The omicron variant has become by far the most prevalent version of the coronavirus spreading in South Africa, replacing the fading delta variant, health officials said today.

Nearly three-quarters of all samples from positive tests in the country that were checked genetically in November were found to involve the omicron variant, the National Institute for Communicable Diseases announced.

New cases are rising sharply in South Africa, after having dropped to low levels in recent months. The institute said that there were 8,651 new cases reported today, almost twice as many as the day before; as recently as early November, the country was averaging fewer than 300 new cases a day.

The share of tests that are coming back positive rose to 16.5%, from 10.2% on Tuesday, the institute said.

The first omicron case detected in the United States was announced Wednesday. The World Health Organization said that at least 23 other countries have reported cases of omicron, and that it expects that number to grow as scientists around the world sequence more test samples.

In a virtual news conference, WHO experts said that “it is early days” in determining whether the mutations seen in the new variant made it more transmissible or better able to evade vaccine protection, as some experts fear. Dr. Maria Van Kerkhove, the technical lead of the WHO’s coronavirus response, said she expected more information on those issues “within days,” but she emphasized that so far, “there is no evidence to suggest that the vaccines don’t work” against omicron.

“Vaccines are saving lives,” she said.

Many questions remain about whether this variant causes more severe illness than others, she said, adding that officials in South Africa had seen reports of omicron cases with symptoms that ranged from “mild disease all the way to severe disease.” To date they had seen no deaths associated with the variant.

Tulio de Oliveira, director of the KwaZulu-Natal Research and Innovation Sequencing Platform, said that the omicron variant “seems to cause much more breakthrough infections than the previous ones.” He said that he was concerned about the threat of such infections among health workers, which would sap the country’s ability to cope with a surge in cases.

But de Oliveira cautioned against reading too much into head-to-head comparisons between omicron and delta. Omicron might swiftly supplant delta in countries like South Africa where the older variant was fading, but perhaps not as readily where delta is still spreading actively, he noted.

The WHO panel emphasized the need for countries to speed up vaccination efforts as much as possible, particularly for vulnerable populations.

The panel also called on health authorities to strengthen sequencing, surveillance and field investigations, including community testing.

The agency strongly advised against imposing “blanket travel bans,” calling instead for “a tailored approach” to travel restrictions that could include quarantine and testing requirements for arrivals.

“Our concern here is that we apply public health principles, not political principles, to select the measures that are used,” said Dr. Michael Ryan, the head of the emergencies program at the WHO.

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This article originally appeared in The New York Times.