What, exactly, is a booster shot? For which groups has the FDA authorized Pfizer boosters? What about boosters for Moderna and J&J? Will we need a fourth shot in the future? Will there be a delta-specific booster? Would it make a difference in world vaccine supply if the US refrained from giving any boosters? Stephanie Desmon interviews cohost Dr. Josh Sharfstein about these and more COVID-19 booster questions.