Vaccines are still likely to work in the face of a new virus variant. The designation, announced following an emergency meeting of the health body, is reserved for dangerous variants that have the potential to spread quickly, cause severe disease, or reduce the effectiveness of vaccines or treatments. Delta, the most recent coronavirus variant to receive this designation, exploded this summer and now accounts for nearly all Covid cases in the United States.

According to the World Health Organization, the new strain, dubbed Omicron, carries a number of genetic mutations that could allow it to spread quickly, possibly even among the vaccinated.

Independent scientists agreed that Omicron required immediate attention, but they also noted that more research would be required to determine the extent of the threat. Although some variants of concern, such as Delta, have lived up to initial expectations, others have had only a minor impact.

“Epidemiologists are trying to tell the tiger, ‘Easy, tiger,'” said William Hanage, an epidemiologist at Harvard T.H. Chan School of Public Health. “This could turn out badly.” This could be disastrous. But we don’t know enough to move that tape.”

Vaccines, according to Dr. Hanage and other researchers, will most likely protect against Omicron, but more research is needed to determine how much of the shots’ effectiveness may be reduced.

As the coronavirus replicates within people, new mutations emerge on a regular basis. Most give the virus no new advantage. When concerning mutations do emerge, the World Health Organization names the variants using Greek letters. Alpha, the first “variant of concern,” debuted in the United Kingdom in late 2020, and was quickly followed by Beta in South Africa.

Omicron was discovered in Botswana, where researchers at the Botswana Harvard H.I.V. Reference Laboratory in Gaborone sequenced coronavirus genes from positive test samples. They discovered some samples that shared about 50 mutations that had never been seen in such a combination before. According to an international database of variants, six people have tested positive for Omicron in Botswana so far.

Around the same time, researchers in South Africa discovered Omicron in a cluster of cases in the Gauteng province. They had 58 Omicron samples on the variant database as of Friday. Tulio de Oliveira, the director of South Africa’s Centre for Epidemic Response & Innovation, said at a news conference on Thursday that “close to two or three hundred” genetic sequences of Omicron cases would be released in the coming days.

The World Health Organization has called for increased surveillance of the variant as well as laboratory experiments to better understand its biology.

“This variant did surprise us,” said Dr. de Oliveira at the press conference. “However, the full significance is still unknown.”

Dr. de Oliveira and his colleagues requested that the World Health Organization convene an emergency meeting on Friday to discuss the variant for two reasons: the mutations in Omicron and what appears to be an alarming spread in South Africa.

The researchers discovered more than 30 mutations on the surface of the coronavirus in a protein called spike. The spike protein is the primary target of antibodies produced by the immune system in response to a Covid-19 infection. Because there were so many mutations, it was feared that Omicron’s spike would be able to evade antibodies produced by either a previous infection or a vaccine.

Dr. de Oliveira and his colleagues devised a quick method for determining how quickly Omicron spread in South Africa. Although sequencing a virus’s entire genome takes time, the scientists were able to identify Omicron using a standard nasal swab test known as P.C.R.