An unvaccinated man in his 50s with underlying health conditions is believed to have been the first American to die from the omicron variant of COVID-19. Harris County Judge Lina Hidalgo confirmed on Monday that he became the first to die from omicron in the county.

The death comes as omicron spreads rapidly throughout the United States, replacing delta as the dominant strain. 

Omicron accounted for 73.2% of new cases in the week that ended Dec. 18, up from just 12.6% the prior week, according to data released by the CDC on Monday. 

A person has a COVID-19 test administered at a walk-up testing site on December 15, 2021 in New York City. (Photo by David Dee Delgado/Getty Images)

Omicron has been reported in nearly every state, and it accounts for about 90% of new cases in some areas of the country, such as New York, the Southeast, and the Pacific Northwest. 

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The omicron variant was first discovered in South Africa less than a month ago and the World Health Organization declared it a "variant of concern" on Nov. 26. 

It's unclear if omicron will cause more or less severe illness, but the CDC says "it likely will spread more easily than" previous variants. 

Current vaccines are likely to protect against severe illness and death from omicron, but breakthrough infections are likely in people who are fully vaccinated, according to the CDC. 

Booster shots may help fend off the new variant. Moderna announced Monday that a half-dose booster shot increased the level of neutralizing antibodies by 37 fold. A full-dose booster caused an 83-fold increase in antibodies. 

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Cases have surged in recent weeks as the 7-day average was 132,659 on Sunday, up from a low in the fall of 64,161 on Oct. 24.