Senator Bernie Sanders, Independent of Vermont, brought up the Children’s Health Defense, which is the organization Mr. Kennedy co-founded that has spread falsehoods about vaccinations for children, pulling up images of onesies sold by the nonprofit that read “Unvaxxed, Unafraid” and “No Vax, No Problem.”

HHS Secretary nominee RFK Jr. and Democratic Sen. Ron Wyden were involved in a heated exchange about Kennedy's past comments during his confirmation hearing on Wednesday.

Over rigorous questioning from senators Wednesday, Robert F. Kennedy Jr. laid out his vision to lead the Department of Health and Human Services, while backtracking on his past statements in support of abortion and against vaccines.

Over many years, Kennedy has been clear about his beliefs on vaccines in dozens of interviews, podcasts and social media posts.

Alexandra Sifferlin, a health and science editor for Times Opinion, hosted an online conversation on Wednesday with the Opinion columnist Zeynep Tufekci and the Opinion writers David Wallace-Wells and Jessica Grose about Robert F. Kennedy Jr.’s first of two confirmation hearings for secretary of health and human services.

Robert F. Kennedy Jr. was accused of contradicting himself on vaccines in a heated exchange during his senate confirmation hearing Wednesday.

Robert F. Kennedy Jr. pushed back on questioning from Sen. Ron Wyden (D., Ore.) about his vaccine views. “I support the measles vaccine. I support the polio vaccine. I will do nothing as HHS secretary that makes it difficult or discourages” for people to get those vaccines,

Robert F. Kennedy's nomination will put Republican lawmakers' loyalty to the test, as the former Democrat holds a range of unorthodox positions that could alienate both conservatives and liberals.

Kennedy Jr. scrapped with senators for more than four hours Wednesday, trying to defend everything from his “conflicting” claims on vaccines to his stance on abortion to past statements that the virus causing COVID-19 was “ethnically targeted” against black and Caucasian people.

Robert F. Kennedy Jr. worked strenuously Wednesday to clarify his views on vaccines after a top Democrat challenged him on past statements and actions that seemed to suggest he was a blanket opponent of shots.

Senators challenged Kennedy, who was nominated as health secretary by President Trump, on his long track record of questioning vaccines.