Mr. Kennedy’s rise as the face of the vaccine resistance movement has tested as never before the solidarity of a family that has for decades remained resolute in the face of tragedy and scandal. It has rattled the Hollywood and entertainment circles that he inhabits, while showing how the vaccine debate is upending traditional political alliances.

And it has left the Kennedys and his friends anguished and mystified about the dramatic turn in the often troubled life of a man who was a pallbearer at his father’s funeral when he was 14, who emerged from drug addiction to become one of the leading environmentalists in the country and who is regarded as among the most politically gifted Kennedys of his generation.

Mr. Kennedy has effectively used his talent and one of the most prominent names in American political history as a platform for fueling resistance to vaccines that could save countless lives.

His conduct “undercuts 50 years of public health vaccine practice, and he’s done it in a way I’ve never see anyone else do it,” said Michael T. Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “He is among the most dangerous because of the credibility of who he is and what his family name has brought to this issue.”