New ‘Omicron’ Variant Stokes Concern but Vaccines May Still Work

Scientific experts at the World Health Organization warned on Friday that a new coronavirus variant discovered in southern Africa was a “variant of concern,” the most serious category the agency uses for such tracking.

The designation, announced after an emergency meeting of the health body, is reserved for dangerous variants that may spread quickly, cause severe disease or decrease the effectiveness of vaccines or treatments. The last coronavirus variant to receive this label was Delta, which took off this summer and now accounts for virtually all Covid cases in the United States.

The W.H.O. said the new version, named Omicron, carries a number of genetic mutations that may allow it to spread quickly, perhaps even among the vaccinated.

Independent scientists agreed that Omicron warranted urgent attention, but also pointed out that it would take more research to determine the extent of the threat. Although some variants of concern, like Delta, have lived up to initial worries, others have had a limited impact.

“Epidemiologists are trying to say, ‘Easy, tiger,’” said William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. “This could be bad. This could be very bad. But we don’t know enough to roll that tape forward.”

Dr. Hanage and other researchers said that vaccines will most likely protect against Omicron, but further studies are needed to determine how much of the shots’ effectiveness may be reduced.

As the coronavirus replicates inside people, new mutations constantly arise. Most provide the virus with no new advantage. When worrisome mutations do emerge, the World Health Organization uses Greek letters to name the variants. The first “variant of concern,” Alpha, appeared in Britain in late 2020, soon followed by Beta in South Africa.

Omicron first came to light in Botswana, where researchers at the Botswana Harvard H.I.V. Reference Laboratory in Gaborone sequenced the genes of coronaviruses from positive test samples. They found some samples sharing about 50 mutations not found in such a combination before. So far, six people have tested positive for Omicron in Botswana, according to an international database of variants.

Around the same time, researchers in South Africa stumbled across Omicron in a cluster of cases in the province of Gauteng. As of Friday, they have listed 58 Omicron samples on the variant database. But at a news conference on Thursday, Tulio de Oliveira, the director of the Centre for Epidemic Response & Innovation in South Africa, said that “close to two or three hundred” genetic sequences of Omicron cases would be released in the next few days.

The W.H.O. called for increased surveillance of the variant and laboratory experiments to better understand its biology.

“This variant did surprise us,” Dr. de Oliveira said at the news conference. “But the full significance is still uncertain.”

Dr. de Oliveira and his colleagues asked the W.H.O. to hold an emergency meeting about the variant on Friday for two reasons: the mutations in Omicron and what appears to be an alarming spread in South Africa.

The researchers found more than 30 mutations on a protein, called…