We review economic arguments for using public policy to accelerate vaccine supply during a pandemic. Rapidly vaccinating a large share of the global population helps avoid economic, mortality, and social losses, which in the case of Covid-19 mounted into trillions of dollars. However, pharmaceutical firms are unlikely to have private incentives to invest in vaccine capacity at the socially optimal scale and speed. The socially optimal level of public spending may cause some sticker shock but — as epitomized by the tagline ‘spending billions to save trillions’ — is eclipsed by the benefits and can be restrained with the help of careful policy design and advance preparations. Capacity is so valuable during a pandemic that fractional dosing and other measures to stretch available capacity should be explored.