President Joe Biden on Tuesday set a new vaccination goal to deliver at least one shot to 70% of adult Americans by July Fourth as he tackles the vexing problem of winning over the “doubters” and those unmotivated to get inoculated. Demand for vaccines has dropped off markedly nationwide, with some states leaving more than half their available doses unordered. Aiming to make it easier to get shots, Biden called for states to make vaccines available on a walk-in basis and he will direct many pharmacies to do likewise.