Which, if any, of these new frontiers might be linked to serious side effects? Which, if any, of the other vaccines could be drawn into this story, too? How can a tiny but disturbing risk be mitigated as we fight our way out of this pandemic? And what might be the implications for vaccine design in the years to come? To answer these questions, scientists will have to figure out the biology behind this rare blood condition: what exactly causes it; when and why it happens. This is not an easy task.