For years, Robert F. Kennedy Jr., has leveraged his famous name, his celebrity connections and his nonprofit, Children’s Health Defense, to spread misinformation about vaccines and call their safety and efficacy into question. Soon, he might have the power to go much further.

If Mr. Kennedy is confirmed by the Senate to be secretary of health and human services, he would be in charge of the nation’s pre-eminent public health and scientific agencies, including those responsible for regulating vaccines and setting national vaccine policy.

Legal and public health experts agree that he would not have the authority to take some of the most severe actions, such as unilaterally banning vaccines, which Mr. Kennedy has said he has no intention of doing.

“I’m not going to take anyone’s vaccines away from them,” he wrote on social media last month. “I just want to be sure every American knows the safety profile, the risk profile, and the efficacy of each vaccine.”

But Mr. Kennedy, who has said that he wants federal researchers to pull back from studying infectious diseases, could exert his influence in many other ways. His actions could reduce vaccination rates, delay the development of new vaccines and undermine public confidence in a critical public health tool.

In the last three decades alone, childhood vaccines have prevented more than 500 million cases of disease, 32 million hospitalizations and more than one million deaths in the United States, according to a recent report from the Centers for Disease Control and Prevention. But vaccination rates have been falling in recently years, and Mr. Kennedy could accelerate the trend, public health experts said.