs as a precautionary measure while they investigate the reports of blood clots. 

Meanwhile, Italy and Austria have suspended certain batches associated with these events as a precautionary measure – different batches for each country. Romania has suspended the same batch as Italy, while Estonia, Latvia, Lithuania and Luxembourg have suspended the same batch as Austria (ABV5300, a batch of one million doses that was sent to 17 European countries).

What are the investigations telling us?

So far, no link between the vaccine and blood clots has been established.

“There is currently no indication that vaccination has caused these conditions, which are not listed as side effects with this vaccine,” the European medicine regulator EMA said in a statement last week. “The information available so far indicates that the number of thromboembolic events in vaccinated people is no higher than that seen in the general population.”

Earlier today, AstraZeneca released a statement saying that their review process found no evidence of increased blood clot risks:

“A careful review of all available safety data of more than 17 million people vaccinated in the European Union and UK with COVID-19 Vaccine, AstraZeneca has shown no evidence of an increased risk of pulmonary embolism, deep vein thrombosis or thrombocytopenia, in any defined age group, gender, batch or in any particular country.”

The review found a total of 37 reports among those vaccinated in the EU and the UK – lower than the background rate of deep vein thrombosis and pulmonary embolism across the population.

“Blood clots are extremely common,” confirms Gerry Fitzgerald, a public health expert from the Queensland University of Technology. “These are more common again in the elderly and those with chronic disease who are the people who are the target of the early vaccination programs.”

Could there be a biological link between the vaccine and blood clots?

Over the past year, research has shown that a protein made by the COVID-19 virus itself has been linked to blood clotting disorders. 

According to Farhid Hemmatzadeh, an Associate Professor in Virology at the School of Animal and Veterinary Sciences at the University of Adelaide, when this nucleoprotein is expressed in high amounts in the body, it can simulate two different pathways in the immune system. These pathways, he says, “are mostly responsible for the stimulation of interleukin 6 and the formation of micro thrombus in COVID-19 patients”.

However, the nucleoprotein doesn’t exist at all in the AstraZeneca vaccine. The vaccine’s viral vector, which simulates COVID-19 to teach the body what it looks like, does not contain the protein, and can’t reproduce in human cells to make more of it anyway. 

Hemmatzadeh concludes that based on the epidemiological evidence from the WHO and EU regulators, “no scientific links exist between the blood clotting disorders and the AstraZeneca vaccine”. 

Fitzgerald adds that “the scientifically conducted clinical trials of the vaccines prior to their approval did not identify any increased risks associated with the vaccine”.

However, both Fitzgerald and Hemmatzadeh note that the scientific community has not had the chance to test the new vaccines over a long time period and in large populations.

Hemmatzadeh says we need “a longer time to see possible links between any disorder and using the vaccine”.

Is pausing the vaccine roll-out an overreaction?

Monitoring potential side effects of a vaccine is a critical part of any vaccine rollout, according to Julie Leask, a social scientist who focuses on infectious disease and immunisation controversies and communication at the University of Sydney.

She explains that countries around the world have set up systems to monitor for any potentially abnormal responses following immunisation.

“Once an adverse event is detected, regulators and vaccine safety experts will investigate whether the event is thought to be caused by the vaccine or not,” Leask says. “This is a normal part of any vaccine program. It demonstrates a system is open, transparent and responsive. Governments want to ensure they are giving people safe vaccines.”

Stephen Evans from the London School of Hygiene & Tropical Medicine says that this is a “super-cautious” approach from European nations based on isolated reports.

“The problem with spontaneous reports of suspected adverse reactions to a vaccine are the enormous difficulty of distinguishing a causal effect from a coincidence,” he explains. “This is especially true when we know that COVID-19 disease is very strongly associated with blood clotting and there have been hundreds if not many thousands of deaths caused by blood clotting as a result of COVID-19 disease.”

Investigating potential links, he says, is a sensible approach to “be sure that the benefit and risk balance is in favour of the vaccine”.

But as the global death toll of COVID-19 passes 2.6 million, most countries maintain that the benefits outweigh the risks and are continuing with the roll-out of the AstraZeneca vaccine.

This includes Australia, which approved the AstraZeneca vaccine last month and is manufacturing doses in Melbourne.