Viruses mutate constantly. If a particular mutation helps the virus replicate more effectively, then that mutation becomes dominant and potentially more dangerous. Multiple variants of SARS-CoV-2 have emerged and circulated globally throughout the COVID-19 pandemic.

In order to better study and understand these variants’ behavior and their relationship to one another, the U.S. SARS-CoV-2 Interagency Group (SIG) group developed a classification code that defines three classes of viral variants:*

• Variants of Interest
• Variants of Concern
• Variants of High Consequence

Using this code enables health, safety, and research organizations, such as the Centers for Disease Control and Prevention (CDC) and the National Institutes of Health (NIH), to coordinate and focus their efforts to identify emerging strains and predict their potential impacts on vaccines, diagnostic abilities, and treatments.

The Variant Classifications

The status of a given variant can change based on updated scientific evidence. Although the World Health Organization (WHO) uses the same general classification code as the United States, the actual variants that fall under those labels on the WHO’s list may differ because WHO is a global organization, and variant importance can differ by location. Variants are referred to by letters of the Greek alphabet in public communications for the convenience of non-scientific audiences. 

Variants of Interest

A variant of interest is a virus strain with specific genetic markers associated with:

• Changes to receptor binding 
• Reduced neutralization by antibodies 
• Reduced efficacy of treatments 
• Potential diagnostic impact 
• A minor increase in transmissibility and/or disease severity

A variant of interest may require public health actions in order to assess how easily the virus spreads from one person to another, the severity of illness, treatment efficacy, and degree of protection provided by vaccines.

Variants of Concern

A variant of concern is one that has provided evidence of:

• Higher transmission rates 
• More severe illness (e.g., resulting in increased hospitalizations or deaths) 
• Significant reduction in neutralization by antibodies (from illness or vaccine) 
• Reduced effectiveness of treatments 
• Failure to diagnose based on previous knowledge of the virus/disease

Classification of a variant of concern might require: 

• Notifying the WHO, under the International Health Regulations, 
• Reporting to the CDC, as well as regional and/or local health organizations, to help:
  • Control spread 
  • Increase testing  
  • Inform research 
• Other public health actions, depending on the circumstances.

Variants of High Consequence

A variant of high consequence has provided clear evidence that either prevention measures or countermeasures are significantly less effective than they were against prior strains. This evidence can include:

• Failure to diagnose based on previous knowledge of the virus/disease
• A disproportionately high number of vaccine breakthrough cases
• Low protection from vaccines against severe illness
• Low response to multiple Emergency Use Authorization treatments
• More severe illness and a higher number of hospitalizations

As with variants of concern, identification of variants of high consequence require notifying the WHO and reporting to the CDC and other health organizations. It also mandates an announcement of strategies to prevent or contain transmission, along with recommendations to update treatments and vaccines. There are currently no SARS-CoV-2 strains classified as variants of high consequence in the U.S.

*Centers for Disease Control and Prevention. (2021, July 6). SARS-CoV-2 Variant Classifications and Definitions: A Guide [Web Page]. https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-info.html

Much about the novel coronavirus, i.e., COVID-19, is still not fully understood. As research progresses and our knowledge of the virus increases, information can change rapidly. We strive to update all of our articles as quickly as possible, but there may occasionally be some lag between scientific developments and our revisions.