Viruses mutate, or change, over time. As the COVID-19 virus spreads, it makes copies of itself that can have different genetic codes — these are called variants. Variants can make the virus easier to spread, more severe or able to evade antibodies from previous infections or vaccines. Scientists monitor these variants to better understand how the virus affects people and ways to stop it.
Every week, samples from people with COVID-19 are tested for the presence of different variants. Those that have features that could reduce the ability of the immune system to prevent infection or to respond to existing treatments or vaccines are called variants of concern (VOC). CDC tracks these and other new variants in cooperation with state health departments.
The most common variant in the United States is delta, followed by omicron and alpha. Omicron has several major offshoots, or sublineages. Together, omicron and its offshoots account for most of the cases reported in the United States.
Researchers and community members are spotting more variants than ever before. These include NB.1.8.1 – unofficially nicknamed Nimbus by community science and viral monitoring groups – and XFG – which isn’t yet designated for monitoring by WHO, but has raised concerns amongst virus spotters because of the effect it may have on vaccine and antibody response in lab tests. These two variants — and others that have been spotted in other parts of the country — are also a reminder that the vaccine we currently have available can still protect against severe illness, hospitalization and death.