The rise of the Delta variant has prompted a thorny question: Do we need a booster dose of the vaccine for Covid-19? Vaccine makers think so, but regulators are yet to be convinced. Principles are also at stake: Should richer countries be talking about administering extra doses when so many people around the world are yet to receive even a single shot? Guest: Rebecca Robbins, a business reporter covering Covid-19 vaccines for The New York Times.