Immunization is one of the "best buys" in global health. However, for the poorest countries, even modest expenditures may be out of reach. The GAVI Alliance is a public-private partnership created to help the poorest countries introduce new vaccines. Since 2008 GAVI has required that countries cover a share of the cost of vaccines introduced with GAVI support. To determine how much countries can contribute to the cost of vaccines--without displacing spending on other essential programs--we analyzed their fiscal capacity to contribute to the purchase of vaccines over the coming decade. For low-income countries, external financing will be required to purchase vaccines supported by GAVI, so co-financing needs to be modest. Relatively better-off "intermediate" countries could support initially modest but gradually increasing co-financing levels. The countries soon to graduate from GAVI can generally afford to follow a rapid path to self-sufficiency. Co-financing for these countries needs to ramp up so that national budgets fully cover the costs of the new generation of vaccines once GAVI support ends.