Updated Dec. 6, 2021

On November 24, 2021, South Africa reported the identification of a new SARS-CoV-2 variant, B.1.1.529, to the World Health Organization (WHO). B.1.1.529 was first detected in specimens collected on November 11, 2021 in Botswana and on November 14, 2021 in South Africa. South Africa has since detected B.1.1.529 in specimens collected on November 8, 2021. On December 1, 2021, the first case attributed to B.1.1.529 was reported in the United States in a person who returned from travel to South Africa. A second case was reported on December 2, 2021 in a person with no international travel history who also attended a convention in the days preceding symptom onset. The Omicron variant has also been detected in travel-related cases in several European countries, as well as Australia, Brazil, Canada, Hong Kong, Israel, Japan, Nigeria, Norway, Sweden, and the United Kingdom. A few countries, including the United States, have reported cases in individuals without travel history to southern Africa.

On November 25, 2021, the United Kingdom Health Security Agency designated B.1.1.529 as a Variant Under Monitoring (VUI-21-NOV-01) 1. On November 26, 2021, the Technical Advisory Group on SARS-CoV-2 Virus Evolution (TAG-VE)2 convened to assess B.1.1.529. The TAG-VE advised WHO that this variant should be designated as a Variant of Concern (VOC), and WHO designated B.1.1.529 as a VOC named Omicron.3

The WHO classification as a VOC was based on epidemiological data indicating an increase in infections in South Africa in recent weeks that coincided with detection of Omicron. Omicron has many concerning spike protein substitutions, some of which are known from other variants to be associated with reduced susceptibility to available monoclonal antibody therapeutics or reduced neutralization by convalescent and vaccinee sera. The European Center for Disease Prevention and Control also classified this variant as a VOC due to concerns “regarding immune escape and potentially increased transmissibility compared to the Delta variant.”4

The SARS-CoV-2 Interagency Group (SIG), established by the U.S. Department of Health and Human Services, is responsible for variant classifications in the United States.i The SIG meets regularly to evaluate the risk posed by SARS-CoV-2 variants circulating in the United States and globally and to make recommendations about the classification of variants. On November 30, 2021, the SIG made the decision to classify the Omicron variant as a Variant of Concern (VOC). This decision is based on a number of factors, including, detection of cases attributed to Omicron in multiple countries, including among those without travel history, transmission and replacement of Delta as the predominant variant in South Africa, the number and locations of substitutions in the spike protein, and available data for other variants with fewer substitutions in the spike protein indicating a reduction in neutralization by vaccinee and convalescent sera and certain monoclonal antibody treatments.

There are two variants classified as a VOC by the United States: Omicron and Delta. As of December 2, 2021, two confirmed cases attributed to the Omicron variant have been detected in the United States and additional possible Omicron cases are being investigated. Delta continues to be the predominant circulating variant. On August 26, 2021, CDC published information on What We Know About the Delta Variant. Importantly, nearly all lineages designated as Delta remain susceptible to available monoclonal antibody therapeutics, and vaccines continue to be highly effective against severe illness, hospitalization, and death among people infected with the Delta variant.