Only 1 in 43 known Omicron cases in the US has been hospitalized, CDC says. Almost all the rest had mild symptoms like coughing and fatigue.

• Of 43 cases detected in the US, only one has led to hospitalization, the CDC said Wednesday.
• The rest of the cases were mild, reporting mostly cough, congestion, and fatigue, it said.

Of the 43 known cases of Omicron in the US, so far only one has been hospitalized, the Centers for Disease Control and Prevention (CDC) said on Wednesday.

Nearly all of the other 42 cases featured mild symptoms, the most common being coughing, congestion (stuffy noses), and fatigue, according to CDC director Rochelle Walensky, who gave an interview to the Associated Press.

The data is very early and provides only the first glimpse of the Omicron situation in the US, CDC officials said.

Most of the cases were in vaccinated people and in young adults, which could explain why they were so mild, CDC officials said.

Here's what we know so far, according to the AP interview and CDC data:

• No deaths and one hospitalization have been reported in Omicron-positive people.
• Cases were mostly among young adults.
• Over three-quarters of those who had caught Omicron were vaccinated, and a third had received a booster.
• Some caught Omicron within two weeks after their booster shoot, during which the protection is partial.
• About a third of cases could be linked back to international travel.
• Cases were spotted in 21 states so far, per CDC data.
• The earliest symptoms after catching Omicron in the US were reported on November 15.
• Omicron makes up only 1% of samples sequenced in the US so far, with Delta making up the other 99%.

More data is emerging but it's still too soon to know how bad Omicron is

Omicron was first named a variant of concern nearly two weeks ago, in part because of its concerning set of mutations.

Since then scientists have been racing to collect more evidence on its characteristics.

Very early lab data from South Africa and from tests by vaccine-makers Pfizer and BioNTech suggest that the variant might be better able to infect people who have been vaccinated, although more data is needed to know that for sure.

The CDC says vaccines "are expected" to continue to protect against severe disease, hospitalizations, and death even in Omicron cases.

Early real-world data is also emerging suggesting Omicron could transmit more efficiently than other variants.

The CDC said the variant is "likely" to be more transmissible than the original coronavirus strain, but whether it spreads more quickly than Delta is "unknown."

It is also not clear whether Omicron will cause more severe or milder symptoms, as hospitalization reports tend to lag behind case reports by a couple of weeks.

Anthony Fauci, the White House Medical advisor, said Tuesday Omicron is "almost certainly" not more severe than Delta.

However, at a press briefing on Wednesday, the World Health Organization warned against inferring too much about Omicron from preliminary data.

"We must be careful about drawing firm conclusions until we have a more complete picture", Tedros Ghebreyesus, the WHO director-general, said on Wednesday.

However, a "consistent picture of rapid increase in transmission" is emerging, he said.

"Any complacency now will cost lives", Tedros said on Wednesday.

Omicron has been detected in 57 countries so far, per the WHO.