A rare side effect has dominated the discussion of the AstraZeneca and Johnson & Johnson vaccines. Some people have used the potential for blood clotting as a reason to "hold out" for other vaccines, even as the wait leaves them vulnerable to Covid-19. How realistic are their fears? What are we learning about this side effect? And what could that learning do to help us understand the virus itself?

GUEST: Roxanne Khamsi, independent science journalist

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