SARS-CoV-2 Variant Classifications and Definitions
There are multiple ways in which SARS-CoV-2 viruses are classified. Each classification type can be appropriate, depending on the context in which SARS-CoV-2 is being communicated. SARS-CoV-2 is often discussed in the context of lineages (and sublineages). The most commonly used classification system for lineages is Pango. Nextclade may also be used in this context. In a larger context, lineages or groups of related lineages may be classified using Greek letters (such as Omicron) by the World Health Organization (WHO). These classification methods enable scientists to communicate similarities and differences between SARS-CoV-2 viruses.
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 characterizes emerging variants and monitors their potential impact on vaccines, therapeutics, and diagnostics.
The SIG meets regularly to evaluate SARS-CoV-2 variants and lineages circulating in the United States and to make recommendations about the classification of variants and lineages. A group of experts assess the available data on variant proportion at the national and regional level. This information is shared with local and state public health officials to aid in their decision-making for community-level guidance. They also assess how these changes may affect vaccines, therapeutics, and diagnostics, as well as transmission and severity of disease. Variants may be reclassified as more information becomes available.
The SIG has updated its classification system and working definitions for variants of SARS-CoV-2, the virus that causes COVID-19, to better reflect the current variant landscape. The previous system classified all Omicron sublineages as part of the Omicron VOC and therefore did not provide the distinction needed to compare new descendent lineages with altered phenotypic characteristics to the Omicron parent lineages (BA.1, BA.2). These classification updates allow for independent evaluation and tracking of Omicron sublineages and forthcoming new variants when required.
Notes: Each variant classification includes the possible attributes of lower classes (for example, VOC includes the possible attributes of VOI). This page will be updated as needed to show the variants that belong to each class. U.S. classifications may differ from the WHO classifications because the impact of variants may differ by location. The labels assigned to each variant are provided in the List of Variants Table.
Pango Lineage System
Lineages are named using an alphabetical prefix (such as B or BA) and numerical suffix (such as “.1” or “.1.1.5”). When a new lineage is defined, the Pango system assigns an additional number to the name of its parent lineage (e.g., BA.2.75 is a sublineage of BA.2). As the virus continues to change, the Pango lineage names can become very long. Lineages with longer names may be given alphabetic aliases and numbering continues (e.g., “BA” stands for “B.1.1.529,” thus BA.2 is the same as B.1.1.529.2).
Members of a clade will share many features with each other, reflecting their common ancestry. Not every SARS-CoV-2 virus is assigned its own clade by Nextclade. Each potential clade must circulate at a certain frequency for a period of time, show consistent growth in a region, or be assigned a WHO classification. The Nextclade team outlines their clade designation rubric in this publication. These broader definitions offer an alternative naming solution that is meant to reflect significant differences in biology or circulation.
Nextclade clade names consist of the two-digit year of assignment followed by a letter indicating the order of assignment within the year. For example, “22A” is the first clade designated in 2022 and corresponds to the group of Pango lineages that descended from BA.4, which was circulating in early 2022. More information about the various clades and their relationship to the various Pango lineages is detailed at covariants.org.