Sen. Bill Cassidy (R-LA), a key Republican at the helm of the top health committee in the Senate, pressed Robert F. Kennedy Jr. on his stance on vaccines during his second hearing to be confirmed as the head of the Department of Health and Human Services.

Cassidy, the chairman of the Senate Health, Education, Labor, and Pensions Committee, is one of a handful of Republicans who could jeopardize Kennedy’s confirmation. Cassidy has declined to indicate his vote intention.