The Food and Drug Administration is expected to formally approve Johnson & Johnson’s Covid-19 vaccine on Saturday, making a third shot available in the United States.
The vaccine will be the first of the approved vaccines to require one dose instead of two. Shipments are expected to start within days, adding to the effort already underway to administer millions of doses of the Pfizer-BioNTech and Moderna vaccines.
“We’re going to use every conceivable way to expand manufacturing of the vaccine — the third vaccine — to make even more rapid progress at getting shots in people’s arms,” President Biden said on Friday in Houston, where he had traveled to showcase the government’s latest mass vaccination site.
The shots can’t come fast enough.
As Saturday dawned, the United States had recorded about 28.5 million coronavirus cases — representing more than 8 percent of the population — and a staggering 510,373 deaths. And it will be many weeks before vaccinations make a dent in the pandemic. Meanwhile, the virus has been mutating, creating variants that may partly sidestep the immune system.
Daily case numbers are about where they were in October, far below the single-day record of about 300,000 infections set in early January. And daily vaccination numbers have started to increase again after a decline brought on by severe weather.
Yet federal health officials warned impatient governors against relaxing pandemic control measures, saying that the recent steep drop in cases and deaths could be leveling off.
The seven-day average for new cases — 69,483 as of early Saturday — has been ticking up. Progress has largely stalled in New York City, where the latest coronavirus variant was discovered only this week. And another concerning version of the virus is spreading at a rapid pace through California.
“Things are tenuous,” Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, said on Friday. “Now is not the time to relax restrictions.”
More than 68 million shots have been given since vaccinations began in December. The country is averaging about 1.5 million reported vaccinations a day.
The Johnson & Johnson shot, which was unanimously endorsed on Friday by a panel of experts advising the F.D.A., had an overall efficacy rate in clinical trials of 72 percent in the United States and 64 percent in South Africa, where a concerning variant emerged in the fall. It also showed 86 percent efficacy against severe Covid-19 in the United States, and 82 percent in South Africa.
Those are strong numbers, albeit lower than the roughly 95 percent efficacy rates of Pfizer-BioNTech and Moderna’s vaccines against mild, moderate and severe cases of Covid.
Trials are underway to determine if a second dose of the Johnson & Johnson shot would increase its protective effects — as scientists found it did during early clinical trials — although results will not be available until July at the earliest.
“The big question mark still is, how long does protection last?” said Dr. Johan Van Hoof, the global head of vaccine research and development at Janssen Pharmaceuticals, the drug development arm of Johnson & Johnson.
Mr. Van Hoof said it would be important to track trial volunteers who receive a single dose to see if their immunity changes.
In Houston on Friday, Mr. Biden described the federal government’s mass inoculation drive as “the most difficult operational challenge this nation has ever faced logistically.” He said he expected challenges in reaching people in remote areas, and in persuading others who are “hesitant to take the shots.”
“We all know there’s a history in this country of subjugating certain communities to terrible medical and scientific abuse,” Mr. Biden said. “But if there’s one message that needs to cut through all this: The vaccines are safe. I promise you.”