The virus that causes COVID-19 is constantly changing and adapting. New variants of the virus are expected to occur, as has been the case in the United States and globally throughout the pandemic. A Variant of Concern (VOC) is "a variant for which there is evidence of an increase in transmissibility, more severe disease (for example, increased hospitalizations or deaths), significant reduction in neutralization by antibodies generated during previous infection or vaccination, reduced effectiveness of treatments or vaccines, or diagnostic detection failures."
Currently, there is one VOC in the United States: Omicron. The Omicron variant was first classified as a VOC in the United States on November 30, 2021. Omicron became the main variant circulating in Colorado starting the week of December 19, 2021, and has been the dominant variant in the United States, including in the Mountain West region.
Omicron spreads more easily than earlier variants. Data indicate that Omicron causes less severe illness and death, in general. COVID-19 vaccination status, other health conditions, age, and history of prior infection can affect the presence and severity of symptoms.
Still, all approved COVID-19 vaccines reduce the risk of severe illness, hospitalization, and death from COVID-19.
For more information about variants, go to the CDC's "Variants of the Virus" webpage.