60 percent more contagious and 67 percent more deadly than the original form of the virus, according to the most recent estimates.

The variant is no different from the original in how it spreads, but infected people seem to carry more of the virus and for longer, said Katrina Lythgoe, an evolutionary biologist at the University of Oxford. “You’re more infectious for more days,” she said.

So contagious is B.1.1.7 that Britain succeeded in driving down infections only after nearly three months of strict stay-at-home orders, plus an aggressive vaccination program. Even so, cases fell much more slowly than they did during a similar lockdown in March and April.

three-quarters of new infections, some hospitals have had to move coronavirus patients to Belgium to free up beds. Roughly as many people are dying each day from Covid-19 in Europe as were this time a year ago.

For too long, government officials disregarded the threat. “Case plateaus can hide the emergence of new variants,” said Carl Pearson, a research fellow at the London School of Hygiene and Tropical Medicine. “And the higher those plateaus are, the worse the problem is.”

In the United States, coronavirus infections began a rapid decline in January, soon prompting many state leaders to reopen businesses and ease restrictions. But scientists repeatedly warned that the drop would not last. After the rate bottomed out at about 55,000 cases and 1,500 deaths per day in mid-March, some states — notably Michigan — began seeing an uptick.

Since then, the national numbers have steadily risen. As of Saturday, the daily count was up to nearly 69,000, and the weekly average was 19 percent higher than the figure two weeks earlier.

Pfizer-BioNTech and Moderna vaccines seem to be slightly less effective against B.1.351, the variant identified in South Africa. That variant contains the Eek mutation, which seems to enable the virus to partly sidestep the body’s immune response. The vaccines made by Johnson & Johnson, AstraZeneca and Novavax were even less potent against B.1.351.

“I think for the next year or two, E484K will be the most concerning” mutation, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.

The mutation slightly alters the so-called spike protein sitting on the surface of the coronavirus, making it just a bit harder for antibodies to latch on and destroy the invader.

The good news is that the virus seems to have just a few survival tricks in its bag, and that makes it easier for scientists to find and block those defenses. “I’m feeling pretty good about the fact that there aren’t that many choices,” said Michel Nussenzweig, an immunologist at Rockefeller University in New York.

The Eek mutation seems to be the virus’s primary defense against the immune system. Researchers in South Africa recently reported that a new vaccine directed against B.1.351 ought to fend off all other variants, as well.

Pfizer, BioNTech and Moderna already are testing newly designed booster shots against B.1.351 that should work against any variants known to blunt the immune response.

Instead of a new vaccine against variants, however, it may be just as effective for Americans to receive a third dose of the Pfizer-BioNtech or Moderna vaccines in six months to a year, said Dr. Anthony S. Fauci, head of the National Institute of Allergy and Infectious Diseases.

That would keep antibody levels high in each recipient, overwhelming any variant — a more practical strategy than making a specialized vaccine for each new variant that emerges, he said.

“My only concern about chasing all the variants is that you’d almost be playing Whac-A-Mole, you know, because they’ll keep coming up and keep coming up,” Dr. Fauci said.

In one form or another, the new coronavirus is here to stay, many scientists believe. Multiple variants may be circulating in the country at the same time, as is the case for common cold coronaviruses and influenza. Keeping them at bay may require an annual shot, like the flu vaccine.

The best way to deter the emergence of dangerous variants is to keep cases down now and to immunize the vast majority of the world — not just the United States — as quickly as possible. If significant pockets of the globe remain unprotected, the virus will continue to evolve in dangerous new ways.

“This might be something that we have to deal with for a long time,” said Rosalind Eggo, an epidemiologist at London School of Hygiene and Tropical Medicine.

Still, she added, “Even if it changes again, which it is very likely to do, we are in a better, much stronger position than a year ago to deal with it.”

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These Covid Vaccines Are Now in the U.S. Others May Be Coming.

About 20 percent of the U.S. population has received at least one Covid-19 vaccine shot as the pace of inoculations in the United States sharply climbs. Here is a look at the vaccines that have been authorized by the Food and Drug Administration and where some other vaccine candidates stand.

Three: from Pfizer-BioNTech, Moderna and Johnson & Johnson. Pfizer’s was the first, in December, with Moderna’s following shortly after; each is given in two shots spaced three to four weeks apart. Johnson & Johnson’s vaccine, authorized last month, is given in one dose.

Not yet. When it was authorized on Feb. 27, Biden administration officials cautioned that supplies would be limited for the first month, with 3.9 million shots initially and 16 million more by the end of March.

Johnson & Johnson pledged last year to deliver 37 million doses by the end of March and a total of 100 million by the end of June, but it is still working on getting production up to that scale. A recent deal with Merck is meant to increase manufacturing and packaging capacity.

another 100 million doses of Johnson & Johnson’s shot.

Novavax could apply for emergency use authorization for its two-shot vaccine in late April. It offers robust protection, though it was not as effective against a variant circulating rapidly in South Africa as it was against other versions. Novavax could deliver 110 million doses by the end of June if the F.D.A. clears the vaccine for use.

AstraZeneca, whose vaccine is authorized in more than 70 countries, has not yet reported results from its U.S. clinical trial, nor has it applied for authorization in the U.S.

The AstraZeneca vaccine, developed with Oxford University, has run into some problems. European countries have suspended use of it over concerns about blood clots, although no evidence has been found of any causal link. Some people in Germany are also declining to receive it because of its lower overall efficacy in clinical trials, compared with other vaccines.

by May 1. No vaccine is authorized yet for children.

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