Science

AstraZeneca vaccine: What are the latest updates on possible blood clot link?


Despite assurances from the World Health Organisation and European medicines regulator that the Oxford-AstraZeneca vaccine is safe to use, certain questions continue to overshadow the jab.

These centre around an extremely rare type of blood clotting within the brain. The condition, known as cerebral venous sinus thrombosis (CVST), prevents blood from draining from someone’s brain and has been recorded in some recipients of the vaccine.

As these cases are investigated, some countries inside and outside of Europe have once again moved to introduce new measures for the administration of the AstraZeneca vaccine.

What’s the latest?

Some countries have paused rollout of the vaccine or are restricting it to certain groups.

Germany has limited use of the jab to people aged 60 and above, as well as high-priority groups. This comes after the country’s medicines regulator reported 31 cases of CVST among the nearly 2.7 million people who have so far received the jab in Germany.

Nine of the 31 people suffering clots have died, and all but two of the cases involved women who were aged 20 to 63, Germany’s Paul Ehrlich Institute said. The two men were aged 36 and 57.

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This policy comes after Germany, along with a number of European countries, originally said the vaccine was not appropriate to use in people aged 65 and over, citing a lack of data on the jab’s efficacy among the elderly.

What have other countries done?

France approved resumed use of the vaccine on 19 March, but said it should only be given to people aged 55 and over.

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Spain said on 30 March it would use the vaccine for people aged 55-65, and a day later said it would extend the vaccination to essential workers aged over 65.

Canada has suspended use of the vaccine in people under 55, citing new concerning data from Europe.

“There is substantial uncertainty about the benefit of providing AstraZeneca Covid-19 vaccines to adults under 55 given the potential risks,” said Dr Shelley Deeks, vice chair of Canada’s National Advisory Committee on Immunisation.

Dr Deeks said the updated recommendations come amid new data from Europe that suggests the risk of blood clots is now potentially as high as one in 100,000, much higher than the one-in-a-million risk cited before.

In Norway, health officials said last week that the AstraZeneca vaccine was responsible for the hospitalisation of three health workers who had developed serious blood clots and low levels of blood platelets. Use of the jab in the country remains suspended as investigations continue.

“Our theory that this is a powerful immune response which most likely was caused by the vaccine has been found,” said Professor and chief physician Pal Andre Holme from Oslo University Hospital. “In collaboration with experts in the field … we have found specific antibodies against blood platelets that can cause these reactions.”

But several European countries are using the AstraZeneca vaccine without any age-related restrictions following the ruling made by the European Medicines Agency (EMA) that it is safe. These include Italy, Greece, Belgium, the Netherlands, Sweden and Portugal.

What does the science say?

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While CVST has so far been reported at a rate of less than one case per one million vaccinated people, the risk of dying from Covid-19 for those aged 40 to 49 is one in 1,000, the UK’s medicines regulator said earlier this month.

The Medicines and Healthcare products Regulatory Agency (MHRA) added that the benefits of the vaccine in preventing Covid-19 far outweigh any associated risks.

Along with the MHRA, the EMA has said there is no evidence to suggest the vaccine has caused these rare blood clots, though it is continuing its investigations into the condition.

The WHO has also urged countries to continue using the jab, as has the International Society on Thrombosis and Haemostasis.

Munir Pirmohamed, chair of the Commission on Human Medicines, suggested that even if a link between the clots and the vaccine was established, the benefits of the jab would continue to outweigh the risk.

“If we feel that there’s causal link then we may need to update the product information, but overall, I don’t think that would necessitate pause to any kind of vaccination programme,” he said.

However a senior official at the EMA said he believed there was a link between the AstraZeneca vaccine and extremely rare cases of blood clots reported in people who recently had the jab. 

In an interview with Italy’s Il Messaggero newspaper, Marco Cavaleri, head of vaccines strategy at the EMA, said it was “clear there is a link with the vaccine” but there was still uncertainty about what exactly was causing such a reaction.

What has AstraZeneca said?

AstraZeneca has previously said its own review found no evidence of an increased risk of pulmonary embolism, deep vein thrombosis or thrombocytopenia, in any defined age group, gender, or in any particular country.

In clinical trials for the jab, the number of clotting incidents was small and “lower in the vaccinated group” than in those who were unvaccinated, it added.

The firm said international regulators had found the benefits of its jab outweigh any risks.

However, it said it will continue to analyse its database to understand “whether these very rare cases of blood clots associated with thrombocytopenia (low blood platelet count) occur any more commonly than would be expected naturally in a population of millions of people”.



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