Omicron Becomes Dominant Strain in US, Health Officials Say

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Much of the U.S. is currently battling a surge in COVID cases spurred by the extra-contagious delta variant, but the new and even more contagious omicron variant has taken over as the dominant strain in the country, health experts say.

Omicron has raced ahead of other variants and is now the dominant version of the coronavirus in the U.S., accounting for 73% of new infections last week, federal health officials said Monday.

The Centers for Disease Control and Prevention numbers showed nearly a six-fold increase in omicron's share of infections in only one week.

In much of the country, omicron's prevalence is even higher. It's responsible for an estimated 90% of new infections in the New York area, the Southeast, the industrial Midwest and the Pacific Northwest.

Since the end of June, the delta variant has been the main version causing U.S. infections. As recently as the end of November, more than 99.5% of coronaviruses were delta, according to CDC data.

"All of us have a date with omicron," said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. “If you’re going to interact with society, if you’re going to have any type of life, omicron will be something you encounter, and the best way you can encounter this is to be fully vaccinated.”

Adalja said he was not surprised by the CDC data showing omicron overtaking delta in the U.S., given what was seen in South Africa, the U.K. and Denmark. He predicted spread over the holidays, including breakthrough infections among the vaccinated and serious complications among the unvaccinated that could stress hospitals already burdened by delta.

CDC’s estimates are based on thousands of coronavirus specimens collected each week through university and commercial laboratories and state and local health departments. Scientists analyze their genetic sequences to determine which versions of the COVID-19 viruses are most abundant.

White House chief medical advisor Dr. Anthony Fauci said last week that the new variant would likely bring another surge on top of the delta surge, making up a majority of cases in the next three weeks.

Centers for Disease Control Director Dr. Rochelle Walensky echoed Fauci's projection during a briefing Friday and Dr. Robert Citronberg, the executive medical director for infectious disease and prevention at Advocate Aurora Health, told NBC 5 a similar time frame.

"Delta is still the predominant strain circulating in our area and throughout the country, though omicron is really catching up pretty quickly," Citronberg said. "Just even a couple of weeks ago, we didn't know about any omicron... and in probably the next few weeks, it will become the dominant strain in our country."

Chicago Department of Public Health Commissioner Dr. Allison Arwady said last week that there is increased confidence omicron "is likely to spread quickly" and even faster than the delta variant.

"The early data suggests that this is probably, you know, maybe two to two-and-a-half times as easy to spread, as contagious as the delta variant," Arwady said.

Health officials have cautioned that while initial cases have appeared mild, the variant could still be dangerous to communities and severe cases are still possible. Dueling surges of the delta and omicron variants could also lead to overcrowded hospitals and dwindling resources.

Arwady said she's "feeling pretty confident" that the variant will lead to more breakthrough cases and reinfections in those who have already had COVID, but the severity of those cases remains unclear despite early breakthrough cases in the Midwest showing mild symptoms, if any.

"We think based on what we're seeing now, omicron is unlikely to be more severe than delta, which is which is very, very good news, but I'm still skeptical of claims that there's significantly reduced severity," Arwady said. "So great news, it doesn't look like it's making people sicker than what we have now, but I do know there are a lot of people who feel like this is really not making people sick at all and that is not yet something that my team feels that the data shows."

For unvaccinated populations, particularly those most susceptible to COVID, Arwady anticipates "accelerated transmission and then a rising number of hospitalizations for severe illness."

Her comments echo the early findings of a study by the U.K.'s Imperial College London, which indicate there is currently no evidence the variant is any less severe than delta.

"The study finds no evidence of omicron having lower severity than delta, judged by either the proportion of people testing positive who report symptoms, or by the proportion of cases seeking hospital care after infection," the research team said Friday in a blog post accompanying the study.

Meanwhile, an analysis last week of data from South Africa, where omicron is driving a surge in infections, found the variant seems to not just spread more easily from person to person, but is better at evading vaccines while causing less serious illness.

