Dangerous coronavirus variants continue to threaten progress the U.S. has made in reducing Covid-19 cases and immunizing the population, according to the nations top infectious disease doctor.

While we are cautiously optimistic about the future, we know that many challenges remain, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in prepared remarks ahead of a congressional hearing on Wednesday.

Faucis agency is racing to understand how emerging mutations of the SARS-CoV-2 virus interact with vaccines and therapies. Its also working with manufacturers to test existing vaccines, as well as new, tailored formulations, against the mutated versions.

Data published Tuesday in the New England Journal of Medicine indicated that

AstraZeneca Plcs vaccine offers little defense against the B.1.351 variant first detected in South Africa.

Moderna Inc. said last month it planned to test a

new recipe of its shot against that mutant.

Fauci is among officials scheduled to testify Wednesday at a

hearing of the House Energy and Commerce Committees Subcommittee on Oversight and Investigations. The hearing comes as vaccines become more widely available and new cases decline steeply since their January peak, although they remain higher than levels seen late last summer.

Also appearing are Centers for Disease Control and Prevention Director Rochelle Walensky and

Peter Marks, director of the Food and Drug Administration division that evaluates vaccines.

The FDA will use every tool in our medical toolbox to fight this pandemic, including pivoting as the virus adapts, Marks said in prepared testimony. The agency has issued guidance to help manufacturers develop vaccines and other products, even as variants emerge.

Headway in reducing cases is fragile in the face of variants that seem to spread more easily, Walensky said. An increase in viral transmission could reverse the progress weve made, according to her prepared remarks.

Fear and Politics

The B.1.1.7 variant that first emerged in the U.K. may now account for as much as 30% of U.S. cases, and the proportion is expected to rise, she said. The South Africa variant has been detected in 81 U.S. cases, and another version called P.1 that surfaced in Brazil has appeared in 15 U.S. cases, according to her testimony.

Data from the CDC and Emory University show antibodies induced from previous infection or vaccination work against the U.K. variant but are less effective at neutralizing the one from South Africa, Walensky said.

Photographer: Chip Somodevilla/Getty Images

It is unclear what impact this will have on the real-world effectiveness of current vaccines, she said in the remarks.

The CDC has boosted surveillance of coronavirus genomes to detect emerging strains. The U.S. is sequencing about 4% of the 400,000 weekly new cases, she said, through a combination of commercial, government and academic labs.

Representative Cathy McMorris Rodgers of Washington state, the House committees ranking Republican member, said the nation has been too slow to re-open schools, according to excerpts of her remarks reviewed by Bloomberg.

Schools remain closed because of fear and politics, not science, she said. McMorris Rodgers blamed President Joe Biden, state leaders, and teachers unions for the delay. Biden has said

states should prioritize educators for vaccines, but that schools with safety measures in place dont need to wait for teachers to be inoculated.

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