The former head of the US Centers for Disease Control and Prevention (CDC) said she was fired last month for refusing to sign off on changes to vaccine policy "regardless of the scientific evidence."

Dr Susan Monarez also told a Senate committee on Wednesday she was sacked for refusing a request from Health Secretary Robert F Kennedy Jr to dismiss CDC vaccine experts "without cause."

"He said if I was unwilling to do both, I should resign," she said.

Kennedy fired Dr Monarez less than a month after she was sworn in as head of the agency that leads the US response to infectious and chronic diseases, adding to a heated political fight over the changes he has made to his department this year.