The World Health Organization's COVID-19 technical lead, Maria Van Kerkhove, said Wednesday that increased transmission will result in increased hospitalizations that burden health-care systems, some of which will fail.

“Omicron is spreading at a rate we have not seen with any previous variant. We are concerned that people are dismissing omicron as mild,” WHO Director-General Tedros Adhanom Ghebreyesus said. “Surely we have learned by now that we underestimate this virus at our peril.”

Even if omicron proves milder on the whole than delta, it may disarm some of the life-saving tools available and put immune-compromised and elderly people at particular risk as it begins a rapid assault on the United States.

And if a dueling surge of both delta and omicron hits, experts fear hospitals and staff won't be able to withstand the increase.

“Our delta surge is ongoing and, in fact, accelerating. And on top of that, we’re going to add an omicron surge,” said Dr. Jacob Lemieux, who monitors variants for a research collaboration led by Harvard Medical School.

“That’s alarming, because our hospitals are already filling up. Staff are fatigued,” leaving limited capacity for a potential crush of COVID-19 cases “from an omicron wave superimposed on a delta surge.”

The White House on Wednesday insisted there is no need for a lockdown because vaccines are widely available and appear to offer protection against the worst consequences of the virus.

Arwady said the city is continuing to push for vaccinations and booster shots for that same reason, though an increase in breakthrough infections is expected and there are heightened concerns for the unvaccinated.

"We do think at this point, vaccines are very likely to continue to protect against severe disease, but because there will be probably more transmission that makes it harder to control efforts," she said. "And it puts everybody at some more risk."

So far, the Pfizer vaccine seems to offer less defense against infection from the variant, but still good protection from hospitalization, according to an analysis of data from South Africa, where the new variant is driving a surge in infections.

But Pfizer and its partner BioNTech noted that while two doses may not be protective enough to prevent infection, lab tests showed a booster increased by 25-fold people's levels of virus-fighting antibodies.

Moderna said Monday its COVID-19 booster does appear to provide protection against the omicron variant.

In an announcement early Monday, the drug company said preliminary data from lab testing found the version of its booster currently in use in the United States and elsewhere provided increased antibody levels to neutralize the virus. But it also found that a double dose of the booster shot provided a much greater increase in those levels.

Fauci said Wednesday there is no need, for now, for an omicron-specific booster shot. The two-dose mRNA vaccines, the Pfizer and Moderna shots, still appear to offer considerable protection against hospitalization from omicron, Fauci said.

“If we didn’t have these tools, I would be telling you to be really, really worried,” Fauci said.

Still, in Illinois, cases and hospitalizations continue to rise leading up to the holiday.

According to the Illinois Department of Public Health, the state has reported 12,328 new cases of the virus in the last day, the most it has recorded in a single day in 2021. In fact, the number is the most in a single day since Dec. 1, 2020, when 12,542 new cases were reported, according to IDPH data.

Across the state, at least 17 omicron cases have been detected so far, according to IDPH, with local health officials confirming at least two in the Chicago area.

At the same time, the state is also seeing a surge in newly-hospitalized coronavirus patients, with more residents seeking emergency medical care than at any point so far this year.

The Cook County Department of Public Health announced the variant was identified in a case last week, but did not specify which suburb the variant was found in. The case was reported in an asymptomatic person who had received at least two doses of the coronavirus vaccine, officials said.

The person had contact with someone else who tested positive for the omicron variant, but officials would not say whether that contact was in the state of Illinois or out of the state or the country.

Additional cases could soon be identified as "there are other cases that are currently being genotyped for omicron," the Cook County Department of Public Health stated.

In Chicago, a case was reported in a fully vaccinated city resident who had also received a booster dose but was visited by an out-of-state traveler who also tested positive for the variant. The resident did not require hospitalization and had been self-isolating since their symptoms began, officials said.

Arwady also said at the time that test results were pending for city residents who are known contacts of out-of-state or out-of-country omicron variant cases.

NBC Chicago/Associated Press
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