Robert F. Kennedy Jr., the health secretary and vaccine skeptic who has used his position to amplify discredited health theories and promote unconventional medical treatments, ducked questions during a congressional hearing Wednesday about whether children should be vaccinated for measles, chickenpox or polio. He said people should not rely on him for medical advice.

Robert F. Kennedy Jr. ducked questions during a congressional hearing Wednesday about whether children should be vaccinated for measles, chickenpox or polio.