Dr. Anthony Fauci, the top U.S. infectious disease official who became the face of America's COVID-19 pandemic response under Presidents Donald Trump and Joe Biden, announced on Monday he is stepping down in December after more than five decades of public service.

Fauci, whose efforts to fight the pandemic were applauded by many public health experts even as he was vilified by Trump and many Republicans, will be leaving his posts as chief medical adviser to Biden and director of the U.S. National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health. Fauci, 81, has headed the institute since 1984.

Fauci has served under seven U.S. presidents beginning with Republican Ronald Reagan in the 1980s, focusing on newly emerging and re-emerging infectious disease threats including HIV/AIDS, Ebola, Zika, monkeypox and COVID-19.

Fauci endured criticism from Trump and various conservatives and even death threats from people who objected to safeguards such as vaccination, social distancing and masking that he advocated to try to limit the lethality of the COVID-19 pandemic.

"As he leaves his position in the U.S. Government, I know the American people and the entire world will continue to benefit from Dr. Fauci's expertise in whatever he does next," Biden said in a statement. "The United States of America is stronger, more resilient and healthier because of him."

In July, Fauci told Reuters he would retire by the end of Biden's first term, which runs to January 2025, adding that his departure could come much earlier than that.

"While I am moving on from my current positions, I am not retiring," Fauci said in a statement on Monday. "After more than 50 years of government service, I plan to pursue the next phase of my career while I still have so much energy and passion for my field."

In the first months of the pandemic in 2020, Fauci helped lead scientific efforts to develop and test COVID-19 vaccines in record time and took part in regular televised White House briefings alongside Trump.

Fauci became a popular and trusted figure among many Americans as the nation faced lockdowns and rising numbers of COVID-19 deaths, even inspiring the sale of cookies and bobblehead dolls featuring his likeness.

However, Fauci drew the ire of Trump and many Republicans for cautioning against reopening the U.S. economy too quickly and risking increased infections and for opposing the use of unproven treatments such as the malaria drug hydroxychloroquine.

Democrats accused Trump of presiding over a disjointed response to the pandemic and of disregarding advice from public health experts including Fauci. Trump in October 2020, weeks before his re-election loss, called Fauci "a disaster" and complained that Americans were tired of hearing about the pandemic. Trump even made fun of Fauci's off-target ceremonial first pitch at a Washington Nationals baseball game.

The U.S. death toll from COVID-19 has exceeded one million https://covid.cdc.gov/covid-data-tracker/#datatracker-home, with more than 93 million recorded cases, according to the U.S. Centers for Disease Control and Prevention.

Asked in a July 2020 interview how he coped with the attacks on his character and allegiance to Trump as president, Fauci told Reuters, "I don't let it bother me. What we're doing with vaccines, what we're doing with therapeutics, what we're doing with clinical trials is the real substance."

Attacks against Fauci by Republican Senator Rand Paul became a regular feature of Senate hearings on the pandemic, with heated exchanges over vaccine boosters and public funding of research. Fauci accused the senator of spreading misinformation.

Paul on his website accused Fauci of "ignoring good advice, and lying about everything from masks to the contagiousness of the virus." At one hearing, the senator accused Fauci of smearing other scientists who disagreed with him.

"He's doing this for political reasons," Fauci told the hearing, citing fundraising appeals on Paul's website next to a call to have Fauci fired.

Fauci will also step down as chief of the NIAID Laboratory of Immunoregulation.