President Donald Trump has signed an order that calls for fewer childhood vaccines and also recommends splitting the mumps, measles and rubella (MMR) shots.

“Decades ago, children received only a small fraction of the vaccines required today,” Trump said on Monday. “In those times, people were much healthier and of course the high rates of autism now observed did not exist.”

The president has long cast doubt on the safety of MMR vaccines, but multiple studies have found no connection between the shots and autism.

The order also recommends that the number of childhood vaccines drop to 11 from the existing 18 recommended by the American Academy of Pediatrics (AAP).

Speaking in the Oval Office, Trump said that his administration “is recognising gold standard childhood vaccine recommendations for only 11 core vaccinations against the most serious and dangerous diseases”.

The immunisations recommended for all children under the order are for measles, mumps, rubella, diphtheria, tetanus, pertussis, polio, Haemophilus influenzae type B, pneumococcal disease, human papillomavirus, and varicella or chicken pox.

“We’re reducing them,” Trump said. “It’s not only that you’re doing fewer vaccines, or jabs, as they say, but you’re doing them in a series of visits to the doctor.”

The president suggested that the MMR vaccine could be “quite lethal” if given at once. He compared it to pouring a bottle of soda into a child’s body.

Experts say that allowing time between jabs means children are more likely to contract diseases in the interim or that they are more likely to miss appointments if there are multiple shots.

According to the Centers for Disease Control and Prevention (CDC), there is “no published scientific evidence that shows any benefit in separating the combination MMR vaccine into three individual shots”.

The agency says “most people who get MMR vaccine do not have any serious problems with it”, and that getting the vaccine “is much safer than getting measles, mumps, or rubella”.

Trump’s Secretary of Health and Human Services (HHS) Robert F Kennedy Jr said the proposed changes were meant to give parents choice, not prohibit access to childhood vaccines.

The federal government does not have the authority to implement the new recommendations. Shots that are required for school attendance are set at the state level.

Dr Andrew Racine, president of the American Academy of Pediatrics, called Trump’s executive order “disheartening” and “dangerous”, pointing out it comes as measles cases reach a 35-year high in the US. Measles cases have also been rising globally.

“Instead of ensuring every family can access life-saving vaccines for measles, influenza, RSV and more, federal leaders are once again spreading misleading claims,” Racine said in a statement on Monday.

He said there was no new evidence to justify the significant changes, adding that the order was not based on “gold-standard science”.

“Delaying or skipping shots is risky, especially as measles continues to spread and children go back to school,” Racine said.

Republican Senator Bill Cassidy also admonished Trump’s executive order on Monday, labelling it “wrong”.

“The President does not have the expertise to make these changes,” Cassidy, who is a physician, wrote on social media. “Vaccines are overwhelmingly safe. Vaccines are effective. Vaccines DO NOT cause autism.”

The lawmaker, who chairs the Senate health committee, urged parents to “listen to their child’s pediatrician about vaccines rather than listening to an inaccurate executive order”.

Experts fear that if parents refrain from getting their children vaccinated as a result of the discredited idea that childhood vaccines are linked to autism, it risks the re-emergence of diseases like measles.

Multiple studies have found no link between the MMR vaccine and autism. A recent high-quality study from Denmark in 2019 looked at 657,461 children and concluded that the data did not support the MMR vaccine causing or triggering autism.

On its website, the US CDC recommends that two doses of the combined MMR vaccine be given to children, starting with the first dose at 12-15 months and the second between the ages of four and six.

How often and how many vaccines children in the US receive is based on the times when vaccines will work best with their immune systems and when they are most vulnerable to certain diseases, according to the AAP.

There is no medical reason to delay or skip recommended immunisations, the AAP says.