WASHINGTON — Dr. David A. Kessler, who for the past two years has been the behind-the-scenes force driving a vast federal effort to develop and distribute coronavirus vaccines and treatments, said on Friday that he was leaving the Biden administration to return to teaching at the University of California, San Francisco.

Dr. Kessler may be best remembered in Washington for a far more public role, as commissioner of the Food and Drug Administration in the 1990s, when he took on the tobacco companies. As chief science officer for the Biden administration’s Covid-19 response, he has operated largely unseen by the public, but his work has affected the lives of millions of Americans.

“Six hundred sixty-five million vaccines, 13 million antivirals,” Dr. Kessler said in a brief interview on Friday, referring to the number of doses that Americans have taken since December 2020, shortly before President Biden took office. “We did what we set out to do.”

Still, millions of Americans remain unvaccinated. In an interview over the summer, Dr. Kessler talked about the mass vaccination sites he had helped set up, his efforts to distribute the shots to pharmacies and nursing homes and his work to make them available to children.

“Our job was getting safe and effective vaccines to be accessible,” he said, “and to make sure that everyone could have it, and that it was easy.”

Dr. Kessler’s departure signals the end of Operation Warp Speed, which was started by the Trump administration to develop and distribute coronavirus vaccines. Now the Biden administration is working to shift Covid vaccination from a government-run effort to one that will be handled by the private sector.

In his role, Dr. Kessler was responsible for negotiating with drug companies to make certain that vaccines were available to anyone who wanted one — at a price for taxpayers that is far lower than the one companies want to charge on the commercial market.

When Mr. Biden was running for office, at the height of the coronavirus pandemic, Dr. Kessler was one half of a two-person team — the other half was Dr. Vivek H. Murthy, now the surgeon general — that briefed the candidate almost daily on matters related to the pandemic. When Mr. Biden became president, Dr. Kessler took over Operation Warp Speed and ran it alongside Gen. Gustave F. Perna, who had worked under President Donald J. Trump.

“For decades, Dr. Kessler has worked tirelessly to address our nation’s most challenging public health issues, and his work during the Covid-19 pandemic has been no different,” Xavier Becerra, the secretary of health and human services, said in a statement.