It's been one step forward, two back for AstraZeneca's COVID-19 vaccine. Even as the company rebutted criticism of its efficacy claims last week, a bigger problem loomed for the vaccine and the many millions depending on it. Evidence continues to accumulate that an unusual clotting disorder seen in dozens of European recipients is a real, albeit rare, side effect. A preprint has detailed a proposed mechanism, and multiple scientific groups have said the worry is legitimate and must be seriously weighed against the vaccine's COVID-19 protection.
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