The delta strain, first identified in India, has rapidly risen to dominance in the United States and in many parts of the world, likely due to a significantly higher transmissibility rate. It can spread through fleeting transmission and required less time post-infection to become transmissible. Delta shows significantly reduced susceptibility to the combination of bamlanivimab and etesevimab monoclonal antibody treatment, though other EUA monoclonal antibody treatments are available. Delta also displays reduced neutralization by convalescent and post-vaccination sera.
The CDC has identified four "variants of concern," which are variants whose mutations significantly alter the virus’s ability to infect cells, enhance viral transmissibility, or cause more severe illness. These include alpha, beta, gamma, and delta.