This thesis investigates the factors driving the U.S. decision to prioritize particular countries over others in its COVID-19 vaccine donations. The United States has claimed that it provides vaccines to countries in need. However, descriptive evidence suggests that factors other than need influence U.S. donations, as apparent in its contributions to high-income nations. The literature presents three theories to explain why the United States gives aid: response to countries' perceived needs, commitment to allies, and quid pro quo for other goals. To examine the extent to which each of these accounts for U.S. vaccine diplomacy, this thesis regresses the cumulative U.S. deliveries of vaccine doses abroad on measures of recipients' needs and their geopolitical importance to the United States. The regression results suggest that: a) the United States donated more vaccines per capita to nations with lower GDP per capita; b) countries in the Middle East and NATO members received fewer doses per capita; c) nations with significant U.S. military presence, major non-NATO allies, and those physically closer to the United States received more doses per capita. These results are robust to other confounders and alternative formulations of the dependent variable. In addition to the usual robustness checks, this thesis conducts sensitivity analyses to test its findings. The results imply that U.S. COVID-19 vaccine diplomacy manifests a dual mandate of charity and self-interest by concurrently donating to countries in need and to those that contemporaneous U.S. foreign policy values more.