EMA says 'possible link' between AstraZeneca's COVID-19 vaccine, rare unusual blood clots

The European Medicines Agency said Wednesday that unusual blood clots with low blood platelets should be listed as "very rare" side effects associated with AstraZeneca's Vaxzevria, although it still believes the overall benefits of the COVID-19 vaccine outweigh any potential risks. The decision stems from a review conducted by the EMA's Pharmacovigilance Risk Assessment Committee (PRAC), which found a "possible link" between the vaccine and these particular thromboembolic events, although the agency's vaccines head Marco Cavaleri was quoted in an Italian newspaper earlier this week as saying there was a "clear" connection between the two.

Multiple countries either halted or limited Vaxzevria's use over the last month amid the blood clotting concerns, with most resuming rollout after the EMA issued an initial assessment on March 18 concluding its benefits outweighed the risks. However, some countries including Germany and France still have age restrictions in place.

Akin to immune response sometimes triggered by heparin

The latest assessment looked at 62 cases of cerebral venous sinus thrombosis (CVST) and 24 cases of splanchnic vein thrombosis reported in the EU's drug safety database as of March 22, with 18 of these proving fatal. The cases mainly came from Europe and the UK, where around 25 million people have received the vaccine, also known as AZD1222.

According to PRAC, most of the cases reported so far have occurred in women under 60 years of age within two weeks of receiving the first dose, noting that there is "limited experienced" up to now with second doses. However, the group indicated that it could not yet outline specific risk factors based on currently available evidence, but suggested that a possible explanation for the combination of blood clots and low platelet counts is an immune response akin to heparin-induced thrombocytopenia. PRAC said it has "requested new studies and amendments to ongoing ones" to provide more information.

UK to offer alternative vaccine for younger people

Meanwhile, the Medicines and Healthcare products Regulatory Agency (MHRA) said Wednesday that people in the UK under 30 will be offered an alternative COVID-19 vaccine to AstraZeneca's product. Lim Wei Shen, COVID-19 immunisation chair for the UK's Joint Committee on Vaccination and Immunisation (JCVI), said the move was being made "out of the utmost caution" rather than any serious concerns. 

As of March 31, the MHRA said there were 79 people in the UK who experienced blood clots with thrombocytopenia after receiving AstraZeneca's vaccine, 19 of whom died, including three under age 30. These included 44 cases of CVST and 35 of thrombosis involving other major veins. Nearly two-thirds of the cases of blood clots were seen in women, while the people who died ranged from 18 to 79 years of age. To date, JCVI says there have been no reports of rare thrombosis/thrombocytopenia events following the second dose of AstraZeneca's vaccine.

'Course correction'

Based on the 20.2 million doses of Vaxzevria administered in the UK as of the end of March, the MHRA estimates the overall risk of these blood clots is approximately four people in a million who receive the vaccine. However, the data suggest there is a "slightly higher" incidence reported in the younger adult age groups, and as such it is recommending that this "evolving evidence should be taken into account" when considering whether to use it. By contrast, the MHRA said it has received two reports of thromboembolism with low platelets for Pfizer and BioNTech's vaccine Comirnaty as of March 31, at which point about 11 million first doses and 3.5 million second doses had been given.

England deputy chief medical officer Jonathan Van-Tam described the move to offer a second-dose alternative for under-30s as a "course correction," noting it is not unusual in medicine to change preferences in this way. He also said the impact on the UK government's promise to offer all adults a COVID-19 vaccine by the end of July should be "zero or negligible" as long as expected supplies of the Pfizer/BioNTech and Moderna vaccines arrived as expected in the coming months.

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