WASHINGTON (NYTIMES) - Federal health officials warned impatient governors Friday (Feb 26) against relaxing pandemic control measures, saying that a recent steep drop in coronavirus cases and deaths in the United States may be levelling off at a very high number - a shift that the director of the Centres for Disease Control and Prevention said "must be taken extremely seriously."

The pleas from the director, Dr Rochelle Walensky, and Dr Anthony Fauci, President Joe Biden's chief medical adviser for the virus, came as the Biden administration scrambled to stay ahead of a possible fourth surge of infections and the spread of worrisome variants, which officials say account for a rising percentage of cases in the country.

Those calls punctuated a face-paced day of pandemic-related developments around the country.

Mr Biden flew to Houston to showcase the government's latest mass vaccination site. The Food and Drug Administration neared emergency authorisation of a third coronavirus vaccine, this one from Johnson & Johnson. And the White House enlisted business groups to help vaccinate their employees and reach Americans resisting vaccinations.

Behind it all were ominous signs after weeks of positive developments.

"Things are tenuous," Dr Walensky said at a White House briefing on the pandemic. "Now is not the time to relax restrictions."

According to a New York Times database, virus cases across the United States appear to be levelling off from the steep decline that began in January, with figures comparable with those reported in late October. The seven-day average of new cases was 69,450 as of Thursday.

Cases have slightly increased week over week in recent days, though severe weather limited testing and reporting in Texas and other states the previous week, and not all states reported complete data on the Presidents Day holiday. Still, the overall numbers remain horrific: More than half a million Americans are now dead of Covid-19, and as of Friday, more than 28 million have been infected.

Yet governors were chafing against coronavirus-related restrictions and itching to take steps to restore a sense of normalcy. In a sign that the partisan divide over the pandemic has not yet abated, Republicans seemed more eager to roll back virus control measures than Democrats, though in New York, Governor Andrew Cuomo, a Democrat, has also been easing restrictions on a variety of activities.

"There's nothing partisan about this virus," Mr Biden said, speaking at a mass vaccination site in Houston with Texas Governor Greg Abbott, a Republican and Senator John Cornyn.

Yet in Texas, Mr Abbott is considering lifting a statewide mask mandate in place since July.

In South Carolina, which has been struggling with some of the highest infection rates in the country, Governor Henry McMaster, a Republican, announced that Monday, restaurants would once again be able to serve alcohol past 11pm and that residents no longer needed to get approval from the state to hold events with 250 people or more. The move lifts orders imposed by the state last year.

A spokesman for Mr McMaster, Mr Brian Symmes, said the governor "appreciates perspectives that differ from his own" but "respectfully disagrees" with Dr Walensky's assessment.

In Mississippi, Governor Tate Reeves said Thursday that he was also considering pulling back some restrictions, particularly mask mandates for people who have been fully vaccinated. Both are Republicans.

In Washington, Mr Biden administration officials pleaded with Americans to be patient. Dr Fauci echoed Dr Walensky's warnings that more rollbacks at state or local levels would be unwise, noting that with coronavirus cases still hovering at around 70,000 per day, the country remains in a "very precarious position."

"We don't want to be people always looking at the dark side of things, but you want to be realistic," he said. "So we have to carefully look at what happens over the next week or so with those numbers before you start making the understandable need to relax on certain restrictions."

While coronavirus deaths tend to fluctuate more than cases and hospital admissions, Dr Walensky said the most recent seven-day average was slightly higher than the average earlier in the week. The seven-day average of newly reported deaths was 2,165, as of Thursday.

"We at CDC consider this a very concerning shift in the trajectory," she said, adding, "I want to be clear: Cases, hospital admissions and deaths - all remain very high, and the recent shift in the pandemic must be taken extremely seriously."

Dr Walensky said some of the rise may be attributable to new variants of the coronavirus that spread more efficiently and quickly. The so-called B117 variant, which first emerged in Britain, now accounts for approximately 10 per cent of all cases in the United States, up from 1 per cent to 4 per cent a few weeks ago, she said.

The United States' ability to track variants is much less robust than Britain's. Even so, data gathered by the CDC shows the number of cases with the variant in the country has risen from 76 in 12 states as of Jan 13 to more than 2,100 in 45 states as of Thursday. But the actual infections may be much higher because of inadequate surveillance efforts.

"I know people are tired; they want to get back to life, to normal," Dr Walensky said. "But we're not there yet."

About an hour before Mr Biden was to speak in Houston, a panel of expert advisers to the Food and Drug Administration voted unanimously to give the green light to Johnson & Johnson's Covid-19 vaccine, clearing the last hurdle before a formal emergency authorisation expected Saturday.

The vaccine will join two others, one by Moderna and the other by Pfizer-BioNTech, that are authorised for use in the United States.

But unlike the first two, Johnson & Johnson's vaccine takes only one dose and has fewer shipping and handling difficulties.

Earlier Friday, the Biden administration announced it has purchased 100,000 doses of a recently authorised Covid-19 treatment from Eli Lilly, increasing the supply of such drugs for patients who are at high risk of becoming seriously ill but are not yet hospitalised.

The treatment is a combination of monoclonal antibodies. The government will pay US$210 million (S$279.80 million) for an initial tranche of 100,000 doses, which the company will ship by the end of March. The administration has an option to buy an additional 1.1 million more doses through November if necessary.

Mr Biden has often compared the fight against the coronavirus to wartime mobilisation, but with the exception of pharmaceutical companies, the private sector has done relatively little in the effort. It has not made a major push to persuade Americans to remain socially distant, wear masks or get vaccinated as soon as possible.

The administration said Friday that it was trying to change that by joining with business lobbying groups, including the US Chamber of Commerce, National Association of Manufacturers and the Business Roundtable, to encourage companies to support workers in getting vaccinated by offering paid time off or benefits.

"There is a light at the end of the tunnel," Mr Neil Bradley, chief policy officer for the US Chamber of Commerce, said in an interview. "We have to get to that light as quickly as possible, and employers can be a huge asset in getting us there."