Washington DC - President Donald Trump signed an executive order Monday pushing for a rethink of school vaccine mandates and repeating calls for the measles-mumps-rubella vaccine to be separated into three individual shots. But New York City's health commissioner says the move is rooted "in politics and conspiracy theories."

Curbing vaccines has been a key priority for Health Secretary Robert F. Kennedy Jr., who founded an anti-vaccine nonprofit and has long promoted a disproven link between immunization and autism.

The federal government does not have direct control over several areas covered by the order. States, for example, have the authority to require vaccinations for students to attend school.

On these points, Trump's move is more of a suggestion than an order.

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"States and territories are advised to review" the Trump administration's recommendations on "immunization requirements for contexts such as school enrollment and attendance," the order states.

In another section, it says the Health and Human Services department will form a task force to offer the MMR shot as individual vaccines "by working with the private sector and other countries as appropriate, while guaranteeing continued availability of combination vaccines and those vaccines recommended for shared clinical decision-making."

Years of scientific evidence support the safety of a combined MMR vaccine.

Public health experts have long advised that the combined shot offers children more protection faster, and also decreases the logistical hurdles for parents who may struggle to make multiple appointments.

Yet during the Oval Office signing, Trump made a series of incendiary and unsupported statements, including: "Together, there could be a possibility they are quite lethal."

In a call with reporters, Andrew Racine, president of the American Academy of Pediatrics, estimated it would take more than a decade to develop approved separate vaccines in the US.

"We're advising that you continue to give children a vaccine that has decades of safety data," added Aaron M. Milstone,  member of the AAP Committee on Infectious Diseases.

The Trump administration's push to overhaul vaccine policy comes as its efforts to implement major changes were blocked by a federal court earlier this year, pending further litigation.

The judge's stay preventing the overhaul sided with major medical organizations, which asserted that the Trump administration's actions were unlawful.

The president's latest order also arrives just as children across the country are readying to return to school, and as the US suffers its worst measles outbreak in 35 years.

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Trump's health secretary, Kennedy, is accused of contributing significantly to the crisis by spreading anti-vaccine sentiment and fueling fears of well-studied and proven immunizations.

Polling indicates that most Americans still support vaccination, and in recent months both Kennedy and Trump had backed off anti-vaccination rhetoric, which could spell negative repercussions in November's midterm vote.

But Monday's efforts show the issue remains a core component of their health policy agenda, as both men remain committed to the false notion that vaccines can cause autism.

Such a potential link has been debunked by years of research.

"The Trump administration's proposed changes to the childhood vaccine schedule illustrate the risks of allowing government recommendations to occupy a privileged position in medical decision-making," said Jeffrey Singer of the Cato Institute in a statement.

New York City's health commissioner, Alister Martin, said Trump's order was based "in politics and conspiracy theories."

"Fortunately, vaccination recommendations are not set by the president, nor should they be," Martin said in a statement.

"Cities and states can set their own recommendations and requirements, and many, including New York City, have moved away from the federal government's recommendations as they've become politicized and unscientific."