A rare clotting disorder may cloud the world’s hopes for AstraZeneca’s COVID-19 vaccine

In the tumultuous rollout of AstraZeneca’s COVID-19 vaccine, all eyes this week were on the United States, where the company had a highly public communication breakdown over the vaccine’s efficacy with an expert panel overseeing a large study in the Americas. But on the other side of the Atlantic Ocean, the vaccine faces new concerns about safety as an explanation gains ground for the unusual strokes and clotting disorders recorded in at least 30 recipients.

Many European countries suspended use of AstraZeneca’s vaccine earlier this month following initial reports of the symptoms, which have led to at least 15 deaths. Most resumed vaccinations after the European Medicines Agency (EMA) recommended doing so on 18 March, saying the benefits of the vaccine outweigh any risks. EMA is continuing to investigate the matter and will convene a wide-ranging committee of experts on 29 March.

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Now, a group of researchers led by German clotting specialist Andreas Greinacher of the University of Greifswald says the highly unusual combination of symptoms—widespread blood clots and a low platelet count, sometimes with bleeding—resembles a rare side effect of the blood thinner heparin called heparin-induced thrombocytopenia (HIT).

The scientists, who first described their findings during a 19 March press conference, recommend a way to test for and treat the disorder and say this can help ease worries about the vaccine. “We know what to do: how to diagnose it, and how to treat it,” says Greinacher, who calls the syndrome vaccine-induced prothrombotic immune thrombocytopenia (VIPIT). Greinacher has posted a manuscript on the preprint server Research Square.

Even if Greinacher’s mechanism isn’t the whole story, multiple researchers told Science they were convinced the vaccine was causing the rare set of symptoms. If that turns out to be true, it could have major consequences for the vaccine, which is one of the cornerstones of the World Health Organization’s push to immunize the world. AstraZeneca is working with partners around the globe to make and distribute billions of doses in low- and middle-income countries, which might have a harder time identifying and treating rare side effects.

Europe is relying heavily on the vaccine as well; the European Union bought 400 million doses. The company’s failure to deliver on time has delayed vaccine rollouts on the continent, and now dented confidence is exacerbating the delays. And even if the risk is very low, it may make sense to use the vaccine only in those who also stand to gain the most from it: elderly people at high risk of dying from COVID-19. Several European countries have started to do this. The situation has scientists walking a tightrope: They want to make the medical profession aware of their concerns without sowing panic.