He did so in a statement of support for Gavi at the public Gavi pledge conference, which was hosted by the United Kingdom, on June 4th. So far, the United States has donated more than $12 billion for the development of COVID-19 vaccines and therapies, and "the U.S. commitment to immunization complements the work of innovators in the United States and other countries who are racing to find a vaccine and treatments for COVID­19." (source)

Bill and Melinda Gates co-founded the Gavi alliance in the year 2000, it's a public-private partnership that claims to support "global health-system strengthening and vaccine deployment for infectious diseases worldwide."  (source)