The Gates Foundation said on Tuesday it will commit $1.6 billion over the next five years to support Gavi, a public-private partnership that helps buy vaccines for the world's poorest children.

Kennedy called on Gavi “to re-earn the public trust and to justify the $8 billion America has provided in funding since 2001,” saying officials must “consider the best science available, even when that science contradicts established paradigms.” Kennedy said until that happens, the U.S. won’t contribute further to Gavi.

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.