WASHINGTON >> The acting director of the National Institutes of Health pushed back Wednesday against Republicans’ assertions that a lab leak stemming from taxpayer-funded research may have caused the coronavirus pandemic, telling lawmakers that viruses being studied at a laboratory in Wuhan, China, bore no resemblance to the one that set off the worst public health crisis in a century.

Those viruses “bear no relationship to SARS-CoV-2; they are genetically distinct,” the NIH official, Dr. Lawrence A. Tabak, told a House panel, using the formal name for the virus. He added that to suggest otherwise would be akin to “saying that a human is equivalent to a cow.”

Tabak’s comments came at a hearing before members of the House Energy and Commerce Committee, with the House newly under Republican control. For two years, Republicans in Washington seethed as Democrats investigated the Trump administration’s coronavirus response. On Wednesday, they flipped the script.

During a public grilling that lasted nearly four hours, Republicans accused Tabak and two other top Biden administration officials — Dr. Robert M. Califf, commissioner of the Food and Drug Administration, and Dr. Rochelle P. Walensky, director of the Centers for Disease Control and Prevention — of stonewalling their requests for information, imposing unnecessary vaccine mandates and eroding Americans’ faith in public health institutions.

“President Biden’s public health leaders are here today because they have broken the American people’s trust,” Rep. Cathy McMorris Rodgers, R-Wash., the committee chair, declared at the outset of the session.

It was only a hint of things to come. Republicans have made clear that they intend to tap into Americans’ frustration with masking, mandates and other coronavirus restrictions to wage a broad assault on Biden and his administration. The House’s Select Subcommittee on the Coronavirus Pandemic expects to hold its own hearings beginning in March.

Beyond hearings, Republicans are passing mostly symbolic measures to signal their discontent with Biden’s COVID policies. On Wednesday, the House passed, by a vote of 227-201, a bill that would end the CDC’s requirement that most foreign air travelers entering the United States show proof of vaccination against COVID-19.

Last week, Republicans pushed through legislation that would repeal the vaccine mandate for health care workers and end the public health emergency for COVID; shortly before those votes, the White House said it planned to allow the emergency to expire in May.

Wednesday’s hearing laid bare the vast gulf in the way Democrats and Republicans perceive the risk posed by the virus and the federal response. That gulf is particularly pronounced in the debate over the so-called “lab leak theory,” which is deeply intertwined with Republican suspicions that NIH-funded research in Wuhan may have led to a laboratory leak that caused the pandemic. There is no direct evidence linking the Wuhan laboratory to the start of the pandemic.

The suspicions revolve around $8 million in grants to EcoHealth Alliance, a nonprofit that was collaborating on coronavirus research with scientists at the Wuhan Institute of Virology, in the city where the pandemic began.

Late last month, an internal federal watchdog agency found that the NIH had made significant errors in its oversight of those grants. In a 64-page report, the Office of Inspector General at the federal Department of Health and Human Services outlined missed deadlines, confusing protocols and misspent funds — raising and reinforcing concerns about the government’s system for monitoring research on potentially risky viruses.

The Wuhan studies looked at how animal coronaviruses, especially bat coronaviruses, evolve naturally in the environment and have the potential to become transmissible to the human population. NIH officials have long maintained that the viruses studied in Wuhan “could not have possibly been the source of SARS-CoV-2 or the COVID-19 pandemic,” as an agency website puts it — the sentiment Tabak reiterated Wednesday.

But Rep. Morgan Griffith, R-Va., who leads one of the two subcommittees that convened Wednesday’s hearing, was not persuaded by Tabak.

While he conceded that he did not have absolute proof that a lab leak caused the pandemic, Griffith said that, as a lawyer, he did not feel the need to eliminate “all doubts.” Rather, he said, he is convinced beyond “a reasonable doubt,” in part because China has withheld information from the United States and in part because of the irregularities uncovered by the inspector general.

“What he has is a lack of evidence,” Griffith said of Tabak. “He does not have evidence that they didn’t study the coronavirus that became COVID-19.”

Rep. Diana DeGette, D-Colo., a longtime member of the committee, has come to the opposite conclusion with the same set of facts. After reviewing reports and attending a classified briefing, she said, she agrees with scientists who say the pandemic was most likely caused by “viral spillover” — the virus jumping from animals to humans — at a wet market in Wuhan.

“I have looked at all the evidence, all of the reports,” she said. “And I haven’t seen definitive evidence that the virus came from a Wuhan lab.”

Several Republicans on the committee who are medical professionals — both doctors and pharmacists — expressed deep frustration with federal officials, including the CDC, over vaccine mandates and messages about masking and vaccines to the public.

“As a public health director, what I conveyed to my staff was that our credibility was the most important thing that we have in public health,” said Rep. Mariannette Miller-Meeks, an ophthalmologist and former director of the Iowa Department of Public Health. She added, “It’s demoralizing, and it is depressing that agencies that were once held in such high esteem cannot translate and transfer research and evidence and respond to real-world evidence when they come up with strategies and policies.”

The hearing brought a moment of satisfaction to one Republican in particular: Rep. Greg Pence of Indiana, brother of former Vice President Mike Pence, who oversaw the Trump administration’s COVID-19 response. The congressman took what he called a “shameless point of personal privilege” to thank his brother for his “humble leadership,” his wisdom and his “clear and transparent communications to the American public.”

This article originally appeared in The New York Times.

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