It’s an idea as popular as it is incorrect: American babies now receive too many vaccines, which overwhelm their immune systems and lead to conditions like autism.

This theory has been repeated so often that it has permeated the mainstream, echoed by President-elect Donald J. Trump and his pick to be the nation’s top health official, Robert F. Kennedy Jr.

“When you feed a baby, Bobby, a vaccination that is, like, 38 different vaccines and it looks like it’s been for a horse, not a, you know, 10-pound or 20-pound baby,” Mr. Trump told Mr. Kennedy on a call in July. “And then you see the baby all of a sudden starting to change radically — I’ve seen it too many times.”

On Sunday, Mr. Trump returned to the theme, saying Mr. Kennedy would investigate whether childhood vaccines caused autism, even though dozens of rigorous studies have already explored and dismissed that theory.

“I think somebody has to find out,” Mr. Trump said on NBC’s “Meet the Press.”

But the idea that today’s vaccines are overtaxing children’s immune systems is fundamentally flawed, experts said. Vaccines today are cleaner and more efficient, and they contain far fewer stimulants to the immune system — by orders of magnitude — than they did decades ago.

What’s more, the immune reactions produced by vaccines are “minuscule” compared with those that children experience on a daily basis, said Dr. Yvonne Maldonado, a pediatrician at Stanford University who advises the Centers for Disease Control and Prevention on vaccines.