This article examines the role played by philanthrocapitalist foundations in impact investing for international development, focusing on the Covid-19 Vaccines Global Access Initiative (COVAX) as a response to the current pandemic. Philanthrocapitalists and development institutions are increasingly turning to ‘blended finance’ and ‘social bonds’ to address the gaps in funding required to meet global development agendas, particularly in the arena of global health. These impact investing mechanisms deploy public or philanthropic money to leverage for-profit investment in development, by ‘de-risking’ (providing guarantees for) interventions that might otherwise put private capital at risk. Via COVAX, the Bill and Melinda Gates Foundation has platformed a pandemic response centred on this approach, resisting alternative responses – such as the proposal for a temporary waiver to pharmaceutical patent rights – that seek to challenge the prevailing trade architecture. The global policy response to Covid-19 thus accelerates the ‘financialization’ of development and cements the role of philanthropy in ‘de-risking’ for-profit impact investment.