Omicron appears to cause mild illness, but experts say too early to know for sure

Since the first case of the new Omicron variant was detected last month, early anecdotal reports indicate people infected seem to be experiencing mild illness -- leading some scientists to wonder if this version of the virus could be less dangerous than prior variants.

But scientists have cautioned it's too soon to know for sure.

It "looks less severe in really early data," Centers for Disease Control and Prevention Director Dr. Rochelle Walensky told ABC News Chief Medical Correspondent Dr. Jennifer Ashton in an interview from CDC headquarters in Atlanta. "We're certainly following and very interested in disease severity."

Omicron was dubbed a "variant of concern" due to its many mutations. As a result, scientists are scrambling to determine if these changes lead to increased transmissibility or weakened response from vaccines. The World Health Organization said several studies are underway and that more information will emerge in the following days to weeks. Preliminary studies show that the Pfizer vaccine may be less effective against the Omicron variant, but, again, more research is needed.

Early clinical data from researchers in South Africa hint that the virus may cause less severe COVID-19 infections. The South African Medical Research Council has reported that very few hospitalized patients with the Omicron variant have required supplemental oxygen or were admitted to an ICU.

Dr. John Brownstein, an epidemiologist and professor with the Department of Pediatrics at Harvard Medical School, said that preliminary results are "very encouraging."

The U.S reported its first Omicron case on Dec. 1 in California. Many subsequent cases in the U.S. have happened among younger, healthier and vaccinated people who already are less likely to become very sick from COVID-19. But there are still too few people with Omicron to draw meaningful conclusions about whether the variant itself causes a more mild illness, experts told ABC News.

With an increase in the number of cases, Brownstein said scientists "will be able to characterize the [variant] better." This information will aid public health officials in establishing better guidelines and preventative measures.

If it turns out Omicron is more transmissible but less severe than other variants, some experts said that could bode well overall -- perhaps signaling the virus will still circulate among people but become less life-threatening.

"I think Omicron may represent the first step in adaptation that you want to see, which is that it's more contagious and less virulent," said Dr. Paul Offit, an infectious disease pediatrician.

Even with all eyes on the Omicron variant, experts are reminding the public that the Delta variant remains the predominant circulating strain in the United States, where it's still responsible for more than 99% of cases. Among best practices for avoiding all variants, officials continue to recommend vaccinations, obtaining booster shots as soon as eligible and following mask-wearing guidelines.

Bernadette Baker M.D., a family medicine resident physician with Emory School of Medicine, is a contributor to the ABC News Medical Unit. Sony Salzman is the unit's coordinating producer.