The race is on for a Covid-19 vaccine, but behind the scenes there’s a secretive group that has the power to stop a vaccine trial in its tracks. That’s the Data and Safety Monitoring Board, or DSMB, and they’re the only ones who get a look at the data during the trials. CNN’s Chief Medical Correspondent Dr. Sanjay Gupta speaks with Susan Ellenberg about her work with Covid-19 DSMBs.

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