Intense research into the new coronavirus variant first identified in southern Africa has just begun. World leaders have urged people not to panic — and to get vaccinated, if they can.

By Andrew Jacobs

What is the Omicron variant?
First identified in Botswana and South Africa, this new iteration of the coronavirus has prompted concern among scientists and public health officials because of an unusually high number of mutations that have the potential to make the virus more transmissible and less susceptible to existing vaccines.

The World Health Organization has called Omicron a “variant of concern” and warned that the global risks posed by it were “very high,” despite what officials described as a multitude of uncertainties. Cases have been identified in dozens of countries on every continent except Antarctica.

What do we know about Omicron’s presence in the U.S.?
It’s too early to say how widespread the variant will become in the United States. As of Dec. 4, cases have been identified in more than a dozen states, but experts say that number is likely to increase in the coming days and weeks. The first person to have tested positive for the variant returned to San Francisco from South Africa on Nov. 22 and tested positive on Nov. 29, according to the Centers for Disease Control and Prevention.

Should we be worried?
Omicron’s discovery has prompted considerable panic across the globe, with a number of countries banning flights from southern Africa, or — like Israel, Japan and Morocco — barring entry of foreign travelers altogether.

But public health experts have urged caution, noting that there is as yet no firm evidence that Omicron is more dangerous than previous variants like Delta, which quickly overtook its predecessors in the United States and other countries.

Although Delta turned out to be much more transmissible than prior variants — and there is some data suggesting it can cause more severe illness in the unvaccinated — there is little evidence it is more lethal or capable of outsmarting vaccines.

Much remains unknown about Omicron, including whether it is more transmissible and capable of causing more serious illness. There is some evidence the variant can reinfect people more readily.

In South Africa, where Omicron is already the dominant form of the virus, scientists have reported a sudden, sharp rise last month in coronavirus cases among people who had already been infected, in a study that has not yet been reviewed and published by a scientific journal. The authors noted that there was no such upswing when the Beta and Delta variants emerged.

The finding hints that Omicron may be less vulnerable to the body’s immune defenses. Researchers in South Africa also reported that the variant appears to be spreading more than twice as quickly as Delta, which had been considered the most contagious form of the virus.

There are some early signs that Omicron may cause only mild illness. But that observation was based mainly on South Africa’s cases among young people, who are less likely overall to become severely ill from Covid.

Dr. Angelique Coetzee, who chairs the South African Medical Association, said that the nation’s hospitals were not overwhelmed by patients infected with the new variant, and most of those hospitalized were not fully immunized. Moreover, most patients she had seen did not lose their sense of taste and smell, and had only a slight cough.

On Tuesday, Regeneron said its Covid antibody treatment might be less effective against Omicron, an indication that the popular and widely beneficial monoclonal antibody drugs may need to be updated if the new variant spreads aggressively.

That said, Omicron’s emergence is so recent that it may be a while before experts know whether it is more pathogenic. Covid hospitalizations lag new infections by two weeks or more.

Tracking Omicron and Other Coronavirus Variants
See which countries have reported cases of the Omicron variant.

Scientists expect to learn much more in the coming weeks. At the moment, they say there is no reason to believe Omicron is impervious to existing vaccines, although they may turn out to be less protective to some unknown degree.

There’s another reason to remain calm: Vaccine makers have expressed confidence they can tweak existing formulations to make the shots more effective against new variants. Also reassuring: Omicron’s distinctive mutations make it easy to quickly identify with a nasal swab and lab test.

Why are scientists so concerned about Omicron?
As the coronavirus replicates inside people, new mutations constantly arise. Most provide the virus with no new advantage, but sometimes mutations can give the pathogen a leg up by allowing it spread more readily among its human hosts or dodge the body’s immune response.

Researchers in South Africa sounded the alarm because they found more than 30 mutations in the spike protein, a component on the surface of the virus that allows it to bind to human cells and gain entry to the body. Some of the samples from Botswana shared about 50 mutations throughout the virus not previously found in combination.

The spike protein is the chief target of antibodies that the immune system produces to fight a Covid-19 infection. Having so many mutations raises concerns that Omicron’s spike might be able to somewhat evade antibodies produced by either previous infection or vaccination.

Those mutations also raise the prospect that the variant will reduce the efficacy of monoclonal antibody treatments — a fear partly confirmed on Tuesday with Regeneron’s announcement.

Still, it is worth remembering the fate of earlier variants that stirred concern: Beta and Mu, for example, evolved the ability to partially evade the body’s immune defenses, but they never became a serious threat to the world because they proved to be poor at transmitting.

What about vaccines?
Vaccines are expected to provide some protection against Omicron because they stimulate not only antibodies but other immune cells that attack virus-infected cells. Mutations to the spike protein do not blunt that response, which most experts believe is instrumental in preventing serious illness and death.

Citing the potential for waning immunity six months or more after vaccination, some health experts are promoting booster shots to increase antibody levels.

Dr. Fauci has urged people to get a booster shot, which he said would most likely provide additional protection against severe disease. “We’ve said it over and over again and it deserves repeating. If you’re not vaccinated, get vaccinated, get boosted if you are vaccinated, continue to use the mitigation methods, namely masks, avoiding crowds and poorly ventilated spaces,” he said on Tuesday.

Moderna, Pfizer-BioNTech and Johnson & Johnson, makers of vaccines approved for use in the United States, and AstraZeneca, which is widely used in Europe, have all said they were studying Omicron, and they expressed confidence in their ability to tailor their formulations to target the variant.

Why is it called Omicron?
When the W.H.O. began to name emerging variants of the coronavirus, they turned to the Greek alphabet — Alpha, Beta, Gamma, Delta and so on — to make them easier to describe. The first “variant of concern,” Alpha, was identified in Britain in late 2020, soon followed by Beta in South Africa.

But veterans of American sorority and fraternity life might have noticed the system has skipped the next two letters in the alphabetical order: Nu and Xi.

Officials thought Nu would be too easily confused with “new,” but the next letter, Xi, is a bit more complicated. W.H.O. officials said it was a common last name, and therefore potentially confusing. Some noted that it is also the name of China’s top leader, Xi Jinping.

A spokesman for the W.H.O. said the organization’s policy was designed to avoid “causing offense to any cultural, social, national, regional, professional, or ethnic groups.”

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