Among the total of 13,052 variants cases reported by the CDC as of Thursday, 12,505 cases were caused by the variant known as B.1.1.7, which was originally detected in Britain.

There were 323 cases of a new strain initially discovered in South Africa, called B.1.351, and 224 cases of the P.1 strain first discovered in Brazil.

In addition, the B.1.427 and B.1.429 variants, two coronavirus strains first detected in the U.S. state of California, are also being closely monitored by the CDC.

The five coronavirus strains are currently classified by the CDC as “variants of concern,” as evidence shows an increase in their transmissibility, increased hospitalizations or deaths, reduced effectiveness of treatments or vaccines, and diagnostic detection failures.