Hospitalizations are rising across South Africa, but it’s still too early to know whether the omicron variant is driving an increase in severe Covid-19 cases, according to the World Health Organization.

Maria Van Kerkhove, Covid technical lead for the WHO, said Wednesday that some patients infected with omicron are showing mild symptoms, but there are also reports of cases in which the disease becomes more severe. Hospitalizations could be rising due to a general increase in Covid cases and not necessarily because omicron is more lethal, Van Kerkhove said.

“With regards to severity, there are studies that are underway looking at hospitalizations, looking at those individuals who are hospitalized, whether or not they have this variant or not,” Van Kerkhove told reporters during an update in Geneva. “We’re also getting a picture of some of the cases that are detected in other countries.”

The WHO reported Wednesday that 23 countries have identified omicron cases so far, up from 18 just two days ago, and that number is expected to rise in the coming days and weeks.

U.S. health officials have confirmed the country’s first case of omicron in California, the Centers for Disease Control and Prevention said 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.

Van Kerkhove said there are early indications that omicron is more infectious, and the WHO expects to have more information on the variant’s transmissibility within days.

“It is certainly possible that one of the scenarios is that the virus, as it continues to evolve, may still have a fitness advantage, meaning that it can become more transmissible than delta, we’ll have to see,” she said. “But we don’t know quite yet about the severity.” Van Kerkhove noted there’s a “surveillance bias” in reported Covid cases that may cloud the early data.

Dr. Scott Gottlieb, a board member at vaccine maker Pfizer and a former commissioner of the U.S. Food and Drug Administration, told CNBC on Wednesday that there was a mini-delta surge in South Africa as well as an uptick in a separate variant, C.1.2, which complicates efforts to gain clarity on omicron’s transmission and virulence.

Stephane Bancel, CEO of vaccine maker Moderna, told CNBC on Monday that omicron symptoms reported in South Africa may not be a good predictor of the variant’s virulence in other parts of the world, because the country has a much younger and healthier population than European nations and the United States. The elderly are typically at higher risk of developing severe Covid than younger individuals.

“The molecular profile of the kinds of mutations that you see (in omicron) would suggest that it might be more transmissible and that it might elude some of the protection of vaccines,” Fauci said Wednesday. “But we don’t know that now.”

Van Kerkhove said Wednesday that the public health measures used to fight delta, which is currently the dominant variant worldwide, should be strengthened to combat omicron.

“That does not mean lockdown. What that means is using proven public health and social measures,” Van Kerkhove said. The WHO recommended last week that people wear masks and socially distance regardless of their vaccination status.

WHO Director-General Tedros Adhanom Ghebreyesus on Wednesday advised countries against imposing “blanket travel bans,” warning that such measures do not prevent the spread of omicron and place a heavy economic burden on the nations that are targeted. The U.S., the European Union and the U.K. restricted travel from southern African nations after South Africa alerted the world about omicron. Botswana said Friday it first detected the variant on four foreign nationals who entered the country on a diplomatic mission on Nov. 7 as part of its regular Covid surveillance.

“I thank Botswana and South Africa for detecting, sequencing and reporting this variant so rapidly,” Tedros said. “It’s deeply concerning to me that those countries are now being penalized by others for doing the right thing.”

Van Kerkhove said placing travel restrictions on countries that report new variants to the international community could make them hesitant to share critical information in the future.

“If there is any disincentive if countries feel like they will be penalized for recording that information, that is of course a worry for us,” she said “We rely on this information, quite frankly.”

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.”

The WHO will hold a meeting on Dec. 6 to discuss how well natural and vaccine-induced immunity is holding up against Covid, including the omicron variant. Dr. Soumya Swaminathan, the organization’s chief scientist, said the primary goal of the world should be to ensure that as many people as possible have received their first vaccination series, particularly those who are vulnerable.

“There are all countries that still have vulnerable populations that have not been vaccinated for one reason or another,” Swaminathan said. “Of course, there are a large number of low-income countries where it hasn’t happened because we haven’t had the supplies.”

Wealthy nations such as the United States have started rolling out booster doses to the general public as vaccine efficacy wanes over time. That has been a source of controversy internationally because many people in poorer nations have very limited access to vaccines.