Gavi has paid for more than 1 billion children to be vaccinated through routine immunization programs, saving an estimated 18 million lives.

Robert F. Kennedy Jr., the health and human services secretary, said the United States would not deliver on a $1.2 billion pledge made by the Biden administration until the organization changed its processes.

Gavi works to expand access to vaccines for children in some of the world’s poorest countries. Health experts say the move could have deadly consequences.