there might be a link between the Oxford-AstraZeneca vaccine and those blood clots after all

AstraZeneca still isn’t out of the woods yet. In fact, the woods seem to be just getting deeper, and now I’m fairly certain it’s not me jinxing things.

Last time we covered AstraZeneca’s blood clot woes, the European Medicines Agency (EMA) had said in a March 18 press conference that “[t]he committee… concluded that the vaccine is not associated with an increase in the overall risk of thromboembolic events or blood clots.” (Thrombosis just means blood clotting.)

But this week, authorities had enough data to posit a possible connection between blood clots known as cerebral venous sinus thrombosis (CVST) and the Oxford-AstraZeneca vaccine. The EMA has now advised, as of April 7, that “that unusual blood clots with low blood platelets should be listed as very rare side effects of Vaxzevria (formerly COVID-19 Vaccine AstraZeneca).” They are still recommending its use given the gravity of the COVID-19 pandemic, but it’s another blow to the vaccine that held much of the world’s hopes in inoculating the entire population. A mechanism by which the vaccine is causing these thromboses has not been discovered.

As of April 4, there had been 222 cases of abnormal thromboses in Britain and the European Economic Area after receiving the Oxford-AstraZeneca vaccine, according to the New York Times. This is out of about 34 million people being vaccinated, and they estimated that the frequency was about 1 in 100,000. In total, the EMA committee looked at “62 cases of cerebral venous sinus thrombosis and 24 cases of splanchnic vein thrombosis reported in the EU drug safety database (EudraVigilance) as of 22 March 2021, 18 of which were fatal.” 

This made us wonder how this compares to the early reports of anaphylaxis (serious allergic reactions) in recipients of the Pfizer-BioNTech and Moderna mRNA vaccines. According to an early report from the CDC, there were 21 cases out of 1,893,360 Pfizer-BioNTech first doses, for a frequency of 11.1 cases per million, or about 1 in 90,000. A later JAMA paper reported updated rates of about 4.7 cases per million doses for Pfizer-BioNTech and 2.5 cases per million for Moderna. To make the scales easier to compare, this works out to about 1 in 213,000 and 1 in 400,000, respectively.

Comparing 1 in 213,000 and 1 in 400,000 cases of anaphylaxis to 1 in 100,000 cases of serious blood clots, it makes sense why some authorities are starting to hit the brakes on the AstraZeneca vaccine. No deaths were reported with the anaphylaxis reactions, but out of the sample of thromboses that the EMA examined, 18 people died. Not only are the numbers worse, but anaphylaxis reactions can also be easier to prepare for. In that early CDC report, 71% of anaphylaxis reactions occurred within 15 minutes of vaccination. For this reason, vaccine clinics monitor you for about 15 minutes (mine sure did). That’s harder to do with blood clots, which take much longer than 15 minutes to manifest and can’t be treated with an EpiPen on the spot. 

Again, this is definitely a blow for what’s still a very effective vaccine. But taking these precautions is how faith in vaccines is earned and kept. We hold vaccines to a high standard for a reason.

More vaccine coverage

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  • How to talk about breakthrough cases
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