Viruses like SARS-CoV-2 continuously evolve as changes in the genetic code (genetic mutations) occur during replication of the genome. A lineage is a genetically closely related group of virus variants derived from a common ancestor. A variant has one or more mutations that differentiate it from other variants of the SARS-CoV-2 viruses. As expected, multiple variants of SARS-CoV-2 have been documented in the United States and globally throughout this pandemic. To inform local outbreak investigations and understand national trends, scientists compare genetic differences between viruses to identify variants and how they are related to each other.
Key Definitions
Mutation: A mutation refers to a single change in a virus’s genome (genetic code). Mutations happen frequently, but only sometimes change the characteristics of the virus.
Lineage: A lineage is a group of closely related viruses with a common ancestor. SARS-CoV-2 has many lineages; all cause COVID-19.
Variant: A variant is a viral genome (genetic code) that may contain one or more mutations. In some cases, a group of variants with similar genetic changes, such as a lineage or group of lineages, may be designated by public health organizations as a Variant of Concern (VOC) or a Variant of Interest (VOI) due to shared attributes and characteristics that may require public health action.
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The U.S. Department of Health and Human Services (HHS) established a SARS-CoV-2 Interagency Group (SIG) to enhance coordination among CDC, National Institutes of Health (NIH), Food and Drug Administration (FDA), Biomedical Advanced Research and Development Authority (BARDA), and Department of Defense (DoD). This interagency group is focused on the rapid characterization of emerging variants and actively monitors their potential impact on critical SARS-CoV-2 countermeasures, including vaccines, therapeutics, and diagnostics.
The SIG meets regularly to evaluate the risk posed by SARS-CoV-2 variants circulating in the United States and to make recommendations about the classification of variants. This evaluation is undertaken by a group of subject matter experts who assess available data, including variant proportions at the national and regional levels and the potential or known impact of the constellation of mutations on the effectiveness of medical countermeasures, severity of disease, and ability to spread from person to person. Given the continuous evolution of SARS-CoV-2 and our understanding of the impact of variants on public health, variants may be reclassified based on their attributes and prevalence in the United States.
Notes: Each variant classification includes the possible attributes of lower classes (for example, VOC includes the possible attributes of VOI); variant status might escalate or deescalate based on emerging scientific evidence. This page will be updated as needed to show the variants that belong to each class. The World Health Organization (WHO) external icon also classifies variant viruses as variants of concern and variants of interest; U.S. classifications may differ from those of WHO because the impact of variants may differ by location. To assist with public discussions of variants, WHO proposed using labels consisting of the Greek alphabet (for example, alpha, beta, gamma) as a practical way to discuss variants for non-scientific audiences. The labels assigned to each variant are provided in the tables below.
CDC monitors all variants circulating in the United States. Variants designated as VBM include those where data indicates there is a potential or clear impact on approved or authorized medical countermeasures or that have been associated with more severe disease or increased transmission but are no longer detected, or are circulating at very low levels, in the United States. These variants do not pose a significant and imminent risk to public health in the United States.
A Variant of Interest or a Variant of Concern may be downgraded to this list after a significant and sustained reduction in its national and regional proportions over time, or other evidence indicates that a variant does not pose significant risk to public health in the United States.
These variants continue to be closely monitored to identify changes in their proportions and new data are continually being analyzed. If the data indicate that a VBM warrants more concern, the classification will be changed based on the SIG assessment of the attributes of the variant and the risk to public health in the United States.
WHO Label | Pango Lineage | Date of Designation | ||
---|---|---|---|---|
Alpha | B.1.1.7 and Q lineages | VOC: December 29, 2020 | VBM: September 21, 2021 | |
Beta | B.1.351 and descendent lineages | VOC: December 29, 2020 | VBM: September 21, 2021 | |
Gamma | P.1and descendent lineages | VOC: December 29, 2020 | VBM: September 21, 2021 | |
Epsilon | B.1.427 B.1.429 |
VOC: March 19, 2021 | VOI: February 26, 2021 VOI: June 29, 2021 |
VBM: September 21, 2021 |
Eta | B.1.525 | VOI: February 26, 2021 | VBM: September 21, 2021 | |
Iota | B.1.526 | VOI: February 26, 2021 | VBM: September 21, 2021 | |
Kappa | B.1.617.1 | VOI: May 7, 2021 | VBM: September 21, 2021 | |
N/A | B.1.617.3 | VOI: May 7, 2021 | VBM: September 21, 2021 | |
Zeta | P.2 | VOI: February 26, 2021 | VBM: September 21, 2021 | |
Mu | B.1.621, B.1.621.1 | VBM: September 21, 2021 |
A variant with specific genetic markers that have been associated with changes to receptor binding, reduced neutralization by antibodies generated against previous infection or vaccination, reduced efficacy of treatments, potential diagnostic impact, or predicted increase in transmissibility or disease severity.
Possible attributes of a Variant of Interest:
A Variant of Interest might require one or more appropriate public health actions, including enhanced sequence surveillance, enhanced laboratory characterization, or epidemiological investigations to assess how easily the virus spreads to others, the severity of disease, the efficacy of therapeutics and whether currently approved or authorized vaccines offer protection.
Currently, no SARS-CoV-2 variants are designated as VOI.
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
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.
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
Attributes:
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
Attributes:
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
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.