US President Donald Trump has signed an executive order endorsing a plan to halve the number of vaccines currently recommended for children, the Guardian reports.

The order directs health agencies to reassess the current vaccination schedule, reducing “non-core” vaccine recommendations. Medical experts have strongly opposed the move, warning it could leave millions of children unprotected against serious diseases such as measles and polio.

The American Academy of Pediatrics issued a statement saying the current vaccination schedule is backed by decades of scientific research and is one of the most effective tools for protecting children’s health.

Public health experts warn the policy change could negatively impact global vaccination confidence.