Almost a month after U.S. vaccination campaigns ramped up to give Covid-19 shots to more than a million people a day, their second doses are coming due. That’s putting a strain on state rollouts, and leaving some people without complete immunizations. John Tozzi reports that as President Joe Biden accelerates purchases and distribution, critical weaknesses in the system are starting to show. Learn more about your ad-choices at https://www.iheartpodcastnetwork.com