• The WHO said Wednesday that 23 countries across the world have reported cases of the highly mutated omicron Covid-19 variant. 
• Director-General Tedros Adhanom Ghebreyesus said they don't yet know how the new variant will effect transmission, the severity of disease, therapeutics and vaccines.
• He added that using the "tools" available to prevent transmission of the highly transmissible delta Covid-19 variant will also stop the transmission of omicron.
• The U.S. has confirmed its first case of omicron, health officials said Wednesday.

The World Health Organization said Wednesday that 23 countries across the world have reported cases of the highly mutated omicron Covid-19 variant. 

"At least 23 countries from five of six WHO regions have now reported cases of omicron and we expect that number to grow," WHO Director-General Tedros Adhanom Ghebreyesus told reporters during an update Wednesday in Geneva. 

"WHO takes this development extremely seriously and so should every country. But it should not surprise us," Tedros continued. "This is what viruses do. And it's what this virus will continue to do, as we long as we allow it to continue spreading."

The U.S. has confirmed its first case of omicron in California, the Centers for Disease Control and Prevention said on Wednesday. The individual, who was fully vaccinated, had returned from South Africa to the San Francisco area on Nov. 22 and tested positive on Nov. 29, White House chief medical advisor Dr. Anthony Fauci told reporters Wednesday.

The variant, which was reported to WHO by South Africa a week ago, has more than 30 mutations to the spike protein alone. Some of the mutations are associated with a decrease in antibody protection and higher transmission, according to the WHO.

Tedros said there is still more to learn about the new variant's effect on transmission, the severity of disease and the effectiveness of tests, therapeutics and vaccines. Several WHO advisory groups have met in the last few days to "evaluate the emerging evidence, and prioritize the studies needed to answer these questions," he said.

He added that the highly transmissible delta Covid-19 variant still accounts for almost all cases globally. Tedros said using the tools available to prevent transmission of delta will also stop the transmission of omicron.

"If countries and individuals don't do what they need to do to stop transmission of delta, they won't stop omicron either," Tedros said.

He urged countries to bolster vaccination efforts and other prevention measures, noting that low vaccine coverage and testing in countries is a "recipe for breeding and amplifying variants."

"We continue to urge countries to fully fund the accelerator to ensure equitable access to vaccines, tests therapeutics, all over the world," Tedros said.

The use of blanket travel bans, however, will not prevent the transmission of omicron and are a "heavy burden on lives and livelihoods," according to Tedros. 

Dozens of countries have imposed travel restrictions on southern African nations since the mutation was reported to WHO a week ago. The U.S., for instance, began restricting travel for non U.S.-citizens from South Africa and seven other countries on Monday.

Tedros said it is "deeply concerning" that Botswana and South Africa were "being penalized by others for doing the right thing."

He called on countries to turn to "rational proportional risk reduction measures." This includes measures screening passengers prior to traveling and or upon arrival in a country, or the application of quarantine to international travelers.

Fauci defended the U.S. travel restrictions on Wednesday, describing them as a temporary measure intended to buy time for health officials to better understand the virus variant.

"No one feels that a travel ban is going to prevent people who are infected from coming to the United States," Fauci said. "But we needed to buy some time to be able to prepare, understand what's going on. So we look at this as a temporary measure."