A decade before he became President Trump’s health secretary, the environmental lawyer Robert F. Kennedy Jr. appeared on a talk show hosted by Dr. Mehmet Oz to promote his latest book, “Thimerosal: Let the Science Speak.”

The book, published in 2014, explored an obscure mercury-based preservative, thimerosal, that was removed from most vaccines, but not flu vaccines, more than two decades ago. Dr. Oz noted that the Centers for Disease Control and Prevention had deemed the vaccines that still contained thimerosal “safe and effective” and said they did “not present a public health risk.”

Mr. Kennedy did not buy it. “We found 500 peer-reviewed studies,” he insisted. “Virtually every one of them said that thimerosal is a potent neurotoxin that should not be in vaccines.”

On Thursday, the new members of the C.D.C.’s Advisory Committee on Immunization Practices, hand-selected by Mr. Kennedy after he fired all 17 members of the previous panel, decided it would longer recommend annual flu shots that contain it. Thimerosal’s appearance on the committee’s agenda in the first place shocked public health leaders, who have long considered the matter settled.

But it was not a surprise to people who have followed Mr. Kennedy closely. Thimerosal started Mr. Kennedy down a path of questioning vaccine safety, and Thursday’s vote was the culmination of a long personal journey. It offers a window into how, as secretary, he is pursuing his own passions and installing old allies in positions of influence.