RFK Jr. announced a halt to all U.S. funding for the global Gavi vaccine alliance. One expert calls it a "travesty and a nightmare."

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.

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