The now "dominant" omicron variant of the coronavirus is causing more than 50% of new COVID-19 cases in Minnesota, state leaders said Wednesday, prompting health systems to suspend use of two monoclonal antibody therapies that are ineffective against the strain.

Omicron is widespread enough that it would be futile to give infusions of antibodies that don't work against it — Eli Lilly's bamlanivimab/etesevimab and Regeneron's casirivimab/imdevimab, said Dr. Ruth Lynfield, state epidemiologist.

"By the end of last week, we were probably about 20 percent" in the rate of infections involving the variant, she said. "So, given the doubling rate of omicron, we do think it currently is the dominant strain."

Federal estimates earlier this week indicated that omicron had caused 73% of recent coronavirus infections nationally and 92% of infections in a Midwest region including Minnesota. Lynfield said that estimate is likely too high for Minnesota, but that the variant makes up the majority of new infections in the state and is replacing the delta variant that caused this fall's pandemic wave.

Minnesota is among the most aggressive states in analyzing samples from patients with COVID-19 to identify variants and reported Wednesday that it had found 65 cases of omicron. State health officials said that is an exponential increase from seven last week and is significant considering how few samples are submitted for genomic sequencing.

"What we are identifying is an undercount of what is circulating in Minnesota," said Kris Ehresmann, state infectious disease director. "This variant is highly transmissible and we are seeing evidence of that when we look at the very speedy spread throughout the state."

Omicron was labeled a variant of concern because it quickly overtook delta as the dominant strain when it was discovered in South Africa last month and showed some ability to evade immunity. It's unclear whether the high number of vaccinations and infections in Minnesota this fall could reduce omicron spread compared with other states that had peaks in their delta waves this summer.

Omicron appears to cause a lower rate of severe illness, but spreads so widely that it could end up as bad or worse than delta, said Dr. Andrew Badley, chair of Mayo Clinic's COVID-19 task force. "If we have more cases but a smaller proportion are sick, the net effect is that the burden on hospitals remains the same."

Health officials stressed that vaccination against COVID-19 remains the best protection along with social distancing and mask-wearing in crowds. The state advises testing before large gatherings so people can try to ensure that they aren't spreading the virus.

Monoclonal antibodies are infusions or injections mostly provided on an outpatient basis in the early stages of COVID-19 to prevent severe illness and hospitalization. The state expects to have 2,000 doses of GlaxoSmithKline's sotrovimab, the antibody therapy that works against the omicron variant, through Jan. 3. Supplies are being conserved for people at greatest risk.

The loss of the two most common monoclonal antibodies in Minnesota's stockpile is significant for a state that was increasing infusion capacity in response to COVID-19. Capacity had risen from 2,000 infusions per week to as many as 3,000 but will decline to less than 1,000.

The loss is offset by Wednesday's news that the Food and Drug Administration granted emergency-use authorization for a Pfizer oral medication to treat COVID-19 — though initial supplies will be limited.

"The availability of that could be a game-changer in terms of our ability to treat COVID-19 in the outpatient setting," Badley said.

The fast-spreading delta variant fueled a prolonged COVID-19 wave this fall in Minnesota, which has reported 10,254 COVID-19 deaths and 992,851 infections overall. That includes 57 deaths and 2,807 infections added Wednesday.

The wave has been declining for two weeks, with the positivity rate of COVID-19 testing dropping below the 10% high-risk threshold to 9%. COVID-19 hospitalizations in Minnesota declined from 1,678 on Dec. 9 to 1,432 on Tuesday. Hospitals remain swamped by COVID and non-COVID admissions and reported just 23 open adult intensive care beds out of 1,012.

A memo to providers of monoclonal antibodies warned them to expect "significant scarcity in the coming weeks relative to demand" for sotrovimab. The memo said providers should "forestall" use of the other types but could still administer them to lower-risk patients with existing appointments for the next few days.

Minnesota for the past month has used a scoring system to prioritize its limited antibody therapies, giving preference to people with COVID-19 who are older, pregnant or minorities or have diabetes or diseases of the lungs, kidneys or heart that elevate their risks for severe illness and hospitalization. Sotrovimab is now being reserved for people with COVID-19 whose combinations of demographics and disease history produce the highest score.

"People who are at some risk, but not necessarily the highest risk, should not expect to get access in the short term, and they should definitely be in touch with their providers in case their symptoms worsen," said JP Leider, a public health researcher at the University of Minnesota. He helped created the Minnesota Resource Allocation Platform, MNRAP, which prioritizes patients for monoclonal antibodies provided by several health care providers.

The expected shortage of sotrovimab means that Minnesota might need to use a lottery system if there are more qualifying high-risk patients than available doses, he added.

The state memo was specific to MNRAP providers, a group that excludes Mayo, CentraCare in St. Cloud and Essentia Health in Duluth. Those providers are required to have their own scoring systems for determining patient need.

Badley said Mayo had suspended use of the other two forms of antibodies. Minneapolis-based Allina Health is part of MNRAP and suspended their use as well. Dr. John Misa, Allina's system clinical officer, said the system had enough sotrovimab to switch and use on high-risk patients with appointments this week.

"If we go on and continue to use the existing antibody therapies until those supplies run out, with the hopes that perhaps they will be effective given that some people may have delta out there ... if may give a false sense of security to people who have omicron that they've been treated appropriately," he said.

Antibodies are more effective when given early, so doctors can't wait for genomic sequencing results to identify the variants involved.

While switching to one monoclonal antibody "is the right thing to do," the other versions might have value regionally, said Dr. Jeremy Cauwels, chief physician of Sanford Health, a large provider of monoclonal antibodies in greater Minnesota and the Dakotas.

"It will be very important to understand how fast omicron takes over individual counties in MN [and elsewhere]," he said in an e-mail. "A county that is still mostly seeing delta cases could treat patients well with any antibody."