With several cases of the omicron variant confirmed in the United States, officials have learned that "many" of those infected are vaccinated against COVID-19, according to Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.

But many of the patients experiencing mild symptoms from omicron are also vaccinated, Walensky said, indicating that the current COVID-19 vaccines are fending off severe disease.

"We've seen omicron in about five states now and we're continuing to do investigations in other states as probable cases emerge. But what we can say, based on what these cases are showing -- some have mild disease, some may have more severe disease, many of them are vaccinated -- and what we're seeing now is that many of the people with mild disease were the vaccinated people," Walensky told ABC News' Cecelia Vega in an interview Friday on "Good Morning America."

"So we still have a lot of science to do to understand how these vaccines are working against omicron, except to say that we know for every variant that we've had it's better to be vaccinated than unvaccinated," she added.

Walensky emphasized that, despite the global frenzy around omicron, delta remains the dominant variant in the U.S.

"We have 90,000 new cases of COVID-19 every day and about 99.9% of them continue to be delta," she said. "So we can't lose sight of the fact that the vast majority of what we have here in the United States is delta, and we know how to tackle delta with vaccines, with boosters, with masking and all of our prevention measures we have been using all along."

Walensky acknowledged that there are still many unknowns about omicron, including the severity of disease, transmissibility and vaccine effectiveness.

"I think we really do need to follow the science here and understand how our vaccines are going to work against omicron," she said. "It may very well be that our vaccines actually work quite well and continue to work quite well against severe disease, and those are the studies that are ongoing."

Hawaii became the fifth state to detect the omicron variant, after confirming a case through expedited genomic sequencing Thursday, health officials said.

The individual is an unvaccinated resident of Oahu who had a previous COVID-19 infection, the state health department said. The person is experiencing "moderate symptoms," the department said.

The resident has no recent travel history, indicating that this is a case of community transmission, health officials said.

The Los Angeles County Department of Public Health said Thursday evening it has confirmed its first case of the omicron variant.

The county resident had recently traveled to South Africa, returning via London on Nov. 22, and the infection "is most likely travel-related," the department said.

The person is fully vaccinated and their symptoms are improving without medical care, health officials said. Several close contacts have all tested negative.

This is the second confirmed case of omicron in California, following identification Wednesday in a resident of San Francisco who had recently traveled to South Africa.

A new study from South Africa suggests that the new omicron variant might be more likely to lead to COVID-19 reinfection than prior variants, though more research is needed.

The study, which is not peer-reviewed, found that in November, there was an uptick in the rate of reinfections seen within three months of a primary infection, compared to prior surges driven by the delta and beta variants.

Researchers, who reviewed records of over 2.7 million people in South Africa with COVID-19 infections in 2020 and 2021, assumed many cases in November were caused by omicron, even though the first cases of the variant were not detected there until late November.

The vaccination status of individuals with suspected reinfections was unknown in the study, so it is unclear if they had immunity from prior infection or vaccination.

-ABC News' Dr. John Brownstein, Dr. Esra Demirel and Sony Salzman