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 Trump administration has long pushed a discredited link between MMR vaccines and autism, but multiple studies have found no link between the two.

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.

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."