Mr. Kennedy made his name in the anti-vaccine movement by contending that there was a link between thimerosal, a preservative, and the “epidemic of childhood neurological disorders” in Rolling Stone and other publications. Indeed, he wrote that the government “colluded with Big Pharma to hide the risks of thimerosal from the public.”

Health and Human Services officials said the postponement was to accommodate public comment ahead of the meeting but experts decried the delay.

To earn the vote he needed to become the nation’s top health official, Robert F. Kennedy Jr. made a promise: He would not change the nation’s current vaccination schedule. But on Tuesday, he vowed