The Senate health committee chairman said new members of a key advisory panel who were appointed by Robert F. Kennedy Jr. “lack experience.”

The chairman of the Senate health committee, in his first significant break with Health Secretary Robert F. Kennedy Jr., called for a delay in this week’s meeting of a panel of vaccine advisers, saying the group Mr. Kennedy appointed lacks the experience and ideological diversity necessary to ensure public faith in its recommendations.

Senator Bill Cassidy, Republican of Louisiana, made his comments in a social media post Monday night, adding to the tensions surrounding the meeting, which is set to take place Wednesday and Thursday. Andrew Nixon, a spokesman for Mr. Kennedy, said the meeting would go on as scheduled.

Mr. Kennedy earlier this month fired all 17 members of the advisory committee on immunization to the Centers for Disease Control and Prevention and replaced them with eight new members, at least four of whom have in some way been critical of vaccination. (On Tuesday, one of the new members withdrew from the panel for reasons that were not immediately clear.)

The immunization committee wields enormous influence, carefully reviewing data about vaccines and making recommendations about who should get them and when. It ordinarily takes months to prepare for its meetings; the newly constituted panel has had only two weeks to do so. Now, an ordinarily routine committee meeting has become a high-stakes affair.

Mr. Cassidy, a physician and a strong proponent of vaccines, had voted reluctantly to confirm Mr. Kennedy after announcing that the secretary had agreed to consult with him on significant matters, and to “maintain” the committee “without changes.” The senator has carefully parsed his words about Mr. Kennedy.

“Although the appointees to ACIP have scientific credentials, many do not have significant experience studying microbiology, epidemiology or immunology,” Mr. Cassidy wrote, using the acronym for the Advisory Committee on Immunization Practices.