Washington DC - The doctor nominated by President Donald Trump to lead the Food and Drug Administration (FDA) gave guarded support to vaccines on Thursday as she faced a contentious Senate confirmation hearing.

Heidi Overton, a current White House adviser, was questioned extensively about health policies put forward since vaccine-skeptic Trump took office last year.

She was notably in the Oval Office last month when Trump signed an executive order urging the combined measles, mumps and rubella (MMR) vaccine be broken up into three shots, falsely alleging the widely used combo could be deadly.

Asked about those comments on Thursday, Overton stated that "the MMR vaccine is currently licensed and approved by the FDA."

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"It is safe and effective, and it is our best tool right now in the public health response to the measles outbreak," she said.

The US is facing its worst measles outbreak in decades – with four deaths this year linked to the vaccine-preventable disease in Pennsylvania – amid a wider backdrop of declining vaccination.

Republican Senator Bill Cassidy, a doctor whose vote will be key for Overton to win Senate confirmation, asked her whether there were "any vaccines on the US market right now which are unsafe?"

"According to the FDA," she said, "every vaccine on the US market right now are safe and effective."

The remark appeared to place some distance between her views and Trump, as well as his Health Secretary Robert F. Kennedy Jr., who for decades has spread conspiracy theories about vaccines.

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Overton, however, refused to say whether the president had been wrong in his remarks about vaccines, nor did she promise to oppose any presidential orders if they ran counter to public health.

Saying she has "had robust discussions with the president," Overton promised to provide Trump with her "best advice."

Senators also pressed the nominee on her views regarding the abortion pill – which she had previously described as "dangerous for women" – and food safety, which the FDA also regulates.

At 37 years old, Overton would become the FDA's youngest administrator if confirmed by the Senate.

Since Trump's return to office, the agency has faced much tumult, with large staff cuts, controversies over policy moves, and the departures of several senior leaders.