Variant Proportions

Monitoring Variant Proportions

Variants of SARS-CoV-2, the virus that causes COVID-19, are expected to continue to emerge. Some will emerge and disappear, and others will emerge and continue to spread and may replace previous variants. For example, the variant of the virus that caused the first U.S. COVID-19 cases in January 2020 is no longer detected among variants circulating in the country.

CDC’s national SARS-CoV-2 genomic surveillance program identifies and tracks SARS-CoV-2 variants circulating in the United States and the proportion of each variant causing infections, by region. The thousands of sequences provided every week through CDC’s national genomic surveillance efforts fuel the comprehensive and population-based U.S. surveillance system required to monitor the spread of variants.

Widespread, rapid genomic sequencing provided by the program, combined with phenotypic data, are used to determine whether COVID-19 tests, treatments, and vaccines authorized for use in the United States will work against each variant.

Nowcast

Nowcast is a model that projects more recent proportions of circulating variants and enables timely public health action.