UPDATE 2-"Things are tenuous," U.S. CDC says as downward trend in COVID-19 cases stalls

(Adds quotes from officials)

By Jeff Mason and Carl O'Donnell

WASHINGTON, Feb 26 (Reuters) - The head of the U.S. Centers for Disease Control and Prevention said on Friday that a recent decline in COVID-19 cases may be stalling, a development she described as concerning while urging that safeguards to fight the virus remain in place.

Dr. Rochelle Walensky told reporters the number of cases had been increasing for the past three days compared to the prior week and that declines in hospitalizations and deaths were also "potentially leveling off at still a very high number."

Walensky painted a critical picture of the current state of the pandemic.

"Things are tenuous. Now is not the time to relax restrictions. Cases, hospital admissions, and deaths all remain very high and the recent shift in the pandemic must be taken extremely seriously."

States and cities have been gradually lifting restrictions in recent weeks. New York City reopened indoor dining earlier this month and Massachusetts plans to remove limits on restaurant capacity starting in March. Montana and Iowa lifted statewide mask requirements earlier this month, while North Dakota's mask mandate expired in January.

The White House urged companies on Friday to join efforts to help fight the pandemic by requiring mask-wearing by employees and educating customers.

Andy Slavitt, a senior adviser on the White House's COVID-19 response team, listed a number of companies that were taking measures to help with the pandemic fight and urged more to join.

He announced what he described as a new partnership between the Biden administration and leading business organizations to spur the private sector to encourage companies to require masks and social distancing to protect employees and customers, make it easier for employees to get vaccinated with incentives, and educate the public about the benefits of masks and vaccines.

"We are asking businesses to amplify CDC messages about masking and vaccinations on their products, properties and web sites," Slavitt said.

Ford and the Gap were producing and donating millions of masks, he said, while Best Buy, Target and Dollar General were giving workers paid time off or compensating them to get vaccines.

Uber, PayPal and Walgreens were teaming up to provide $10 million in free rides to vaccination sites, he said, while Lyft had partnered with CVS and the YMCA to provide $60 million free or discounted rides to vaccination sites.

The White House is working on a broad campaign to educate Americans about the vaccine as it seeks to bring the pandemic that has killed more than 500,000 people in the United States under control.

President Joe Biden on Thursday noted concerns that later this spring supply of the vaccines would outstrip demand because of vaccine hesitancy.

Moderna Inc said on Wednesday it is working with U.S. government scientists to study an experimental booster shot that targets a concerning new variant of the coronavirus.

Infectious disease expert Dr. Anthony Fauci said the study, which will be run by the National Institutes of Health, will start in mid-March and that there was no plan yet to begin manufacturing the experimental booster at scale as it is unclear whether the new B1351 variant first found in South Africa will become a dominant variant in the United States.

(Reporting by Jeff Mason in Washington and Carl O'Donnell in New York; additional reporting by Lisa Lambert Editing by Chizu Nomiyama and Mark Heinrich)