President Trump issued an executive order this week calling for immunizations for measles, mumps and rubella, a mainstay of vaccination programs worldwide, to be offered as three individual jabs given at separate times, instead of in one combination shot.

In an Oval Office signing ceremony, the president made a frightening and unsubstantiated claim about the combination shot. “Together, there could be a possibility they’re quite lethal,”

Researchers have not linked the combination M.M.R. shot to a risk of death, and there is no evidence that it is dangerous or that splitting it into its three components is any safer or more effective, vaccine experts said. Combination shots for the diseases have been used in the United States since 1971, and were introduced only after careful comparison against individual shots. They have been given to hundreds of millions of children.

What’s more, separate vaccines for measles, mumps or rubella have not been available in the United States for many years. Vaccine manufacturers said it would take a minimum of five to 10 years for them to be tested and made available.

“We don’t have any way to implement what he’s suggesting,” said Dr. Kathryn Edwards, a retired vaccinologist and professor emerita at Vanderbilt University who studied individual and combination vaccines.

“It’s just simply more confusion for the poor parents,” Dr. Edwards said. “And it’s just showing the world that our health system and our H.H.S. and our president are totally uninformed,” she said, referring to the Department of Health and Human Services.

Mr. Trump’s executive order does not carry legal weight and does not affect the availability of the current combination vaccines or their reimbursement by insurance companies. States set requirements for which shots children should get, and vaccine mandates across Republican and Democratic states have been remarkably similar over decades.

If some states opt to follow Mr. Trump’s executive order and require the shots to be given individually, parents would need to bring their children to three separate appointments. And the children would receive even more needle pricks than they currently do. (Mr. Trump and Mr. Kennedy have repeatedly said American children receive too many shots — although the numbers they give are vastly inaccurate. Children in the United States may receive 35 shots by age 5, including a seasonal flu shot, not 72 as the officials have repeatedly said.)

Combination shots were developed in part to address parents’ worries that their children were becoming “pin cushions” for vaccines, said Dr. Frank DeStefano, who led the vaccine safety office at the Centers for Disease Control and Prevention from 2009 to 2021. No safety concerns emerged in the years of clinical trials that compared the two approaches.

“It’s never been demonstrated that separating the vaccines is any safer than combined vaccines,” he said.

Mr. Trump said his executive order was based primarily on unsubstantiated theories that childhood vaccines can cause autism. Those concerns have been thoroughly researched and debunked since a 1998 study, which was later retracted and exposed as fraud, floated the idea that the measles vaccine caused autism. But Health Secretary Robert F. Kennedy Jr. and others have nevertheless fueled the theory for years, at times pointing to the combination M.M.R. vaccine, the hepatitis B vaccine or the entire childhood vaccination schedule as possible culprits.

“These ideas never die,” Dr. DeStefano said. “They’re zombie ideas. They keep popping up.”

The combined M.M.R. vaccine is associated with a very small risk of seizures if a child develops a fever, but that type of seizure — which can occur after any high fever, including one that may develop after receiving a single shot — does not cause long-term harm, according to the C.D.C.

“Most people who get M.M.R. vaccine do not have any serious problems with it,” the agency’s website says. “Getting M.M.R. vaccine is much safer than getting measles, mumps or rubella.”

Requiring parents to bring their children in for three separate visits to receive that protection may deter some from getting the required shots and decrease immunization rates, or at least delay protection for vulnerable young children. In studies that compared timing of shots, parents who had to bring their children in more often ended up dropping out because of the inconvenience, Dr. Edwards said.

Japan offers a cautionary tale for countries that rescind combination shots. In 1993, the country paused use of an M.M.R. vaccine containing the Urabe mumps virus — which is different from the one used in the United States and Europe — because it caused aseptic meningitis in 0.05 percent of the recipients.

That type of meningitis resolves on its own and does not cause long-term damage. Still, responding to parents’ concerns, Japan switched to recommending individual shots for measles and rubella, and made the stand-alone mumps shot voluntary. Use of the mumps shot quickly “dropped to almost nothing,” said Dr. Margie Andreae, a pediatrician at the University of Michigan who studied the events.

Cases of mumps soared, and caused aseptic meningitis in 1.24 percent of the children, as well as more harmful types of meningitis, brain damage and hearing loss.

And vaccination for measles and rubella also plummeted, leading to surges in those illnesses. “There’s strong evidence that splitting multicomponent vaccines into single components leads to decrease in immunization coverage rates,” Dr. Andreae said.

If immunization rates for measles in the United States drop to a similar level, according to one modeling study, the country may see 11.1 million cases of measles over 25 years. This year, a much smaller drop in the vaccination rate has already led to the highest number of measles cases observed in more than 25 years.

The M.M.R. vaccine relies on weakened live viruses, which change over time, so companies cannot simply resurrect the individual shots, which were phased out in 2009.

“Essentially, these would be considered three investigational vaccines and would require full clinical trial testing and regulatory review for each new product to be licensed,” said Dr. Paula Annunziato, who heads vaccine clinical research at the pharmaceutical company Merck, which manufactures one of the two M.M.R. shots available in the United States.

Alison Hunt, a spokeswoman for the pharmaceutical company GSK, which makes the other one, said, “We stand by the safety and effectiveness of our combination vaccines, which are backed by decades of global use and robust scientific evidence.”

Combination shots support public health by “reducing missed doses, improving immunization coverage and minimizing the burden on families and health care systems,” she said.