U.S. Department of Health & Human Services

12/02/2021 | Press release | Distributed by Public on 12/01/2021 19:12

SARS-CoV-2 Variant Classifications and Definitions

Variant of Concern (VOC)

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.

Possible attributes of a variant of concern:

In addition to the possible attributes of a variant of interest

  • Evidence of impact on diagnostics, treatments, or vaccines
    • Widespread interference with diagnostic test targets
    • Evidence of substantially decreased susceptibility to one or more class of therapies
    • Evidence of significantly decreased neutralization by antibodies generated during previous infection or vaccination
    • Evidence of reduced vaccine-induced protection from severe disease
  • Evidence of increased transmissibility
  • Evidence of increased disease severity

Variants of concern might require one or more appropriate public health actions, such as notification to WHO under the International Health Regulations, reporting to CDC, local or regional efforts to control spread, increased testing, or research to determine the effectiveness of vaccines and treatments against the variant. Based on the characteristics of the variant, additional considerations may include the development of new diagnostics or the modification of vaccines or treatments.

Current variants of concern in the United States that are being closely monitored and characterized are listed below. This table will be updated when a new variant of concern is identified.

(*) = detected in some sequences but not all

a - Phylogenetic Assignment of Named Global Outbreak (PANGO) Lineages is software tool developed by members of the Rambaut Lab. The associated web application was developed by the Centre for Genomic Pathogen Surveillance in South Cambridgeshire and is intended to implement the dynamic nomenclature of SARS-CoV-2 lineages, known as the PANGO nomenclature.

b - Nextstrain, a collaboration between researchers in Seattle, USA and Basel, Switzerland, provides open-source tools for visualizing the genetics of outbreaks. The goal is to support public health surveillance by facilitating understanding of the spread and evolution of pathogens.

Characteristics of Selected SARS-CoV-2 Variants

WHO Label: Delta

Pango Lineage: B.1.617.2 and AY lineages(Pango lineageexternal icon )a

Spike Protein Substitutions: T19R, (V70F*), T95I, G142D, E156-, F157-, R158G, (A222V*), (W258L*), (K417N*), L452R, T478K, D614G, P681R, D950N

Nextstrain clade (Nextstrainexternal icon )b: 21A/S:478K

First Identified: India


  • Increased transmissibility29
  • Nearly all lineages designated as Delta are susceptible to Emergency Use Authorization (EUA) monoclonal antibody treatments. AY.1 and AY.2 lineages are not susceptible to some monoclonal antibody treatments.7, 14
  • Reduction in neutralization by post-vaccination sera21

WHO Label: Omicron

Pango Lineage: B.1.1.529 (Pango lineageexternal icon )a

Spike Protein Substitutions: A67V, del69-70, T95I, del142-144, Y145D, del211, L212I, ins214EPE, G339D, S371L, S373P, S375F, K417N, N440K, G446S, S477N, T478K, E484A, Q493R, G496S, Q498R, N501Y, Y505H, T547K, D614G, H655Y, N679K, P681H, N764K, D796Y, N856K, Q954H, N969K, L981F

Nextstrain clade (Nextstrainexternal icon )b: 21K

First Identified: South Africa


  • Potential increased transmissibility
  • Potential reduction in neutralization by some EUA monoclonal antibody treatments
  • Potential reduction in neutralization by post-vaccination sera

Variant of High Consequence (VOHC)

A VOHC has clear evidence that prevention measures or medical countermeasures (MCMs) have significantly reduced effectiveness relative to previously circulating variants.

Possible attributes of a variant of high consequence:

In addition to the possible attributes of a variant of concern

  • Impact on MCMs
    • Demonstrated failure of diagnostic test targets
    • Evidence to suggest a significant reduction in vaccine effectiveness, a disproportionately high number of infections in vaccinated persons, or very low vaccine-induced protection against severe disease
    • Significantly reduced susceptibility to multiple EUA or approved therapeutics
    • More severe clinical disease and increased hospitalizations

A variant of high consequence would require notification to WHO under the International Health Regulations, reporting to CDC, an announcement of strategies to prevent or contain transmission, and recommendations to update treatments and vaccines.

Currently, no SARS-CoV-2 variants are designated as VOHC.