Retired immunologist Anthony Fauci, 85, has declined to answer questions from United States senators about his handling of the COVID-19 pandemic, invoking protections under the Fifth Amendment of the Constitution.

On Wednesday, Fauci appeared before a Republican-led committee on homeland security and government affairs, led by Senator Rand Paul of Kentucky, who has been critical of the physician’s record during the pandemic.

Repeatedly, Fauci offered the following line: “On the advice of counsel, I respectfully decline to answer based on my rights under the Fifth Amendment.”

In his opening statement, he explained that it “pains” him to invoke the Fifth Amendment, but he said he feared Paul might twist his words to seek a criminal prosecution.

“The sole reason he is calling me before this committee is to prompt me to say something, anything, that could vindicate his repeated public pledges that I end up, in his words, ‘behind bars’,” Fauci said.

Fauci led the National Institute of Allergy and Infectious Diseases at the time when the COVID-19 crisis began in late 2019 and early 2020.

The pandemic catapulted Fauci into the public eye, making him a household name. But his medical guidance led him to clash with President Donald Trump during the waning days of his first term.

After Trump’s defeat in the 2020 elections, Fauci became chief medical adviser to Trump’s successor, Democrat Joe Biden, for the first half of his presidency. He stepped down from both the National Institute of Allergy and Infectious Diseases and his White House role in 2022.

Republican lawmakers have long criticised Fauci for his handling of the coronavirus pandemic, and some have accused him of lying about the origins of the pandemic, which he denies.

Though he received a presidential pardon from Biden, some of Fauci’s critics have suggested he could still face charges if he perjures himself under oath.

In June, Paul subpoenaed Fauci to appear before the Senate committee, citing the immunologist’s decision not to voluntarily testify.

In Wednesday’s opening statement, Paul sought to build the case that Fauci helped cause the COVID-19 pandemic by approving funding for research on virus mutations and then covered up his complicity.

“I hope today Dr Fauci will come clean, admit it was a mistake to fund research in China, admit that the risk of this research exceeded any possible benefits,” Paul said.

Other Republicans, including Senator Josh Hawley of Missouri, criticised Fauci for avoiding their questions.

“Well, nothing says honesty like taking the Fifth, huh, doc?” Hawley said.

Using the Fifth Amendment does not imply guilt or innocence. Rather, it is a constitutional mechanism to protect individuals from being compelled to provide potentially self-incriminating testimony, in cases where there is reasonable fear of prosecution.

Experts say that Fauci’s decision to invoke the Fifth Amendment during his testimony allows him to sidestep calls for his prosecution, if he is accused of lying at Wednesday’s hearing.

Still, Fauci’s refusal to answer questions on Wednesday may inflame partisan divides that have festered for years about the origins and handling of the pandemic.

The most common theory about the origins of COVID-19 is one of “zoonotic spillover” — that the virus was transmitted from animals to humans.

But some believe the virus could have come instead from a laboratory setting, a theory that has been embraced by Trump’s White House and Senator Paul.

However, there is no new scientific information supporting that “laboratory leak” theory.

Fauci himself has long maintained that he is open to both theories but that there’s more evidence supporting the idea that COVID-19 jumped from animals to humans, the way other coronavirus strains like SARS and MERS did.

Fauci has long been a target of COVID-related frustrations. In his opening statement, Fauci said Paul has an “obvious obsession with calling for my prosecution”.

Experts in the US medical community have largely rallied around Fauci, calling the accusations against him “baseless”.

In a public letter earlier this month, more than 150 infectious-disease experts and other scientists wrote that “no credible evidence has been produced to support” the “absurd charges” that Fauci covered up the origins of the COVID-19 pandemic.

“These conspiracy theories have led to a five-year period of harassment and threats of violence directed at Dr Fauci and his family,” the letter said. “It is time to stand up for our scientists and say no to these witch-hunts.”

A Republican-led subcommittee that studied the origins of COVID-19 in 2024 did not find any evidence linking Fauci to wrongdoing.