Evidence mounts that Omicron is more infectious, less severe than Delta—but Fauci, other experts warn against premature optimism

Omicron cases are driving a COVID-19 surge in South Africa, but hospitalization rates remain low.

Early studies of the Omicron COVID variant suggest that the highly mutated COVID-19 strain may be producing less severe infections than previous variants like Delta, but White House medical adviser Dr. Anthony Fauci warned against making definitive conclusions about a strain that the world learned about only 12 days ago.

Subscribe to unlock this article and get full access to Fortune.com

Early studies of the Omicron COVID variant suggest that the highly mutated COVID-19 strain may be producing less severe infections than previous variants like Delta, but White House medical adviser Dr. Anthony Fauci warned against making definitive conclusions about a strain that the world learned about only 12 days ago.

On Saturday, the South African Medical Research Council published a report about an Omicron-driven outbreak in the Tshwane district in South Africa’s northern Gauteng province, one of the first areas in the world where Omicron has overtaken Delta as the dominant strain. The researchers wrote that in the past two weeks there has been an “exponential” rise in caseloads, but the surge has not corresponded to a significant uptick in hospitalizations and deaths.

“The relatively low number of COVID-19 pneumonia hospitalizations in the general, high care, and ICU wards constitutes a very different picture compared to the beginning of previous waves,” the report said, examining data from the Steve Biko and Tshwane District Hospital complex.

The hospital complex said it admitted 166 COVID-19 patients between Nov. 14 and Nov. 29, a “sharp rise” from the previous two weeks. But the report said the majority of them were “incidental” diagnoses, stemming from patients who were not suffering from COVID-related symptoms but made to take a test due to the hospital’s policy to screen all patients for COVID-19. On Dec. 2, the complex said that 38 patients had been admitted to the COVID-19 ward, six of whom were vaccinated, 24 unvaccinated, and eight with unknown vaccination status. Of the cases, only one COVID-19 patient required oxygen, but doctors said the patient needed the oxygen for a non-COVID-related illness.

Fauci, the director of the U.S. government’s National Institute of Allergy and Infectious Diseases, says he is encouraged by the preliminary figures coming from South Africa.

“Though it’s too early to really make any definitive statements about it, thus far it does not look like there’s a great degree of severity to it,” Fauci said on CNN on Sunday.

The South African hospital says that it will be at least two weeks before experts there can draw firmer conclusions about the severity of Omicron cases since hospitalization numbers could “significantly change” before then.

But for now at least, South African officials are optimistic that the variant may not devastate the country’s health care system.

“Our hospital admissions are not increasing at an alarming rate, meaning that whilst people may be testing positive, they are not in large numbers being admitted into hospitals,” South African President Cyril Ramaphosa said in an address on Saturday. “We should not panic.”

Scientists elsewhere have posited a theory that may explain why Omicron appears to be less severe than previous variants. At Nference, a U.S.-based biomedical data platform, researchers sequenced Omicron and found that part of its genetic code, which is not in other variants, is also present in the common cold. That strand could be a sign that Omicron is evolving to become less severe and more transmissible similar to the evolution of other viruses, the researchers told the Washington Post.

The variant appears to be spreading fast.

South Africa is now recording an average of 10,055 cases per day compared to 300 cases per day three weeks ago. South Africa’s government reported on Thursday that 74% of the 249 cases it sequenced in November were Omicron cases, suggesting that the variant may already have displaced Delta in the country.

On Friday, a group of researchers led by Carl Pearson, a research fellow at the London School of Hygiene and Tropical Medicine, published a non-peer-reviewed study finding that Omicron may be spreading twice as fast as the Delta variant. But the researchers said they were uncertain whether Omicron was more infectious than Delta or if the variant is simply better than previous strains at evading immune defenses established by previous infections or vaccines.

“We’re not sure what that mixture is…It’s possible that it might even be less transmissible than Delta,” Pearson told the New York Times.

Researchers in Hong Kong say that a case of Omicron transmission in one of the city’s quarantine hotels is fueling worries about the variant’s potentially high degree of transmissibility. Hong Kong University researchers reported that Omicron likely spread between two fully vaccinated people across the hall from one another even though the patients never left their respective rooms.

“It is not known whether [Omicron’s] detected mutations might have affected the effectiveness of existing vaccines and virus transmissibility,” the researchers wrote in a post published on the U.S. Centers for Disease Control’s Emerging Infectious Diseases Journal. “However, detection of Omicron variant transmission between [two] fully vaccinated persons across the corridor of a quarantine hotel has highlighted this potential concern.”

Researchers are concerned that Omicron’s numerous mutations on the spike protein will make it more resistant to vaccines and natural immunity acquired from previous COVID infections. In the coming days and weeks, scientists will likely release neutralization studies projecting how much Omicron may evade existing COVID-19 vaccines.

For natural immunity, emerging evidence suggests that Omicron is bypassing the immune defenses built up from earlier infections. Last week, a group of researchers in South Africa posted a preprint study finding that the risk of reinfection was 2.4 times higher with Omicron compared to Beta and Delta.

“We find clear, population-level evidence to suggest substantial immune evasion by the Omicron variant,” writes lead author Juliet R.C. Pulliam, director of the South African DSI-NRF Centre of Excellence in Epidemiological Modelling and Analysis, in the study.

“This set of real-world data on the ability of the Omicron variant provides us with the first indication that it is indeed able to evade immunity conferred by previous COVID-19 infection,” Dr. Simon Clarke, associate professor in cellular microbiology at the University of Reading, tells the U.K.’s Science Media Centre.

But given South Africa’s low vaccination rate—25% of South Africa’s population is fully vaccinated—the researchers said it’s too early to tell how much Omicron has been able to evade vaccines. Scientists say that existing vaccines will provide a level of protection against hospitalizations and death against Omicron, but the prospect of diminishing efficacy against Omicron has prompted vaccine makers like Pfizer and Moderna to begin developing vaccines that specifically target the variant.

“We await a further indication as to whether Omicron has any ability to evade vaccine-induced immunity,” Clarke said.

Subscribe to Fortune Daily to get essential business stories straight to your inbox each morning.