If a virus gets changed in a way that affects how it causes disease, doctors call it a variant. So far, COVID-19 has spawned five variants: alpha, beta, gamma, delta and omicron. Variants can make people severely ill and lead to symptoms that last months or years, and can even be fatal. They can also spread the virus more easily, make vaccines less effective and cause medicines used to treat or prevent COVID-19 to stop working or work differently.

Coronaviruses, like the original COVID-19 strain, have crown-like spikes on their surface that help them attach to and infect cells. When mutations in the spike protein occur, it can make a virus more contagious, cause more severe illness and reduce the effectiveness of treatments or vaccines. That’s why scientists are avidly tracking these variants.

A variant is identified when a DNA sample shows the new genetic sequence of the virus. CDC’s national network of labs collects these samples, including throat swabs, sputum, blood and urine. They’re sent to a central laboratory for analysis. Vaccine efficacy and other public health information are then tracked using the COVID-19 Nowcast system.

A negative antibody test means a person has antibodies against COVID-19 that they created either from previous infection or vaccination. However, antibodies levels decrease over time, so getting vaccinated and staying up to date on boosters is important.